## 1. The Metaphysics of Natural Kinds



The principal metaphysical questions concerning natural kinds are
fourfold. First, are the kinds that we think of as
'natural' kinds genuinely *natural*? Or are our
classification processes more anthropocentric than that? Secondly, are
our natural kind classifications really classifications into
*kinds*? Thirdly, what are natural kinds? Are natural kinds any
sort of entity at all? Are they basic ontological entities or are they
derived from or reducible to other entities (e.g., universals)?
Fourthly, do natural kinds have essences?



As regards the first question, the general problem is to determine
which of the kinds to which science makes appeal, if any, correspond
to real natural kinds--those existing in nature, so to
speak--and which of these kinds are merely
conventional--those whose boundaries are fixed by us rather than
nature. The second question builds on the first by noting that in
addition to classifying things in a natural way, classification by
natural kinds, is also classification of things into kinds. What then
is it to classify by kind? Then, if natural kinds are genuinely
natural and genuinely kinds, what are they? Are there entities that
are the natural kinds, and if so what can we say about the nature of
such entities? Are they irreducible, basic, *sui generis*
entities (alongside, for example, particulars and universals) (Ellis
2001; Lowe 1998)? Or can they be reduced, say to universals (Armstrong
1978, 1997; Hawley and Bird 2011) or to clusters of properties (Boyd
1991, Millikan 1999)? The third question asks whether there are
properties that might be essential for kind membership. Natural kind
essentialists hold that natural kinds have essences (Ellis 2001, 2002;
Devitt 2008), where the essence of a natural kind is a property or set
of properties whose possession is a necessary and sufficient condition
for a particular's being a member of the kind. That fact is a
so-called essential fact concerning the kind; it is a fact that, in
Fine's terms, stems from the identity or nature of the kind
(Fine 1994).



We can use answers to these questions to articulate the metaphysical
commitment of a theory of natural kinds (Bird 2018). At one extreme
the out-and-out conventionalist may claim that there is no natural
sameness and difference in nature (or at least that our human
classifications do not capture any such natural sameness and
difference). Afortiori, there are no classifications into natural
*kinds*, no entities that are natural kinds, and no natural
kinds essences. At the other extreme, one might hold that there are
natural distinctions between things, some of these distinctions are
distinctions of kinds, that the natural kinds are a certain species of
entity, that these entities are irreducible, and that they have
essences. Intermediate views tend to be more popular. For example, a
common view is that there are natural classifications by kinds, but
there are no entities that are the natural kinds. Some hold that view
and accept essentialism about natural kinds, while others hold that
view and reject essentialism.


### 1.1 Natural Classifications


#### 1.1.1 Naturalism (weak realism)



A common view concerning our so-called natural kind classifications is
that there are genuinely *natural* ways of classifying things.
Intuitively, to group all instances of the metal zinc together is to
engage in a natural classification, whereas to group together
Trajan's column, the number two, and Julius Caesar is to
classify things in an arbitrary manner. The classifications
'cars manufactured in Germany', 'culinary
vegetables', 'altars' are not arbitrary, but neither
are they natural, since they reflect human interests. This view we
call *naturalism*. It is often called *realism* because
of the close affinity to scientific realism; indeed naturalism may be
regarded as a consequence or component of standard scientific realism.
Scientific realists hold that successful scientific theories represent
theory-independent phenomena. Thus the theoretical posits of science
reflect natural divisions in nature; for example, the periodic system
for classifying the chemical elements reflects natural divisions
between the elements. However, since 'realism' is also
associated with ontological doctrines (e.g., realism about universals,
modal realism etc.) it would be preferable to use *weak
realism* (or, as above, *naturalism*) to label the
ontologically uncommitted view that our classifications are often
natural. (We will note below that not all natural classifications are
classifications into natural kinds.)



Naturalism need not maintain that all the classifications we in fact
take to be natural are indeed natural. Sometimes scientists and others
err in the natural kinds they take to exist. There is no kind that is
phlogiston or phlogisticated air. Many might suppose jade to be a
natural kind of mineral, but it is not because there are two distinct
minerals called 'jade'. (See LaPorte 2004, 94-100
and Hacking 2007b for detailed discussions of jade.) Similarly,
biologists now believe that reptiles do not form a natural kind.
Naturalism is the view that there are in fact natural divisions among
things, so that when we attempt to make a natural classification,
there is a fact of the matter as to whether that classification is
indeed genuinely natural.



Assuming that one is a naturalist about (some of) our classifications,
what makes a classification into one of natural kinds? The following
are often suggested criteria or characteristics of a natural kind
classification (some of which are more contentious than others):


1. *Members of a natural kind should have some (natural) properties
in common.*
  

This is a necessary condition of kindhood, but not a sufficient
condition, for objects of very different kinds may share some natural
property. As Mill (1884) remarks, white objects do not form a natural
kind. Natural properties here are often taken to be *intrinsic
properties*. The latter claim, as will be discussed in Section
2.1, leads to problems with biological kinds. Some, it should be
noted, hold that not all natural properties are intrinsic.
2. *Natural kinds should permit inductive inferences.*
  

This is a point emphasized by Whewell (1860) and Mill (1884), and is
central to Quine's (1969) discussion of natural kinds (see
Section 1.2.1). On its own, this criterion also does not help
distinguish natural kinds from sets of objects sharing some natural
property, and in that sense does not go beyond Criterion 1. Criterion
2 is also a necessary but not sufficient condition on natural kind
classifications.
3. *Natural kinds should participate in laws of nature.*
  

This may be considered as a stronger, metaphysical version of
Criterion 2. It is correspondingly more contentious: if something is a
duck, then it has webbed feet; but one need not hold that such
inductively obtained conditionals regarding biological kinds must
constitute laws of nature. Even if this is a necessary condition, like
Criteria 1 and 2, it fails to be a sufficient criterion. For laws of
nature may concern natural qualities and quantities that do not define
kinds (e.g. Newton's law of gravitation).
4. *Members of a natural kind should form a *kind*.*
  

Criteria 1-3 supply related conditions that fail to be
sufficient for kindhood. We may expand on Mill's example of
white things, which do not form a natural kind; neither do negatively
charged things, nor do things with a mass of 1 kg. It is not
immediately clear what this criterion requires, but we may say that an
adequacy condition on a satisfactory account of what kinds are must
address this 'kindhood' question (Hawley and Bird
2011).
5. *Natural kinds should form a hierarchy*.
  

If any two kinds overlap at all, then one is a subkind of the other,
or they are identical (Kuhn 2000; Ellis 2001, 2002). This is
exhibited, for example, by Linnaean taxonomy in biology. No organism
is a member of two distinct species or genera. If organisms from
different species are members of the same genus, then all members of
both species are members of that genus. (One might add that every
organism is a member of some species and some genus. In which case,
the Linnaean taxonomy provides a partition into species among all
organisms and also within each genus.)
6. *Natural kinds should be *categorically* distinct*.
  

There cannot be a smooth transition from one kind to another (Ellis
2001). For then the borderline between them could not be one drawn by
nature but is one that is somehow or other drawn by us. In which case
the kinds would not be genuinely natural. This is exhibited by the
chemical elements. Chlorine and argon are neighbours in the periodic
table. There are no atoms that are intermediate between chlorine atoms
and argon atoms, for the nucleus of an atom cannot have a number of
protons between seventeen (chlorine) and eighteen (argon).


#### 1.1.2 Conventionalism



Naturalism about natural kind classifications contrasts with
*conventionalism* (also called *constructivism* or
*constructionism*), the view natural kinds don't exist
independently of the scientists and others who talk about them. John
Stuart Mill expresses the contrast thus: "In so far as a natural
classification is grounded on real kinds, its groups are certainly not
conventional; it is perfectly true that they do not depend upon an
arbitrary choice of the naturalist" (Mill 1884, bk. IV, ch. II).
While the sciences do, it appears, employ classifications according to
kinds, the conventionalist will maintain that the purposes of
scientists are just one species of interest among many: others will
include the interests of automobile enthusiasts, of cooks, and of
religious people or sociologists of religion. Conventionalists
(constructivists, constructionists) deny that any of our
classifications, including those of science, are naturally privileged
forms of classification. The classifications of botanists do not carve
nature at its joints any more than the classifications of cooks.



We may distinguish between weak conventionalism and strong
conventionalism. Weak conventionalism asserts that our actual
classifications are not, or are very unlikely to be, natural. The
principal ground for weak conventionalism is scepticism about the
ability of science to uncover the natural principles of
classification. We may attribute this reason for conventionalism to
Locke (1690: III). It appears (although this is a matter of debate)
that Locke thought that if there were real essences of species, these
would be found in the shared imperceptible micro-structural properties
of things, and that there may be such genuinely natural species but
that science was ill-equipped to do better than speculate about them.
(Locke also held, employing Criterion 6 above, that there would be
'gaps' or 'chasms' between genuine species but
that observation showed only smooth gradations between the kinds we
actually employ, which therefore must be conventional.)



Strong conventionalism, however, goes on to deny even that
conventionalism is forced upon us simply by ignorance. It denies that
there are any such natural divisions, known or unknown. Strong
conventionalists claim that the differences and similarities that we
attribute to things exist in virtue of, for example, the social
function of the relevant concepts rather than in natural facts. For
example, the concept of gender is used to mark a distinction of social
role. The conventionalist treats natural kinds in the same way. The
chemical elements, biological species, quarks, neutrinos and bosons
are equally constructed by the activity of scientists. The key claim
is that natural kinds are constructed rather than discovered. As
Woolgar claims "there is no object beyond discourse"
(1988, 73).



Hacking (1999) argues against the strong version of conventionalism
(which he terms 'universal constructivism'). The point of
social constructivism is to reveal that some of our classificatory
categories and practices, though they may appear inevitable, are
actually contingent and relative to the practice of classification in
the context of social institutions and norms. However, according to
Hacking, some constructed categories reflect real divisions and so we
need not be constructivists in the strong sense. There is still room
to distinguish between constructed kinds that reflect real categories
and those that do not.



For example, Hacking (1999, Ch. 5) distinguishes between 'child
abuse' and 'satanic ritual abuse'. Satanic ritual
abuse is a socially constructed idea, but not a social kind. In the
1990s, there was an exhaustive investigation into satanic ritual abuse
in Great Britain after a number of reported cases. However, an
independent commission claimed that none of the charges were
substantiated by evidence. Thus, our constructed categories are
subject to empirical investigation. We must establish a referent class
for our constructed categories. For example and in contrast, a kind
such as child abuse is considered real. The emergence of the category
can be traced to a definite time (1961) at a definite place (Denver)
in the discussions of paediatricians. Moreover, the reference of the
category was abused children. This reference dynamically changed as
the idea became embedded in new legislation, incorporated in practices
involving social workers, police, schoolteachers etc. (see Hacking
1999, Ch. 5.) But, importantly real instances of the category were
found through empirical investigation.



The chief claim made by all varieties of conventionalism is that facts
about the structure of the world are in some sense dependent on human
beings, their concepts and their activities in society. The denial of
this view, which we will examine in Section 1.2, is the claim that
facts about the world are independent of human beings, their concepts
and their activities, that is, that there are real independent natural
kinds in the world. The claim that scientific facts about the world
are *dependent* on human activity has been analysed in
different ways by constructivists. Kukla (2000, Ch. 3) distinguishes
(i) material, (ii) causal, and (iii) constitutive dependence. The
first, and weakest, claim is material dependence, namely that
scientific entities are constructed by the activities of scientists in
the laboratory. Chemical reactions are brought about by scientists in
the laboratory using purified chemicals in a controlled environment
(Knorr-Cetina et al. 1983). The resultant kinds are merely the
constructs of this highly artificial environment, along with the
pragmatic interests of the scientists (i.e., what they are testing for
and the methodology they choose to use). However, it is clear that
scientists still abstract from a complex reality and in some
circumstances the materials used are purifications of some naturally
occurring materials. Indeed, material dependence is a rather weak kind
of conventionalism and could even be made compatible with some
versions of naturalism/realism (cf. Hacking 1983). Therefore, no
general conclusion may be drawn about the natural world and its kinds,
from the fact that scientists study scientific kinds in a highly
idealized and controlled setting. The second element in Kukla's
three-fold distinction is causal dependence, which occurs when the
relevant facts are caused to be true by the fact that they are
believed to be true (e.g. belief that there will be a run on the bank
can cause a run on the bank, Merton 1948). Hacking's (1995)
human, interactive kinds, such as person with multiple-personality
disorder, may be regarded as instances, where the kind and its members
are brought into existence by the belief that there are such kinds and
people do belong to them. Lastly, the constitutive dependence thesis
claims that convention is constitutive of the physical facts.
Plausibly, conventions exist in virtue of people holding that they do.
Woolgar's constructivism is of the type that treats all
scientific facts like conventions in this respect.


#### 1.1.3 Promiscuous Realism



Dupre (1993) *promiscuous realism* claims that there are
countless ways of taxonomizing the world into kinds. The structure of
the world is vastly complex and can be categorized in many different
cross-cutting ways, according to the different theoretical interests
we happen to be pursuing. Nevertheless, Dupre insists that his
view is not conventionalist, but rather that he is committed to
naturalism and realism about natural kinds. He accepts that there are
properties that members of natural kinds have in common, but denies
that these are intrinsic properties of kinds (see Criterion 1 above).
He also denies that there is a hierarchical structure of natural kinds
in the world (see Criterion 5 above) and that kinds are categorically
distinct (see Criterion 6 above).



Dupre (1993, 28) argues that promiscuous realism about natural
kinds reflects our classificatory practices within the context of both
common sense and particular sciences. The kinds classified by common
sense and scientific practice often cross one another. For example, in
common sense we classify lilies as a certain kind of flower. However,
in biological classification the *genus* Lilium comprises over
one hundred species, including bulbs such as garlic and onions.
Moreover, species with the name "lily" in common sense do
not all in fact belong to the lilium *genus* in biological
classification (e.g., Calla Lilies and Canna Lilies). Therefore,
common sense and biological science provide us with pluralistic ways
of taxonomizing lilies. Each is equally legitimate, according to
Dupre, depending on our interests. Furthermore, Dupre
(1993, 38) argues that cross-classification occurs even within the
context of a science. Taking the biological classification of species
as an example, he argues that there are countless ways of taxonomizing
species, depending on the model of biological systematics that is used
(see Section 2.1 below). There are equally legitimate, objectively
grounded ways of classifying natural kinds. There is accordingly no
unique way of carving nature at its joints; different systems of
classification are equally acceptable, scientifically, and
metaphysically.


### 1.2 Natural Kind Realism



*Realism* about natural kinds is the view that there exist
entities that are the natural kinds. While naturalism about natural
kind classifications is a matter of there being natural groupings and
distinctions among things and as such has no ontological commitment,
realism is an ontologically committed view. Since '(weak)
realism' is sometimes used to denote naturalism, we may
distinguish by using the term *strong realism* for the
stronger, ontological, claim that the natural divisions between kinds
reflect the boundaries between real entities; *i.e.* the
difference between silver and gold is not just a difference between
two natural groups of thing, but is a difference between two distinct
entities *silver* and *gold*.



The traditional realism versus nominalism debate is an ontological
debate over whether in order to account for properties we need to
invoke a class of entity, universals. There is an analogous debate as
regards natural kinds, whether in order to account for our natural
kind talk and our natural classifications, we need a special sort of
entity in our ontology. The natural kind nominalist may accept that
there are genuinely natural classifications, but will reject the idea
that we should invoke any object as a consequence. For example, the
nominalist might take the world to be made up of individuals, which
can be classified as kinds in a natural way. But, the nominalist
claims, there is no kind of entity beyond the individual instances of
each kind. The (strong) realist maintains that we cannot explain the
distinction between natural and non-natural classifications without
appealing to certain entities, the natural kinds. The naturalism
versus conventionalism debate discussed in the preceding section asked
whether there is a genuine metaphysical difference between natural
classifications and non-natural classifications; it is not obvious
that the naturalist, who maintains that there is a metaphysical
difference, is obliged to understand that difference in terms of an
ontological category of entities that are natural kinds. The analogue
in the debate over properties is whether there is a metaphysical
distinction between natural properties and non-natural properties,
between, in David Lewis's terminology, sparse and abundant
properties (Lewis, 1986). There is no immediate contradiction in the
position of a nominalist who rejects universals, but who maintains
that there is a metaphysical difference between natural properties and
other properties. The realist in both cases argues that, ultimately,
naturalism (about properties and about classification) requires a
commitment to entities (universals and natural kinds
respectively).


#### 1.2.1 Quine on Natural Kinds and Induction



Quine is an example of a philosopher who accepts, to some extent,
natural classifications into kinds. He holds (1969) that kinds are
sets, so he does think that there are entities that are kinds, and to
that extent he is a realist. That said, his realism is minimal, since
sets are ubiquitous entities. Any coherent classificatory principle
will determine a set of things thus classified, whether natural or
artificial. Thus the ontology of natural kinds does not of itself
explain there being *natural* kinds. Quine regards the notions
of *kind* and *similarity* as "substantially one
notion" (1969, 119) but denies that it is straightforward to
give a definition of one in terms of the other. He does consider
defining relative similarity thus: *a* is more similar to
*b* than to *c* iff *a* and *b* share more
properties than *a* and *c* do. But this will work only
if properties are less abundant than sets are (since any two things
are jointly members of any number of sets). And Quine regards such a
notion of property as no clearer than the notion of kind.



Quine opens his discussion of natural kinds by asking "what
tends to confirm an induction?" (see Criterion 2 above) (1969,
114). With Goodman's new riddle of induction and Hempel's
paradox of confirmation in mind, Quine answers that it is the
similarity or sameness of kinds between instances that permits an
induction: two green emeralds are more similar than two grue emeralds
when one of them is green and the other blue. It is the "dubious
scientific standing of a general notion of similarity, or of
kind" (1969, 116) that prompts the discussion alluded to above.
An intuitive notion of similarity is used in our ordinary inductions.
And the force of Darwinian processes gives us some reason to think
that we have evolved an innate similarity space that corresponds to
some natural similarities. At the same time, natural science, which
developed from these primitive inductions, supported by similarities,
is able to reveal deeper similarities that may contradict superficial
similarities. For example, we have strong sense that things of the
same colour are objectively alike, but science tells us that there is
no deep similarity among like-coloured things. Quine thinks that in
due course science will obviate the need for a general notion of
similarity or kind: in each area of science more specific notions will
take the place of the generic notion; this is a sign of the maturity
of a branch of science. For example, in zoology we may replace talk of
the similarity between two animals by discussion of the historical
proximity of their closest common ancestor.



Quine's conception of natural kinds is liberal. Not all sets are
natural kinds, but any set whose members share a natural property are
a natural kind. So the set of white objects is at least a candidate
natural kind and Quine asserts that positively charged objects form a
natural kind. One might wonder whether an up quark, an uranium
nucleus, a hydronium ion, a charged water droplet, and a balloon that
has been rubbed on someone's jumper all belong to one kind. The
literature on natural kinds (cf. Mill 1884) has tended to adopt a more
restricted conception of kinds, although articulating that conception
is not straightforward. This question is related to the link that
Quine draws between kinds and what it is that confirms induction. If
one adopts the narrower conception of kinds, then one will hold that a
law such as 'positively charged objects repel one another'
does not concern kinds at all, but nonetheless may be confirmed by
instances of objects that are positively charged repelling one
another. So the real answer to "what tends to confirm an
induction?" is not "natural kinds and their
instances" but is "natural properties and their
instances". A further reason for taking this view is that
inductions often concern relations (such as the repulsion between two
objects). But we do not suppose that objects that enter into a natural
relation form a natural kind, nor that there is a natural kind of
ordered sequences of objects satisfying a given natural relation.


#### 1.2.2 Cluster Kind Realism



Following Quine, some realists have expanded upon his intuition that a
natural kind's members share a natural property by counting
families of such properties that are contingently clustered in nature
(Boyd 1991, 1999a; Millikan 1999). These families of properties
cluster together over time either because the presence of some
properties in the family favours the presence of others or because
there are underlying internal mechanisms and/or extrinsic contextual
mechanisms which tend to secure the co-occurrence of the properties. A
natural kind is any such family of co-occurring properties that may be
employed in inductive inferences or for the purposes of scientific
explanation. Cluster kind realists will readily concede that,
depending on the case, environmental pressures may affect and alter
the set of properties associated with a kind over time. Therefore, in
such cases, none of the properties themselves need be individually
necessary for kind membership.



Richard Boyd (1999) considers biological species to be paradigmatic
natural kind clusters, where the clustering is due to a
*homeostatic* mechanism. Homeostatic property clusters (HPCs)
occur when mechanisms exist that cause the properties to cluster by
ensuring that deviations from the cluster have a low chance of
persisting; the presence of some of the properties in the cluster
favours the presence of the others. A homeostatic mechanism thereby
achieves self-regulation, maintaining a stable range of properties. In
the case of species, the homeostatic mechanisms may be intrinsic (e.g.
gene exchange within a population or developmental factors) or
extrinsic (e.g. a species's evolutionary niche provides common
selective factors). Since individuals inherit their basic
characteristics from their parents, and very few people are taller
than 2.5m, we can expect the next generation to contain few people of
that height. Height can, however, be affected by environmental
factors; a mutation can cause offpring to have genes different from
their parents that might cause greater than usual height, and
malfunctioning of the pituitary gland can cause an excess of growth
hormone. So there can be individual cases of excessive height. But
they will remain rare because such gigantism and acromegaly are
accompanied by serious complicating disorders and often result in
premature death. Consequently, these individuals are less likely to
reproduce than others. Even greater extremes of height (e.g. 4m) are
just not possible, since the same developmental factors would prevent
the individual from being viable at an earlier stage in life.



Colouration across a species is often uniform when it is a camouflage
against predation. The more specific the ecological niche occupied,
the more uniform will colouration and other features be. Deviations
will lead to decreased fitness and will be selected against. Obligate
mutualistic symbiosis often provides very specialized ecological
niches; consequently certain key properties must be present in order
for an individual to survive. Certain beard worms, such as *Riftia
pachyptila*, live at ocean floor thermal vents and have no
digestive tract; instead they supply hydrogen sulphide and methane to
bacteria in their trophosomes, which oxidise the hydrogen sulphide and
methane on behalf of the worm. As a result the worm cannot lack any
property essential for the survival of the bacteria to which it is
host. Such homeostatic mechanisms do not preclude deviations from the
standard combinations of properties--mutations do occur.
Homeostasis just means that usually such mutations will not persist.
The rare benevolent, or even merely neutral, mutations may persist and
flourish, in which case there will be variation and change.
Environments may change, perhaps locally, also leading to variation
and change (e.g. the famous peppered moth). Consequently, it will not
always be the case that property P in a homeostatic property cluster F
is a necessary condition of an individual belonging to the relevant
kind.



One potential problem with the HPC view account is that homeostatic
mechanisms are not clearly delineated in the account. In other words,
the role of mechanisms in maintaining the stability of property
cluster is not addressed in Boyd's original account. Craver
(2009) is skeptical about our attempts to give a fully realist account
of the homeostatic property clusters because conventional elements are
involved in deciding when two mechanisms are of the same kind and also
in identifying when one mechanism ends and the other begins. Tobin
(2017) argues that there is an overlapping problem for mechanisms
which is similar the overlapping problem often discussed in the
natural kinds literature.



Slater (2015) claims that the HPC view can be made clearer by focusing
on the special sort of stability a cluster of properties might makes
it successful for inductive and explanatory purposes, in favour of an
appeal to mechanisms. Slater calls this view the Stable Property
Cluster account of natural kinds.



In such cases, the boundaries of the natural kind are vague. Cluster
kinds nevertheless do form a kind (Criterion 4 above). Members will
have at least some properties in common (Criterion 1). Since the
emergence of cluster kinds is not entirely random and a
causal-historical account can be provided in virtue of stable
homeostatic mechanisms, cluster kinds are sufficiently robust to
support induction (Criterion 2) and perhaps even laws of nature in
some cases (Criterion 3). Conversely, because the boundaries between
natural kinds are vague, a cluster kind realist will reject the claim
that natural kinds are categorically distinct (Criterion 6). She may
be neutral on whether they always form hierarchies (Criterion 5). That
said, not all clusters need exhibit the flexibility and vagueness of
these biological, homeostatic cases. In other cases the clustering may
be directly due to the laws of nature. Thus the laws of physics rule
out fractional nuclear charge, with the consequence that the chemical
elements are kinds that have clearly defined boundaries and nuclear
charge is a necessary and sufficient condition of kind membership.



Chakravarrty (forthcoming) argues that natural kind realism has run
its course and that attempts to be more liberal about multifarious
natural kinds have meant that more recent natural kind realist
accounts dissolve into conventionalism. Alternatively, many more
recent realist account of natural kinds attempt to cling on to natural
kind realism by buttressing it to other forms of realism. For example,
theorist have identified what is (mind-independently) real with other
things (property clusters, correlations, mechanisms, structures and so
on). According to Chakravarrty this is to conflate *sui
generis* natural kind realism with other kinds of realism (e.g.
realism about property clusters or realism about mechanisms).


#### 1.2.3 Fundamentalism and Reductionism



As mentioned above (introduction to Section 1.2), natural kind realism
may be understood as an analogue of realism about universals. A
further question is whether these are simply analogues or in fact
related positions. Perhaps natural kinds just are (a species of)
universals or are reducible to universals. Within realism, that
reductionist view can be contrasted with natural kind
*fundamentalism*, which holds that not only are there natural
kinds (as entities), but also that natural kinds find a basic and
*sui generis* place in our ontology. We illustrate three
positions within naturalism about natural kinds: (i) naturalism
without (strong) realism; reductionist realism; fundamentalist
realism.



According to David Armstrong (1997, 67-8) "The kinds mark
true joints in nature. But it is not clear that we require an
independent and irreducible category of universal to accommodate the
kinds". Armstrong's view is naturalistic, since there are
genuine natural divisions; but he is also resistant to realism. If
there were natural kinds they would be universals. But the properties
shared by all and only the members of a kind will typically be very
complex. For example, he considers whether there is some description
of human DNA. There may be, but it would be too complex to qualify as
a genuine universal. And there is no reason to suppose that this
complex universal would do any causal or nomic work. So there is not
simply an identity between natural kinds and a subcategory of
universals, the natural kind universals (e.g., the universal of
*being gold*, or of *being a human*)--according to
Armstrong's argument there are no natural kind universals. He
does concede that the case is stronger for basic kind universals, such
as electronhood, where a few properties (the electron's mass,
charge, and spin) are individually necessary and jointly sufficient
for kind membership.



Hawley and Bird (2011) argue that natural kinds are complex
universals. A complex universal is a universal whose parts are
universals (just as a complex particular is a particular whose parts
are particulars). In the case of the electron the fundamental
universals (the electron's mass, charge, and spin) do combine to
form a complex universal. Since an universal is *wholly*
located where the universal is located, every entity that instantiates
electronhood also instantiates its component universals, the
electron's mass, charge, and spin. Not every collection of
universals constitutes a natural kind, just as not every collection of
particulars constitutes a natural (complex) particular. The facts that
explain the clustering of properties explain why these universals
compose a complex one. Not every complex universal is, like
electronhood, precise. Some are vague, and this fact can accommodate
the indeterminacy of kind constitution implied by Boyd's view of
the clusters in the case of biological kinds.



Like Hawley and Bird, E. J. Lowe takes natural kinds to be entities
(1998, 2006a, 2006b). Thus Lowe is a natural kind realist.
Furthermore, he takes natural kinds--*substantial
universals* in his terminology--to constitute an irreducible
ontological category (one category in his four-category ontology).
Hence he is a natural kind fundamentalist. Lowe argues that
recognizing the ontologically fundamental distinction between
substantial universals (natural kinds) and non-substantial universals
(properties) accounts for the laws of nature in a manner that, while
avoiding the problems of a regularity account, may nonetheless do
without Armstrong's second order relation of necessitation,
itself a relational universal (Armstrong 1983, 75-110). Consider
the law that planets travel in ellipses. According to Lowe's
account, this should be understood as a matter of the kind
*planet* being characterized by the property *travels in an
ellipse*. (One might query whether all laws can be given this form
and so whether the fact that some general truth is a law should be
explained this way. For what makes planets travel in ellipses is the
fact that they have mass and hence little to do with their being
*planets*. For example, Kepler's law is an application of
Newton's laws, but the latter concerns quantities (mass,
distance), which should be understood as universals not kinds.)



As in Lowe's ontology, natural kinds play a fundamental role in
the ontology of Brian Ellis (2001, 2002). For Ellis the term
'natural kind' covers three hierarchical categories:
natural kinds of objects (substantive kinds), natural kinds of
processes and events (dynamic kinds), and natural kinds of properties
and relations (property kinds). The chemical elements and compounds
are substantive kinds; the different chemical reactions are dynamic
kinds; a tetrahedral molecular structure is a (structural) property
kind (whose species include the particular tetrahedral structures of
methane, carbon tetrachloride, silicon tetrafluoride and so forth).
Ellis is uncommitted on whether the dynamic kinds are independent of
the other two types of kinds; but these are independent of one
another. Each kind is a universal, and the three categories of kind
therefore form three categories of universal. Thus the substantive
kinds--the familiar kinds of thing and stuff--are
universals, but these are not reducible to property universals. Ellis
notes that laws of nature have a hierarchical structure:
Kepler's laws of planetary motion are an application of
Newton's laws. He argues (2001, 90) that the only satisfactory
account of this is fact that natural kinds also exist in hierarchies
(see Criterion 5).


### 1.3 Essentialism



Do natural kinds have essences? When discussing kinds and essences we
need to distinguish two quite different claims. The first claim is
that the kind a particular belongs to is essential to that particular:
if *a* belongs to kind *K*, then it is an essential
property of *a* that it belongs to *K*. The second claim
is that the kinds themselves have essential properties: for each kind
*K* there is some property Ph of the kind such that it is
essential to *K* that Ph(*K*). What is the logical
relation between these two claims? The first claim may be strictly
consistent with the denial of the second, but it may be difficult to
motivate the first without having reason to accept the second. Would
one think that some object's nature requires it to belong to
kind K without thinking that there is some distinctive (essential)
property characteristic of all Ks?



The second claim, essentialism about kinds, does not obviously imply
the first, individual essentialism. To see this, consider the
following two cases that would seem to show that the latter does not
imply the former (Bird 2018). The first case is chemical/physical. A
nucleus of neptunium-239 may undergo beta decay, in which one of its
neutrons emits an electron leaving a proton. As a consequence the
nucleus now has one more proton and so is a nucleus of neptunium no
longer but is now a nucleus of plutonium. This description is
consistent with the claim that it is essential to neptunium that
nuclei of neptunium atoms have 93 protons whereas it is essential to
plutonium that its nuclei have 94 protons. But one may also claim, as
the description implicitly suggests, that one and the same nucleus
persists through this transformation. If that is so, then a particular
has retained its identity while undergoing a change of kind. That kind
essentialism does not entail essentialism of kind membership is also
demonstrated by a second, biological, example. According to
Mayr's biological species concept, a species is a reproductively
isolated breeding population. Thus a new species can be created by the
advent of reproductive isolation, which will typically split existing
populations (Matthen 2009). And so the existing organisms in at least
one (and arguably both) of the newly isolated (sub-)populations will
belong to a new species. And if species are natural kinds, this is an
example of particulars changing their kinds. But this account of
species is consistent with the view that species have essences
(LaPorte 1997, 2004, 11-12). (As a matter of fact, most
essentialists accept that an object's kind is essential to it.
The point being advanced here is that this claim does not follow
*immediately* from the claim that kinds have essences.)


#### 1.3.1 Arguments for Essentialism



It has been widely held that essentialism, and natural kind
essentialism in particular, is a consequence of the semantic arguments
concerning reference put forward by Kripke (1980) and Putnam (1975a).
These arguments support semantic externalism, the view that the
reference of a speaker's term can depend on facts external to
the speaker herself (e.g. facts concerning prior uses of the term).
However, Salmon (1979, 1982) has argued persuasively that only trivial
essentialism can be inferred from such arguments; stronger
essentialist theses are derived only on the basis of unstated
essentialist premises. For example, consider the following argument,
reconstructed by Donnellan and discussed in Salmon (1982,
161-75):



> 
> 
> 
> |  |  |
> | --- | --- |
> | P1: | It is necessarily the case that: something is a sample of water
> if and only if it is a sample of dthat (the same substance that this
> is a sample of). |
> | P2: | This [the liquid sample] has the chemical structure
> H2O. |
> | P3: | Being a sample of the same substance as something consists in
> having the same chemical structure. |
> |  | *Therefore* |
> | C: | It is necessarily the case that: every sample of water has the
> chemical structure H2O. |
> 
> 
> 
> (Salmon 1982, 166)
> 
> 
> 



P3, however, is not free of non-trivial essentialist import. According
to Salmon, deriving essentialism via the theory of reference is
question-begging. Nonetheless, the fact that essentialist conclusions
seem so plausible may be taken as evidence that there exist strong
essentialist intuitions, and the arguments from Kripke and Putnam can
be framed in non-semantic terms and thereby motivate essentialism
directly--indeed many of Kripke's arguments are not framed
in semantic terms at all.



Several of Kripke's exemplary cases concern theoretical
identities; these are typically identities of the form *C* =
*T*, expressed using a common name for a kind,
'*C*', and a technical identification of the kind,
'*T*', typically thereby telling us what that kind
*is*. Examples: light is a stream of photons; water is
H2O; lightning is an electrical discharge; gold is the
element with atomic number 79. This might give the impression that
Kripkean essentialism is just a matter of the necessity of identity.
However, in several of his discussions Kripke does provide separate,
independent arguments on the one hand for the (necessary) necessity of
being *T* for being *C* and on the other hand for the
(necessary) sufficiency of being *T* for being *C*. In
general, where *K* is a kind and *P* is some property,
arguments for #[?]*x*(*Kx*-*Px*)
can be decoupled from arguments for
#[?]x(*Px*-*Kx*).



For example, in his discussion of gold, Kripke argues that something
that is superficially like gold in appearance but lacks the property
of having atomic number 79, would not be gold. This establishes that,
of necessity, having atomic number 79 is necessary for being gold, but
does not show that possession of this property is sufficient for being
gold. Similarly, argues Kripke, if we were to discover a population of
animals with the appearance of tigers but which turn out to be
reptiles, we would deny that such creatures are tigers. Being
mammalian is necessarily a necessary condition of being a tiger, but
of course not a sufficient condition



Putnam's (1975a) Twin Earth thought experiment, in a
non-semantic guise, leads to a similar conclusion. We imagine there is
a planet, 'Twin Earth', exactly like Earth in all readily
observable respects. Every person and thing on Earth has a twin
equivalent on Twin Earth. However, there is no H2O on Twin
Earth. Instead, there is a liquid that shares the manifest properties
of water (e.g., its being colourless, tasteless, odourless, good for
drinking, etc.). This liquid is composed not of H2O
molecules but of very different molecules to which we assign the
made-up formula 'XYZ'. Despite its superficially
water-like properties, intuitively XYZ is not water. What prevents XYZ
from being water is the fact that XYZ is microstructurally different
from H2O. If intuition is reliable in this respect, then
(i) possessing water's superficial properties is not sufficient
to make something a sample of water, and (ii) being composed of
molecules of H2O is necessary for something's being a
sample of water. (For more on Putnam's Twin Earth thought
experiment, see Section 4.2.3.)



It is natural to use the expression '*the* essence of
...', which implies sufficiency as well as necessity:
possession of the essence of *K* suffices for membership of
*K*, as well as being necessary for it. According to the
foregoing, not all the arguments of Kripke and Putnam establish what
*the* essence of some kind is, rather they establish only what
we may call a *partial* essence, conditions that are necessary
but not sufficient. This is clear in some other of Kripke's
arguments. Thus it is only a partial essence that someone originates
with the gamete (sperm and egg) from which they in fact originate,
since they might have a monozygotic (identical) twin who has the same
origin. A further question is whether there is a full essence
(*the* essence) for everything that has a partial essence.



The fact that such arguments show only that *F* is *an*
essential property of *K* means that they are invulnerable to
complaints that not everything with *F* is *K*. For
example it is sometimes complained that water cannot be essentially
H2O for the following reasons (a) water is a liquid,
whereas a substance comprised of H2O can exist as a solid
or a gas; (b) a single H2O molecule is not a sample of
water because it does not have the properties we can ascribe to
samples of water (such as a temperature); (c) water is not simply
H2O but is a polymer-like substance held together by
hydrogen bonds. These observations are consistent with the claim of
Putnam's argument that necessarily all water is composed of
H2O molecules.



The arguments so far considered show that the superficial properties
associated with a kind are, in those cases, not sufficient for kind
membership, and so some further property (often a microstructural
property) is a necessary condition. Kripke also provides arguments
that show that many of the superficial properties typically associated
with a kind are not even necessary. Kripke examines Kant's claim
that it is *a priori* that gold is a yellow metal (Kripke 1980,
118). Let us imagine that the yellow appearance of what we have always
taken to be samples of gold is just an optical illusion. Intuitively,
we would conclude that we had previously erred in thinking that gold
is yellow--we would not conclude that there is *no* gold.
Likewise, one might ask whether animal species are characterized
(necessarily) by superficially available qualities, for example,
whether a tiger is, by definition, a large carnivorous quadrupedal
feline, tawny yellow in colour with blackish transverse stripes and a
white belly (Kripke 1980, 119-121). Again, we may consider that
these features of the individual animals we have held to be tigers are
in fact illusory; that those individuals and others of the same
species are, for example, three-legged but that an optical
illusion made us think otherwise. This suggests that satisfying a set
of readily observable 'superficial' properties is not
necessary for membership of the tiger kind.



The foregoing arguments from Kripke lead to the conclusion that the
relevant kinds are not characterized of necessity by the readily
observable properties of their instances. They do not show what does
characterize those kinds. Kripke takes the view that, in most cases,
it is certain *microstructural* properties that determine the
nature of these natural kinds, as evident in his endorsement of the
identities: 'water is H2O'; 'gold is the
element with atomic number 79', 'light is a stream of
photons'. Such essential properties of kinds are features,
therefore, that may be unknown even to competent users of a relevant
kind concept. Consequently, these are essences that, when known at
all, are known *a posteriori*.



While the arguments just considered suggest a microstructural essence
for the chemical and physical kinds considered, they do not have the
consequence that every natural kind must have a microstructural
essence. Kripke regards material individuals as having historical
essences. In the light of cladism in biology, one may regard
biological taxa as having historical essences also, and this is
consistent with the general picture given to us by Kripke and Putnam,
even if they themselves once thought that biological kinds had
microstructural essences (McGinn 1976; Kornblith 1993, 111; LaPorte
2004, 64). Cladism is the most commonly accepted system of biological
classification that groups organisms according to their shared
phylogenetic ancestry. Cladism has replaced the Linnean hierarchical
system of biological classification where organisms were grouped
according to their shared morphological characteristics (see Section
2.1 below).



Kripke does not distinguish 'essential property' from
'necessary property'. Fine (1994) notes that the claim
that P is a necessary property of X is *not* the same as the
claim that P is an essential property of X: the latter implies the
former but the former does not imply the latter. The arguments for
essentialism discussed above do not note this distinction and move
quickly from claims about necessity to claims about essence: having
shown that necessarily anything that is gold is made up of an element
with atomic number 79, Kripke concludes that having atomic number 79
is an essential property of the kind gold. Fine's point may not
be an insuperable obstacle to establishing essentialism via the
arguments of Kripke and Putnam. For it may be held, as Fine holds,
that essences concern the nature or identity of things. One may then
argue that the arguments of Kripke, Putnam, and others do appeal to
intuitions concerning the nature and/or identity of natural kinds, and
so the inferences to essentialism from claims about necessity are
legitimate in these cases.



The Kripke-Putnam arguments for essentialism are, in effect,
direct appeals to intuition. For Ellis and Lowe, the dialectical basis
for essentialism is that essentialism is a core part of what they each
take to be the optimal metaphysical system. Ellis (2001, 145-50)
regards his essentialist metaphysics as supported by his argument that
it yields a stronger argument for scientific realism. In particular,
for Ellis the essences of natural kinds explain the laws of nature:
laws are grounded in the world's natural kind structure, which
includes not only natural kinds of thing but also natural kinds of
process and event. Above it was mentioned that for Lowe (2006, 144)
laws concern natural kinds; by contrast, for Ellis laws are grounded
in essential facts about kinds. As a consequence, the laws of nature
are metaphysically necessary. This has the advantage of explaining
what natural necessity is (it is just metaphysical necessity), but at
the cost of denying the intuitive contingency of laws. Lowe, on the
other hand, holds that laws can be grounded in the contingent
possession of properties by kinds, and so need not be metaphysically
necessary.

 In conclusion we note that some anti-essentialists
argue that there is no non-question begging way of motivating the
appeal to essences. Hugh Mellor (1977, 309) argues that the existence
of essences in essentialist accounts of natural kinds is simply a
gratuitous assumption. Others use examples from the empirical sciences
such as biology to argue that essentialism is too limited to capture
the kinds we find in the special sciences (Dupre 1981, 1993).
In particular, essentialist accounts of kinds seem to construe them as
immutable or static, whereas examples from the natural sciences
include mutable and dynamic kinds, as we see in Section 2.1 below.

### 1.4 Natural Kind Eliminativism



Recently, there has been a debate about the usefulness of natural kind
classification in philosophy of science. Some philosophers have argued
that the concept of natural kinds should be eliminated on the grounds
that it does not play a useful role in scientific classification.
David Ludwig (2018) and Ian Hacking (2007a) have argued for
eliminating the concept of natural kind altogether. The arguments here
are twofold: firstly, the concept of natural kind is applicable to a
heterogeneous set of practices of natural classification. The usage of
the natural kind concept is so multifarious that it is misleading to
practicing scientists if they use the natural kind concept in
scientific classification. On these grounds, the concept of natural
kind should be eliminated. Indeed Hacking (2007a, 229) does not mince
his words: "Delete every mention of natural kinds". The
second, related, argument is on the grounds that there are so many
different theories of natural kinds in the literature, of which some
apply more readily to some scientific disciplines, while others seem
to apply more readily in others. For example, the grounds for
essentialism seem more warranted in chemistry, while Boyd's HPC
view seems more readily applicable in biology. Indeed, a homogeneous
theory of natural kinds that would apply in all disciplines looks very
unlikely. On these grounds, so the argument goes, the search for a
unitary theory of natural kinds for all scientific classification is
futile. Conix and Chi (2021) argue that these arguments for
eliminativism do not provide a sufficient justification to eliminate
the natural kind concept altogether. They argue that the notion of
'natural kind' plays another epistemic role in
philosophical research, because it enables fruitful investigation into
non-arbitrary classification.



## 2. Natural Kinds in the Special Sciences



Our interest in natural kinds is generated by the fact that the
particular sciences make frequent use of what, on the face of it, seem
to be natural kinds. So an important question is whether the kinds of
the special sciences (e.g. psychology, economics, biology, chemistry
and so on) do in fact satisfy the conditions laid out by
metaphysicians for natural-kindhood (Fodor 1974; Dupre 1981,
1993; Millikan 1999; Ellis 2001). And to the extent that they do not,
does that show that these kinds are not genuine natural kinds after
all, but are something different? Or does it show that the
metaphysicians need to revise their theories of natural kindhood? The
metaphysical issues raised by the special sciences vary. For example,
philosophers of biology ask whether species would be better understood
as being individuals rather than natural kinds. The so-called
*species problem* (Ghiselin 1974, 1987, 1997; Hull 1976, 1978;
Kitts and Kitts 1979, LaPorte 1994) asks which are the appropriate
criteria to use in order to decide which particular species an
organism belongs to. This is particularly challenging because species
evolve over time, which makes it difficult to determine when we should
recognize a new species and distinguish it from a distinct, older
ancestor species or from distinct sister species. Furthermore, the
criteria advanced by most of the various species concepts on offer
involve relational rather than intrinsic properties of organisms. So
either species are not natural kinds or the view that kindhood is
fixed by the intrinsic properties of things must be revised. In the
philosophy of chemistry, a key question is whether chemical kinds
provide as much support for microstructuralism as the stock examples
discussed by metaphysicians suggest (Hendry 2006; LaPorte 2004;
Needham, 2000, 2002; Van Brakel, 2000). Microstructuralism is the view
that chemical kinds can be individuated solely in terms of their
chemical microstructure. Chemical elements would appear to support
microstructuralism since atomic number is sufficient to individuate
any element. However, the extension of this view to more complex
chemical structures (such as molecules) is much discussed in the
literature. In the philosophy of mind, the ontological status of
psychological kinds has been questioned in the light of modern
advances in neuroscience (Churchland 1981; Fodor 1974). In particular,
action is causally overdetermined by mental and neurophysiological
kinds. If neuroscience can provide a sufficient account of action,
then the role of our common-sense folk psychological concepts, such as
*belief* and *desire* is called into question. Maybe
there are no psychological natural kinds corresponding to those
concepts. Natural kind realists reject the conventionalist view that
in all cases the boundaries of 'natural' kinds are drawn
by human interests. But is it plausible to draw a distinction between
genuinely natural and conventional kinds when it comes to the social
sciences? In particular, is it important that, via a feedback effect,
human perceptions of what kinds there are can have an effect on the
composition and even existence of those kinds (Hacking 1995)?


### 2.1 Natural Kinds and Biology



One central issue in the philosophy of biology concerns the nature of
biological species, which have traditionally been held to be
paradigmatic natural kinds. The traditional Linnean binomial system of
classification groups organisms into species and genera in virtue of
their overall physical similarities (their morphology). However, only
the taxa *species* and *genus* were held to reflect
ontological divisions in nature. The higher taxa (e.g.,
*family*, *order*, *class*, *phylum*,
*kingdom*) are merely conventional divisions, which are of
heuristic use in biology.



Biologists offer many different species concepts, which disagree on
how species are individuated; indeed the different species concepts
will disagree about the extensions of species and about the number of
species. For example, according to the interbreeding species concept
(e.g. Mayr 1969), species are groups of interbreeding natural
populations, that are reproductively isolated from other groups.
Alternatively, according to the phylogenetic approach to species (e.g.
Cracraft 1983), species are classified according to common ancestral
descent. These two approaches carve species differently. Some
phylogenetic species fail to be interbreeding species. For example,
organisms that reproduce asexually may nevertheless have a common
descent. They will be grouped together as species by the phylogenetic
approach, but not by the interbreeding approach (see Ereshefsky 1998
for discussion). Philosophers question whether the multiple divisions
used by biologists reflect an ontological pluralism in the world or,
alternatively, whether there is a privileged conception of species
that captures ontological reality. Furthermore, some philosophers
argue that species are not to be considered as natural kinds at all,
insisting instead that species are individuals.


#### 2.1.1 Species: Individuals or Kinds?



Despite its long history and intuitive appeal, the conception of
species as natural kinds is difficult to sustain while also
maintaining a traditional view of what a natural kind requires: a set
of intrinsic natural properties that are individually necessary and
jointly sufficient for a particular to be a member of the kind.



The fact that lineages evolve more or less gradually over time and
that this process leads to new species and other taxa, has two
consequences: first, that species are spatio-temporally restricted in
the sense that the species to which a particular organism belongs
depends on its being related to a specific lineage; and secondly, that
the characteristic properties of a species may change over time.



The first consequence of evolution implies that, contra the
traditional view, intrinsically identical organisms may not be members
of the same species: a cat-like organism independently evolved on a
distant planet would not be a cat. (Cf. Dummett 1973: 144 'even
if creatures *exactly* like men arose from dragons'
teeth, they would not be men, because not children of
Adam'.)



The second consequence implies that intrinsic similarity is not
necessary for membership of the same species. There exists a high
degree of variation in intraspecific morphology and genetic makeup.
While members of kinds need not be intrinsically identical (e.g.,
there may be isotopic variation between samples of a chemical
element), there will nonetheless be certain distinctive intrinsic
natural properties common to all members of the kind (e.g., nuclear
charge in the case of atoms of the same element). That, it is claimed,
is absent for species. Thus there is no genetic material or sequence
of genes that all and only members of the species *Drosophila
melanogaster* possess, and likewise for all other species. Nor can
we turn to larger-scale phenotypic properties (which may nonetheless
be hidden), since evolutionary change may eliminate such features
without a new species arising (Sober 1980). Furthermore, gradual
change, even through speciation, means that species will not be
categorically distinct (Criterion 6), which for Ellis (2001) is an
additional reason to conclude that species are not natural kinds.



These problems for the thesis that species are natural kinds may lead
one to conclude that classification by species does not correspond to
any real division of things in nature any more than the higher taxa
do. Darwin himself expressed this kind of conventionalism, in taking
species to differ only in degree from varieties on the one hand and
genera and higher taxa on the other hand:



> 
> Varieties have the same general characters as species, for they cannot
> be distinguished from species,--except, firstly, by the discovery
> of intermediate linking forms...and except, secondly, by a
> certain amount of difference, for two forms, if differing very little,
> are generally ranked as varieties, notwithstanding that intermediate
> linking forms have not been discovered. (Darwin 1859, 57)
> 



(Mishler 1999 contains a modern version of this view.) The rejection
of species as natural kinds need not lead to the rejection of realism
about species altogether. Indeed, one can commit to species realism in
a specific way: Ghiselin (1974, 1987, 1995, 1997), Hull (1976, 1978,
1980, 1987), and many other philosophers and biologists accept the
claim that species are *individuals*, not kinds. This claim
would appear to explain why, it is said, there are no serious
candidates for biological laws, at least concerning members of
particular species (Beatty 1997). Individual organisms are
*parts* of species, not members of the species-kind. Speciation
creates a new individual (or possibly two new individuals and the
cessation of the preceding species-individual) but not a new kind.
Organisms that are parts of the same species may share common
features, but that is not what makes them parts of that species.
Rather the explanation is the reverse: because the organisms are parts
of the same species they are parts of the same lineage and so they
will probably (but not inevitably) share features in common.



Does individualism about species imply that species are not natural
kinds? One might suspect as much, but perhaps the two views are
compatible. LaPorte (2004), for example, believes that one can
consistently hold both views. Where one theorist will see a
species-as-kind and an organism-as-member of that kind, another
theorist will see a species-as-individual and an organism-as-part of
that kind. And so even allowing that individualism is correct, we can
construct a kind: if *S* is a species-individual and *x*
is an organism, then *x* is a member of the species-kind
*S*\* iff *x* is a part of *S*. For
example, an individual organism (a radish plant) is a part of the
species-individual *Raphanus Sativus* if and only if it is a
member of the species-kind *Raphanus Sativus* (LaPorte 2004,
16). LaPorte's view does entail giving up on certain features
that natural kinds are often thought to have; one cannot still hold
that kind membership is necessarily intrinsic, since 'being a
part of *y*' is not an intrinsic property and parts of
the same entity do not necessarily have any intrinsic properties in
common. For that reason, it is also less plausible that kinds must
play a role in laws of nature.



If one retains the view that species are natural kinds one must
confront the fact that a plethora of species concepts are available
and as seen earlier, these species concepts do not always delineate
the same species. For example, Mayr's (1969) *biological
species concept* (BSC) regards a species as a group of
"interbreeding natural populations that are reproductively
isolated from other such populations" (1969, 26). The
*phylogenetic species concept* (PSC) holds that a species is
"the smallest diagnosable cluster of individual organisms within
which there is a parental pattern of ancestry and descent"
(Cracraft 1983, 170). These definitions are not perfectly coextensive.
(For a detailed discussion of species concepts see the entry on
 species.)



This diversity may be thought to support conventionalism about natural
kinds (Section 1.1.2). But supporters of naturalism about our
classifications (Section 1.1.1) will deny that the dispute in
systematics reflects an indeterminacy in the natural world. A
*monist* will hold that one of the existing systems is a
superior account of the natural world. A commitment to one of the
different systems over another will result in a different ontological
account of species. The most prevalent system of classification in
contemporary systematics is cladistics, which classifies organisms
phylogenetically, in virtue of shared derived features or an
organism's place in the genealogical tree.



An alternative view to monism, motivated by the variety of species
concepts, is *pluralism*, which holds that the different
accounts of species in systematics reflect equally legitimate ways of
carving up nature subject to our pragmatic and theoretical interests.
However, these systems do carve nature in genuinely natural ways. The
species and higher taxa that they delineate reflect real features of
the world from the point of view of different theoretical interests.
Pluralism (Kitcher 1984; Dupre 1993; Ereshefsky 1992) is the
weakest form of realism about kinds in the philosophy of biology.


#### 2.1.2 Biological Essentialism



Kripke (1971, 1972) and Putnam (1975a) use animal kinds as examples of
natural kinds for which *a posteriori* essences can be found.
There is some implication that these essences are microstructural,
intrinsic properties, which will be, of necessity, individually
necessary and jointly sufficient for an entity to be a member of a
kind. However, if species are individuals, then it is not true that
species may be individuated on the basis of the intrinsic properties
of their members. The various species concepts tend to offer
relational criteria of species membership (see above). According to
the BSC, for example, membership of a species depends on relational
properties, such as membership of a certain population and
interbreeding. Alternatively, the PSC refers to shared descent.



However, Kripke (1980) himself argues that a person's parentage
is essential to them. If that is so, then if individual *S* is
descended from *N*, then *S* is necessarily descended
from *N*. McGinn (1976) suggests that this extends to species
also. LaPorte (2004) argues that essentialism holds with respect to
facts relating individuals, species, and other taxa to the higher taxa
(genus and above) within which they are nested. These taxa are clades,
that is to say kinds defined by shared descent from a common ancestral
group: an individual or group that is a member or part of clade is
necessarily a member or part of that clade. Thus biological kinds
(species, genera, etc.) do have essential properties, and these are
historical rather than intrinsic properties.



So one option for biological essentialism is to drop the traditional
view that the natural properties that are essential for membership of
a natural kind are intrinsic: natural, *extrinsic* (relational)
properties can play this role (Okasha 2002). The categorical
distinctness criterion must be dropped also.


### 2.2 Natural Kinds and Chemistry



On account of the problems for natural kinds in biology as well as the
development of modern chemistry, chemical kinds have replaced
biological kinds as the paradigms of natural kinds. The chemical
elements and chemical compounds appear to be *bona fide*
natural kinds. We refer to chemical kinds in laws, explanations, and
inductions: that a certain item is iron explains its behaviour and
that behaviour is predictable; that iron objects are magnetizable is a
law of nature (criteria 2 and 3 above). Moreover, we can induce that
all iron is magnetizable, from the observation of particular instances
of iron objects that are magnetic. And chemical kinds appear to obey
the categorical distinctness requirement: iron is clearly distinct
from its neighbours in the periodic table (manganese and cobalt); no
elements are intermediate kinds (criterion 6 above). Furthermore,
microstructural essentialism seems to be a *prima facie*
plausible option for chemical kinds: it is essential to iron that
something made of pure iron is constituted by atoms that have
precisely 26 protons in their nuclei.


#### 2.2.1 Microstructuralism



Microstructuralism in the philosophy of chemistry is the thesis that
chemical kinds can be individuated solely in terms of their
microstructural properties (Hendry 2006). As exemplified above by the
case of iron, the chemical elements provide paradigmatic kinds that
may be individuated microstructurally, since the atomic
number--the number of protons in the nucleus--suffices to
identify the element. It is true that macro-level chemical and
physical properties can also serve to individuate chemical elements,
and in the nineteenth century chemists were able to individuate
elements without knowing what we do about nuclear structure. However,
it is held that atomic number has explanatory priority: the number of
protons in the nucleus, and hence the nuclear charge, explains the
structure of electrons surrounding the nucleus, which, in turn,
explains the chemical behaviour of the element.



The microstructuralist can extend this approach from elements to
compounds. Compounds are identified principally by their constituent
elements. Thus carbon dioxide is that compound of carbon and oxygen
with the molar proportion 1:2. In more detail, molecules of carbon
dioxide consist of two oxygen atoms and a single carbon atom. The
practice of identifying a chemical compound only by its composing
elements was the norm in chemistry, until the discovery of isomerism
by Friedrich Woehler in 1827. Isomers are distinct compounds that
nonetheless share the same constituent elements in the same
proportion. Thus fulminic acid and cyanic acid may both be expressed
in terms of constituents in the empirical formula CHNO, but their
distinct chemical and physical properties identify them as different
substances. The explanation of isomerism (and many other facts in
organic chemistry), provided first by August Kekule and
Archibald Scott Couper, is the fact that certain compounds exist in
molecular form and these molecules have internal structure. Thus
molecules of isomers have the same atoms in different spatial
arrangements, e.g., CHNO may be arranged as H-O-C[?]N
(cyanic acid) or as H-C=N-O (fulminic acid). Isomerism
means that specifying the chemical composition alone is not sufficient
for classification. The microstructure of a compound concerns not just
the elemental atoms in its molecules, but also their spatial
arrangement.



Needham (2000) and van Brakel (2000) have argued that compounds such
as water are dynamic structures whose natures cannot be given in
static accounts of their composing elements. Molecules of
H2O are polar, with the consequence that the
electropositive hydrogen atoms in one H2O molecule will
bond with the electronegative oxygen atoms in another H2O
molecule, such bonds being *hydrogen bonds* with the result
that in liquid water H2O molecules will form polymer-like
chains known as *oligomers*. The hydrogen bonds and the
consequent chains are responsible for the fact that water is liquid at
room temperature (whereas compounds with similarly sized molecules,
such as hydrogen sulphide, methane, and carbon dioxide, are all
gaseous at room temperature). These oligomers are constantly forming,
breaking, and reforming. The rate of such changes and the mean length
of such oligomers are dependent on the thermodynamic context (in the
extreme case, an ice crystal may be considered as a single such
oligomer, the crystal structure being dependent on the fixed, strong
hydrogen bonds). Thus, say Needham and van Brakel, we cannot consider
water to be just a compound composed of a collection of H2O
molecules. Furthermore, anything that is to be water must be a
macroscopic entity, since only macroscopic bodies can bear
thermodynamic properties, such as melting point, which we use to
identify water. Hence, a single H2O molecule is not
water.



These considerations need not undermine the microstructuralist claim.
Many chemical kinds may exist in any of the solid, liquid, and gaseous
states while remaining the same substance, including water. But, water
vapour will not possess the oligomers present in liquid water and ice.
So their presence, however characteristic of water in the liquid and
solid phases, cannot be a necessary feature of all bodies of water.
Above we saw that atomic number is regarded as the essential feature
of an element because it explains the other characteristics an element
has. Likewise, the structure of the H2O molecule explains
why it is a strongly polar molecule, which in turn explains why it
tends to form oligomers. As regards thermodynamic properties, Hendry
(2006) points out that we say that water is contained in a
person's body, without thinking that this water has any
thermodynamic properties. More generally, thermodynamic properties can
be ascribed only to entities in equilibrium, but not all bodies of
water are in thermodynamic equilibrium or even nearly so. We may very
well use such thermodynamic properties in identifying something as
water, but that does not mean that it is in virtue of such properties
that something is water.



Hendry (2006) points to a potentially more difficult problem, which is
that any liquid sample of water, however pure, contains not only
H2O molecules but also H3O+ and
OH[?] ions. These ions cannot be regarded as
impurities, since they are also an inevitable consequence of the polar
structure of the H2O molecule. Hendry's solution is
to regard H2O molecules as the constituents of water in
rather the same sense that eggs and flour are the constituents of a
cake--they go into making the cake but need not retain exactly
the form they started with. Water is made *from* bodies of
H2O molecules, but not all those retain precisely that
structure.



The taxa in the classical Linnean system of biological classification
are nested in a hierarchy, as required by Criterion 5 in Section
1.1.1. But, in certain respects, chemical classification fails to meet
this requirement. The phenomenon of *allotropy* is exhibited
when an element exists in two or more distinct forms. The element
carbon has several, including the allotropes diamond and graphite. Tin
has two allotropes at room tempreature, white tin (which is metallic)
and grey tin (which is non-metallic). Some instances of tin will fall
into one category and other instances will fall into the other
category. Consequently, classification by element and classification
by metal or non-metal cannot be combined hierarchically.



Such cross-cutting classifications are frequently found in organic
chemistry, where compounds can be classified according to their
so-called functional groups. Functional groups are specific
combinations of atoms within a molecule that will cause the molecule
to engage in certain reactions and to have other physical and chemical
properties that are characteristic of that group. For example,
alcohols are organic compounds containing a hydroxyl group [?]OH
bound to a carbon atom of an alkyl group or derivative of an alkyl
group. Alcohols undergo characteristic reactions such as
esterification. Since other hydrogen atoms in an alcohol molecule may
be substituted by another functional group, the resulting molecule
will have properties characteristic of both functional groups and may
be classified accordingly. Benzyl alcohol,
C6H5CH2OH (or BnOH), is obtained from
methane, CH4, by replacing one hydrogen atom by an
alcohol-forming hydroxyl group, [?]OH, and another by the phenyl
group [?]C6H5 (Ph) (the phenyl group plus
the CH2 from the methane is the benzyl group, *i.e.*
Bn is PhCH2). Thus benzyl alcohol may be classified either
as an alcohol, or as an aromatic benzene derivative, since it
participates in the characteristic reactions of the latter, such as
electrophilic aromatic substitution or hydrogenation of the benzene
ring. If the hierarchy requirement on a system of natural kinds is
correct, then not all these cross-cutting classifications pick out
natural kinds. The claim that the hierarchy requirement is too
stringent for scientific kinds has been defended by Khalidi (1998) and
Tobin (2010b).



One may have to deny that metals form a natural kind, and that
classification by functional group is a classification into natural
kinds. (Not all classifications need to be into kinds in order for
them to be useful.) Note that in both cases, there is room for
vagueness. Some elements, such as germanium and antimony, are classed
as metalloids, with characteristics between metals and non-metals. The
impact of a functional group diminishes with the size of the molecules
of which it is a part and in the presence of competing functional
groups, and so in certain cases, classification according to
functional group will be vague. Such vagueness falls foul of the
categorical distinctness requirement (Criterion 6 above).



Philosophers have also discussed classification at the interface of
biology and chemistry. Goodwin (2011) has argued that structure is
fundamental and sufficient for classification of biomolecules, such as
proteins. Tobin (2010a) and Slater (2009) appeal to developments in
the relationship between protein structure and function in a number of
cases such as moonlighting proteins, intrinsically unstructured
proteins, and enzymes with multiple structural variants. They claim
that the path from structure to function in not straightforward and so
undermines the fundamental individuating role of structure in protein
classification. Slater (2009) takes these developments in the practice
to constitute an argument in favour of macromolecular pluralism.
Goodwin (2011) claims that these developments do not undermine the
fundamental role of structure in protein classification, but instead
suggest a more nuanced understanding of this role. (Goodwin, 2011: p.
534) In a similar vein, Tahko (2020) argues that biochemical kinds
pose a challenge for ontological reductionism and for
microstructuralism, which have led some to motivate a dual theory of
chemical and biological kinds and others to argue for pluralism about
natural kinds. Tahko argues that the main challenge is multiple
realization, which he nevertheless, thinks can be addressed. The
upshot is that there are reasonable prospects for ontological
reductionism about biochemical kinds, which in his view corroborates
natural kind monism. Thus, even if there are indispensable
classificatory practices at the higher level, this need not entail
pluralism about biochemical kinds.


### 2.3 Natural Kinds and Psychology



Do the different sorts of mental state form natural kinds? We
certainly think that our minds make a difference to what happens in
the physical world and we think that we act *because* we have
certain beliefs, desires, hopes, fears etc. The distinctive
explanatory roles performed by these different states in folk
psychology certainly suggest that these beliefs, desires, hopes and
fears constitute distinct mental natural kinds. Note that in
considering natural kinds in chemistry, biology, and physics we have
thought of kinds principally as kinds of thing (kinds of stuff,
organism, particle, etc.) whereas we are now considering the
possibility of certain kinds of *state*. (Ellis (2001, 2002) is
clear that he does not limit natural kinds to kinds of thing, but
includes kinds of state and process also.) A Cartesian dualist holds
that mental states are distinct from any physical state of the
subject, as they are states of an immaterial thinking substance. The
problem for dualism however is in finding the ground of the difference
between fearing, believing, hoping etc. i.e. what difference is there
to the immaterial substance when a subject is in different mental
states? In principle this problem could be avoided by the dualist if
mental kinds are regarded as immaterial substances (Shoemaker
2003).



There are many ways to understand mental natural kinds, and theories
have been proposed by eliminativists, identity theorists,
functionalists and many others. We will treat each of these in turn.
Physicalists hold that the differences in mental kinds relate somehow
to differences in states of the subject's brain, or, more
generally, the subject's body. At one extreme, the (type)
identity theory of mind holds that types of mental state are identical
with types of brain state. If so, we may expect the kinds of
psychology to be identical with the kinds of neuroscience.



The identity theory faces the problem of multiple
realizability--the idea that it is possible for physically
diverse creatures to be in the same kind of mental state. It is
possible, for example, that Martian neurophysiology might be entirely
different from that of humans while Martians nonetheless have the same
kinds of mental state as humans have; if a Martian's body is
injured, it writhes, groans, and avoids the causes of such
injuries--the Martian is in pain (Lewis 1980). Indeed it is
possible that cognitive scientists might design a machine that is
capable of higher-level thought (implemented on, for example, a
silicon architecture). It is even possible that the same mental kinds
be realized by distinct neurophysiological systems in one and the same
organism over time. Consequently, it is at least an open empirical
possibility that mental kinds may correspond to a widely disjunctive
and heterogeneous set of neurophysiological kinds, and hence that
there is no one-to-one correlation between them.



Eliminativism argues that the *prima facie* failure to
straightforwardly reduce mental kinds to neurophysiological kinds
ought to lead to the elimination of mental kinds altogether
(Churchland 1981, 1988). This is to claim that there are no mental
natural kinds. On this view, the mental kinds in folk psychology are
comparable to the kinds delineated by discredited folk theories from
the past e.g., the humours in medieval medicine. The underlying
principle of medieval medicine was the balancing of so-called four
humours. According to the theory, illness was caused by an imbalance
of these four humours. From the point of view of modern medicine this
theory is radically false and so the only option is to eliminate the
humours as putative natural kinds. Analogously, mental kinds ought to
be eliminated in favour of those kinds uncovered by recent research in
neuroscience. Eliminativists agree that human beings have invented a
very successful methodology for describing their mental lives. This
methodology is folk psychology, a common sense theory of how mental
states are causally related to human action. However, folk psychology,
they argue, is merely a heuristic device invented by human beings in
order to make the explanation of behaviour easier. The posits of the
theory are not real natural kinds. Once a better explanation of human
behaviour in terms of neurophysiological kinds is available, then
mental kinds will be eliminated.



Functionalists argue that the irreducibilty of mental kinds is
nevertheless compatible with a token-token identity theory (Fodor
1974, 1997). Mental kinds are not type-identical with
neurophysiological kinds (*i.e.* there is not one type of
neurophysiological correlate for each type of mental state, so we
cannot make general identity statements such as "pain =
*C*-fibres firing"). A Martian and I, both in a state of
fear at a time *t*, may be in different physiological states.
Nevertheless, my token instance of the kind fear at time *t*
will be identical with the token physical state in my brain at that
time. Likewise, the token instance of the Martian's fear at time
*t*1 is identical with his token physical state.



It has been argued by Kim (1992, 1993) that multiply realized mental
kinds of the latter kind can in principle be locally reduced to their
realizing states. Mental kinds are analogous to mineral kinds such as
jade, which can be locally reduced to nephrite or jadeite depending on
the sample. Importantly, a sample that does not consist of either
nephrite or jadeite is simply not jade. Nevertheless, there is an
important disanalogy between mental kinds and locally reduced kinds
like jade. If you want a sample of jade, then it must be composed of
either jadeite or nephrite. In other words, if *J* (jade) is
realized by the disjunction *P* (Jadeite) or *Q*
(Nephrite), then it is metaphysically necessary that the properties a
thing has *qua* *J* are either properties it has
*qua* *P* or properties it has *qua* *Q*.
So, something with all the other qualitative properties of jade in
another possible world, but which lacked the essential properties
(i.e. of jadeite or nephrite) would simply not be an instance of
jade.



In contrast, for the functionalist (Fodor 1997) mental kinds are
defined by their functional roles, rather than by the essential
properties of the neurophysiological kinds that realize them. It does
not matter which physical kind does the realizing, be it silicon
chips, neurons, *C*-fibres or whatever. Rather, what matters is
that the psychological state (e.g., pain) plays a certain functional
role. The whole point of multiple realizability is that it is easy to
imagine some possible world where a token mental kind is realized by
silicon chips, *C*-fibres firing, or whatever. So, a mental
kind is not essentially realized by a certain neurophysiological kind;
rather it plays the same functional role independently of what
underlying kind is realizing it. This appears to violate Criterion 1
for natural kinds (in Section 1.1.1), that members of a natural kind
should have some natural properties in common.



The functional role of mental kinds is explained by some
functionalists in terms of the modularity of the mind (Fodor 1983).
This is the claim that the mind has different innate modules which
play a certain functional role. These modules are domain-specific,
operating only on certain kinds of inputs. So, for example, there will
be distinct modules for mental tasks like mind-reading, speaker
recognition and facial recognition. The functional role associated
with mental kinds might for example be given an evolutionary account
in terms of adapted modules (Cosmides and Tooby 1992). On this view,
mental kinds in folk psychology are successful for the purposes of the
explanation of human behaviour because they refer to the functional
modules of the mind. Such views may motivate *emergentism*. One
interpretation of the emergentist account of mind is that there are
emergent mental kinds that have no straightforward correlation with
the kinds identified by neuroscience. Nor are mental kinds and
neurophysiological kinds either identical to or subkinds of one
another; consequently emergentism is in conflict with the (contested)
taxonomic hierarchy criterion (Criterion 5). (See the entry on
 emergent properties.)



The type-identity theory, if true, might seem to motivate essentialism
about psychological kinds, by analogy with other theoretical
identities (see Section 1.3.1 and Section 4.3). Kripke (1980),
however, denies that his arguments concerning other theoretical
identities apply here. Let us say that (slow) pain for us humans is
correlated with the firing of *C*-fibres. If this were a
genuine theoretical identity, then in all possible worlds, when
someone undergoes *C*-fibre firing, then they are in a state of
pain. But it seems plausible that there could be worlds where
*C*-fibre stimulation is not accompanied by any painful
sensation. If there are such worlds, then the theoretical identity
implies that such a world is one where people can be in pain without
having any painful sensation; indeed the pain might yield a pleasant
sensation. Kripke regards this as counterintuitive. The concept of
pain is necessarily tied to the phenomenal nature of the pain
sensation, whereas the concept of gold is not tied to the colour
yellow. Consequently, there is no identity between pain and
*C*-fibre firing (nor any other physical state); likewise there
is no physical essence to pain. *A posteriori* natural kind
essentialism fails in this case.



Recently, there has been some discussion of whether mental concepts
are themselves natural kinds. The claim that concepts form a natural
kind has been accepted by philosophers of psychology and cognitive
psychologists alike (Margolis, 1994, 1995; Fodor, 1998, 2008). Recent
literature has argued against that assumption (Griffiths 1997; Machery
2005, 2009; Piccinini and Scott 2006). Griffiths 1997 has argued that
individual and familiar concepts such as emotion do not correspond to
genuine natural kinds, and should therefore be eliminated from
scientific vocabulary. Machery's (2009, Ch. 3) heterogeneity
hypothesis claims furthermore that the class of concepts do not
delineate a homogeneous class; the kind 'concept'. Rather,
they divide into several distinct kinds that actually have little in
common. Thus, it is a mistake to assume that there are many general
properties of concepts, and that a theory of concepts should attempt
to describe these. In general, then the notion of concept is not part
of an adequate taxonomy of our mental representation.


### 2.4 Natural Kinds and Social Science



The question as to whether there are social kinds has a long history.
In the sociology of science, functionalists such as Durkheim (1897)
have argued that there is empirical support for the autonomy of social
facts. For example, statistical analysis shows that suicide rates
differed radically depending on social factors, (e.g. religious
background, gender, marital status etc.) This analysis led to the
possibility of social science being taken in a quantitative direction.
Even if every individual commits suicide as a result of psychological
factors, the statistical facts about the "reasons" for
suicide and how they change with place and over time are distinctly
social. One question then is whether one should be naturalist about
such social facts. If so, how could naturalism be reconciled with the
claim that social facts are dynamic and changing in so far as they
concern human beings in social interactions?



Because social kinds are distinctly anthropocentric, some theorists
claim that the only plausible view we can hold is some form of
constructivism about them. Hacking (1995, 1999) argues that in social
contexts the object of classification itself changes in being
classified. The fact that the people being classified are conscious
makes for dynamic interactions between the classification and the
classified. The latter may resist or embrace certain classifications
('child abuser', 'hip') and modify their
behaviour accordingly. This leads to a change in the extensions of
the relevant kind concepts. Furthermore, as the extensions change, the
criteria for membership may change also. Many of our classifications
in social science are evaluative. A social situation looks deviant
from the norm and people make an evaluative judgment about this.
Sometimes this means that people passively accept what experts say
about them, and see themselves in that light. But feedback can direct
itself in many ways. Alternatively, some people seek to reclassify
themselves. Thus, classification in social science is interactive.
Hacking argues that this is in marked contrast to the indifferent
kinds that are found in natural science. An electron is indifferent to
being classified as an electron.



*Pace* Hacking, Khalidi (2010) argues that in fact the contrast
between natural and social kinds is not so stark: even many chemical
kinds would not have been instantiated in the world were it not for
the fact that a scientist conceived of them and acted on that
conception. So in both cases the existence of a classification scheme
can cause changes to the extension of the kinds in question. The
difference between natural kinds and social kinds is a difference of
the degree of control we have over the object of classification. In
the chemical domain, for example, the nature of the phenomena
themselves imposes greater restrictions on the kinds that we can
create. (See also Cooper 2004.)


## 3. The Epistemology of Natural Kinds



Many of these considerations from the practice of classification
in the special sciences have led to an emerging naturalist approach to
the topic of natural kinds. It is clear from the examination of
classification in the special sciences, that it is difficult to find a
concept of natural kind that fits all of the different practices of
classification. This has sometimes been used as a core argument in
favour of pluralism about natural kinds (Dupre 2001) or to
eliminate the need for natural kinds altogether (Hacking
2007a). Additionally, this first argument has been used to support the
homeostatic property cluster view of kinds, because this takes a more
liberal approach to classification. Indeed one of the motivations for
Boyd's original HPC view was to outline an account of natural
kinds that was liberal enough to accommodate a diversity of natural
kinds, also including the kinds in the biological and social
sciences.



Many of the claims made in support of pluralism stem from concerns
about natural kind classification in the context of scientific
practice. This approach tends to examine specific scientific case
studies and derive philosophical implications about classification
from the examination of these classificatory practices (Kendig 2015).
It is important to point out that many of the discussions are
epistemic in nature; that is to say the claim that there are different
ways to classify in scientific practice, which are all equally
legitimate ways of generating knowledge for practising scientists
depending on their theoretical interests and research questions.



In recent literature, some have argued for a move away from the
metaphysics of natural kinds to an epistemology-first approach to
classification. Slater (2015), Ereshefsky and Reydon (2015) argue that
the epistemic value of natural kinds can be considered independently
from the metaphysics of natural kinds. They claim that it is more
fruitful to be led by classification in scientific practice first and
foremost. A philosophical account of our classificatory practices
should describe how kinds are made in the context of scientific
practice, we might refer these as acts of "kinding".
Slater claims that what matters for classification is the role that
our classes play in inductive inferences within the context of
scientific practice. Slater argues that this requires a minimal
metaphysical commitment "that these properties be sufficiently
stably co-instantiated to accommodate the inferential and explanatory
uses to which particular sciences are put" Slater (2015), p.
396). Similarly, Magnus (2012) argues that a natural kind in a domain
allows enquiry in the domain to achieve inductive and explanatory
success, a success which would be impossible without a pre-existing
commitment to a natural kind classification.



Additionally, those that take an epistemology first approach are
critical of metaphysical accounts of natural kinds which are overly
prescriptive and homogeneous about what counts as a natural kind. The
consequence of taking a metaphysics first approach is that many of the
important and heterogeneous practices of kinding that are used in the
context of scientific practice do not count as natural kind
classifications in the metaphysical sense. The implication here is
that an epistemology first approach is more consistent with a
descriptive and naturalistic account of classification.
Epistemological accounts of natural kinds must come first and our
metaphysical accounts must be informed by these epistemological
considerations.



Ereshefsky and Reydon (2015) have argued that this has led to a
fundamental mismatch between scientific kinds and natural kinds. In
the homeostatic property cluster view of kinds, the overarching
principles of similarity and homeostatic mechanisms lead to many
examples of scientific kinds which do not then count as natural kinds.
For example, according to Ereshefsky and Reydon (2015), non-causal,
functional and heterostatic kinds would not count as natural kinds
despite the fact that many successful scientific kinds are non-causal,
functional or heterostatic.



In contrast, Lemeire (2018) argues that a purely epistemic approach to
natural kinds is misguided. The epistemic approach rests on the
assumption that for kind categories to be epistemically fruitful, they
have to get something right about the world. However, epistemic
success is constrained by deference to the world not only in cases of
natural kind classification, but also in cases of conventional and
pragmatic classification, where the product of kinding is not
considered to be a natural kind. For this reason, Lemiere argues that
a purely epistemic approach fails to give us a principled way of
distinguishing between the natural kinds and their conventional
counterparts. A metaphysical account of which features individuals
require to be considered as natural kinds is required to lead our
categorization practices. An epistemic approach alone cannot provide a
principled way of doing this.



Kendig and Grey (2021) are also critical of purely epistemic
approaches to natural kinds. They accept that there is value to
epistemology first accounts of natural kinds in so far as they
highlight the epistemic value of natural kind classifications to
scientists. Nevertheless, they argue that doing this successfully
requires accurate metaphysical presuppositions. Their argument focuses
on Slater's HPC view of natural kinds to expose why metaphysical
presuppositions are necessary for our categorisation practices. They
claim that it is not possible to evaluate the stability of a cluster
of properties without taking stock of the metaphysical picture used to
account for that stability



Reydon (2016) has argued for a model which attempts to reconcile and
integrate the metaphysics and the epistemology in the act of
classification. On his co-creation model of natural kinds, both nature
and the classifier are constitutive of the creation of kinds'
(p. 70); kinds are 'co-determined by aspects of the state of
affairs in nature as well as by background assumptions and decisions
by investigators' (p. 59).


## 4. The Semantics of Natural Kind Terms



Much recent interest in natural kinds has been stimulated by debates
concerning the semantics of natural kind terms, originating especially
with the work of Saul Kripke (1972) and Hilary Putnam (1973, 1975a).
Typical natural kind expressions fall into one of two categories:
predicates (*is water*, *is gold*, *is a tiger*,
and *is an oak*) and singular terms (*water*,
*gold*, *the tiger*, and *the oak*). In this
section we survey some of the debates concerning the meaning of such
terms, what--if anything--they refer to, and how they do so.
Many discussions focus on the predicate kind terms, often considering
them alongside other general terms, taking the principal issue to be
the relationship between a natural kind term and its extension. Such
an approach will appeal to conventionalists, but it may appeal also to
those naturalists who are nominalists about natural kinds or otherwise
hostile or indifferent to (strong) realism about natural kinds.
However, we shall see that there may be reason to take the singular
kind terms as primary. (One difference between Putnam and Kripke is
that while Putnam tends to focus on the natural kind predicates,
Kripke's arguments are centered primarily on the singular
natural kind terms.) Kripke and Putnam attacked a pair of related
views about natural kind terms: descriptivist semantics (the meaning
of such a term is a description satisfied by all and only members of
the kind) and internalism (the meaning of such a term, the relevant
description, is fully grasped by competent users of the term so that
what fixes the extension of the term supervenes on what is internal to
the user). Putnam's rejection of descriptivism is a component of
his argument that 'meanings ain't in the head'
whereas for Kripke the goal is to show that natural kind terms are
much like proper names, for which, he argues, a descriptivist
semantics fails. Members of the extension of a natural kind share
properties in common. If descriptivism were true, then one would
reasonably believe that those properties would be the properties
mentioned in the description with the possible addition of further
properties nomically correlated with those. But if descriptivism is
false, it would appear that the characteristic properties of a kind
will not typically be knowable a priori, but must be uncovered by
scientific investigation. If so, then those properties, being
necessary conditions of kind membership but discoverable only a
posteriori, look very much like *real essences*. At first sight
it seems then that the semantic arguments of Kripke and Putnam have
metaphysical consequences. This is highly controversial (Salmon 1979,
1982). Do the arguments of Putnam and Kripke really support an
essentialist metaphysics? Or do they illegitimately assume it? Or do
they, more innocently, show that we as users of natural kind terms
implicitly assume such a metaphysics?


### 4.1 Descriptivism and Internalism



The modern tradition of natural kinds begins with J. S. Mill (1884)
who talked of 'Kinds' (the term 'natural kind'
is traced by Hacking (1991a) back to Mill's contemporary, the
logician John Venn (1866)). According to Mill, typical singular
(individual) names such as 'Scott' *denote* their
referents but lack any *connotation*, that is to say the names
do not imply any attribute. Some singular names do have connotation as
well as denotation, for example 'the author of Waverley'.
A general name, such as 'white', denotes all the things
they hold of (its extension) while also connoting the attribute they
all share in common. Natural kind terms are a special case of general
terms in this respect



The idea that something such as a connotation determines the extension
of a term is elaborated in Frege's notion of *sense* and,
later, that of *intension*. Frege and later philosophers have
applied this view to singular proper names, and views of this sort
fall under the general heading of 'descriptivism'. Russell
is also often referred to in this regard, but it should be remembered
that Russell's view is that grammatically proper names are
abbreviations of quantified expressions and so do not have any
referring role at all (and so *a fortiori* they do not have a
referring role that is meditated by a connotation/sense/intension).
Some neo-Fregeans nevertheless deny that Fregean senses should be
cashed out as descriptions (e.g., McDowell 1977; Evans 1982).



Mill's descriptivism implies that for each natural kind term
there is an associated set of properties such that for some item to be
a member of that natural kind, it should possess those
properties--these are properties that describe objects belonging
to the kind in question. Implicit in Mill and explicit in Frege is the
claim that these descriptions are constitutive of the meaning of the
term in the following way: someone who understands the term adequately
will be in a position to grasp *a priori* that something falls
under the kind term if and only if it satisfies that description. For
this reason the relevant properties will normally be readily
observable properties of things; they will not be properties
associated with the kind only by *a posteriori* investigation.
For example, as mentioned above, Kant held the view that it is *a
priori* that something falls under the kind term *gold*
only if it is a yellow metal. Because the extension-fixing meaning of
a term is thus graspable *a priori* this view is
*internalist* about extension fixing. The extensions of kind
terms of any two subjects with the same internal mental states will
have (in the same context) the same extensions.


### 4.2 Arguments Against Descriptivism



Putnam (1973, 1975a) and Kripke (1972) reject the descriptivist view
of natural kind terms. Putnam's principal aim is to undermine
internalism about reference and extension ("meanings just
ain't in the head"). Kripke's aim is first to
restate a Millian view of proper names (they denote without connoting)
and then to extend that view to natural kind terms.
('Millian' will be used to characterize any account of the
semantics of a term that claims that it denotes without connoting,
even when applied, *pace* Mill, to kind terms.) According to
Kripke, Mill was right about proper names but wrong about kinds;
Millianism--Mill's denotation-without-connotation
view--is right for both sorts of term. Salmon (1982) categorizes
Kripke's arguments against descriptivism for singular terms
under the headings 'the modal arguments', 'the
epistemological arguments', and 'the semantic
arguments'--this categorization is useful also for the
corresponding arguments against descriptivism for general terms.


#### 4.2.1 The Modal Arguments Against Descriptivism



The modal arguments against descriptivism take the following form. Let
*C* be some natural kind and *D* be some complex set of
readily observable properties that would, on a descriptivist view, be
most naturally regarded as providing the content of the description
that constitutes the meaning of '*C*'. Now
consider:



> 
> (K) [?]*x*(*x* is an instance of the kind
> *C* - *D**x*).
> 



Since *D* is constitutive of the meaning of
'*C*', (K) should be analytic and so necessary. The
descriptive view would thus be refuted either by (i) metaphysically
possible instances of the kind *C* that do not possess the
property complex *D* or by (ii) metaphysically possible items
that do possess the property complex *D* without being
instances of *C*.



Some proposed cases of (i) require the laws of nature to be
contingent. Although the laws of nature are widely held to be
contingent, some philosophers hold that the laws are necessary, on the
basis of essentialist arguments that are much of a piece with
Kripke's essentialist arguments, even if strictly independent of
them. The anti-descriptivist arguments regard the observable
properties of instances of a kind *C* as nomic consequences of
the essence of *C*. If the laws of nature are metaphysically
necessary then *D* will be a necessary property of instances
of *C* also. Thus, instances of (i) will not be available. If
the laws are necessary, then the melting point of gold will be
necessary just as the atomic number of gold is. However, not all
observable properties are consequences of the laws alone. Some are
consequences of the laws plus contingent environmental factors. Thus
the perceived colour of an object may change with environmental
differences. Thus if *looking yellow* is a component of
*D*, then a possible world in which gold does not look yellow
(because of unusual atmospheric effects)--surely a genuine
possibility--would be a counterinstance of type (i). On the other
hand, it might be argued, that the proper component of *D*
would be *being yellow*, in which case we will have the makings
of an epistemological argument against descriptivism (see below).



Other cases of (i) might be thought to be generated, irrespective of
the necessity of the laws, by noting that a kind might be manifested
in two or more superficially distinct species. Thus allotropes are
distinct types of a single chemical kind that may differ considerably
in superficial properties (for example, graphite and diamond, both
types of the element carbon). Similarly, a caterpillar and a butterfly
can be members of the same species. For known cases, *D* would
have to be disjunctive. While that may not itself be problematic, it
raises the epistemic possibility that the same phenomenon may be
(nomically and metaphysically) possible for many kinds that we think
exist in only one form.



Cases of (ii) may be more readily available, since the necessity of
laws may rule out a particular kind having different possible sets of
manifest properties, but does not rule out two distinct possible kinds
having the same manifest properties. Indeed, the case of jadeite and
nephrite is often referred to as an example of the latter. Along the
same lines, we may imagine that something like fool's gold, iron
pyrites, is just like gold in observable properties, differing only in
chemical constitution. We may imagine that there were creatures to all
appearances like cats, but which are internally robots; or just like
tigers but had reptilian innards. These are all metaphysically
possible, so the arguments go; hence, possession of the relevant
properties cannot suffice for membership of the corresponding
kind.



The modal argument does not refute descriptivism applied to every
description--after all, Kripke and Putnam do both think that the
description "is constituted by H2O" holds
necessarily of anything that is water. Rather, the modal argument is
intended to exclude as possible descriptions those that could be
constitutive of a sense or connotation that is grasped by any
competent user of the term. Such a sense, if it exists, would employ
descriptions that refer to the observable features of the members of a
kind, not those features uncovered by science.


#### 4.2.2 The Epistemological Arguments



The epistemological arguments against descriptivism have a similar
structure to the modal arguments. Consider again the proposition (K).
Someone who uses the term 'C' competently should be
thereby able to come to know that (K) is true, in virtue of grasping
the meaning of 'C'. Thus (K) should be *a priori*.
So counterexamples to descriptivism would be cases (i) where although
one knows that some thing is of the kind *C* one cannot thereby
come to know that it possesses the property complex *D*, or
(ii) where although one knows that some thing possesses the property
complex *D*, one cannot thereby come to know that it is of the
kind *C*.



To generate epistemological type (i) cases, Kripke asks us to imagine
that gold might be blue but only appears yellow thanks to an optical
illusion, or that tigers do not normally have four legs, but only
appear that way, again thanks to an illusion. However, if the
descriptions associated with 'gold' were to include
'is yellow' and the descriptions associated with
'tiger' include 'is four-legged' then we would
know *a priori* that these possibilities do not obtain. But
that seems to be a mistake. Although we may reasonably be confident
that gold really is yellow and not blue, and not just appearing that
way, this confidence is not *a priori* but is instead a
consequence of *a posteriori* experience of the conditions
under which we have observed samples of gold. It is a matter of
discovery whether we are subject to this kind of illusion, or whether
something that is in fact white or grey could be gold; for example it
is only *a posteriori* that we know that gold does not have
white and grey allotropes, as tin does. (What is called 'white
gold' is in fact an alloy of gold; but given a description of
its appearance, it is not *a priori* that it is not gold. This
is a contrapositive version of an epistemological type (i) case.)
Likewise, turning to epistemological type (ii) cases, if descriptivism
were correct, we could rule out *a priori* the possibility that
something has all the superficial properties of gold, but is not gold.
But again we do not have such *a priori* knowledge (again we
note that jadeite is superficially similar to nephrite but not the
same kind).



Putnam's Division of Linguistic Labor may be thought of as
furnishing an epistemological argument. Putnam's idea is that
the referent of a term used by some individual may be fixed not by
their own knowledge or beliefs, but rather by the knowledge of certain
experts to whom that individual defers. Since aluminium and molybdenum
are different kinds, then according to descriptivism there should be
some *a priori* knowable features that will distinguish between
them for anyone who is capable of using 'aluminium' to
refer to the one kind and 'molybdenum' to the other kind.
Likewise there should be some mental difference between a speaker in
the actual world who uses 'aluminium' to refer to
aluminium and a speaker in another world, where the instances of
aluminium and molybdenum are swapped (pans and cans are made of
molybdenum there), who uses 'aluminium' to refer to
molybdenum. But, says, Putnam, no such difference need exist: ordinary
speakers have these differentiated referring capacities without any
difference in knowledge of the referents. That is because those
referential capacities are parasitic on those of the experts who do
have the required discriminating knowledge (Putnam, 1975b, 228). Their
knowledge fixes the referents of their use of 'aluminium'
and 'molybdenum'. Our deference to them as those people
who could tell us which things are aluminium and which are molybdenum,
means that our referents of 'aluminium' and
'molybdenum' are the same as theirs.


#### 4.2.3 The Semantic Arguments



The modal and epistemological arguments concern the modal and
epistemic properties of propositions of the form of (K). If
descriptivism were correct then there should be for each kind,
*C*, a proposition of that form employing a manifest property
complex *D*, such that the proposition is analytic and hence
both necessary and *a priori*. The modal and epistemological
arguments deny this.



Note that the modal and epistemological arguments are most naturally
presented in the material mode, concerning the objects themselves and
their relations. (K) is in the material mode and we ask about its
modal status, by talking about certain stuff and asking whether it is
or would be gold if it had such-and-such properties, and so forth.
Likewise, *mutatis mutandis*, for the epistemological
arguments. The semantic arguments differ in that they are presented in
the formal mode, concerning the linguistic expressions in question.
This is because we are asking what a term's reference or
extension would be in certain counterfactual circumstances. To use an
example concerning a proper (singular) name, we start by imagining
that it turns out that Johannes Kepler had not been the first to
postulate the laws of planetary motion, whereas it was an unknown
student of Kepler's who did this first. Many people have no
beliefs about Kepler other that that he was the first person to
postulate those laws. If our story were true, would the name
'Kepler', as used by such people, refer to the student or
to the teacher? Our intuition, so anti-descriptivists maintain, is
that the name would still refer to the teacher, the man we (falsely)
believed to be responsible for the discovery of the laws of planetary
motion. Yet, if the descriptivist account were correct for proper
names, then with respect to the people under consideration, their use
of 'Kepler' would either refer to the student, or would
fail to refer at all.



Both Putnam and Kripke take arguments of this sort to generalize to
natural kind terms. The best known of the semantic arguments as
applied to natural kind terms is Putnam's Twin Earth thought
experiment (see Section 1.3.1). Putnam asks us to imagine that
elsewhere in the universe there is a planet exactly like Earth in
virtually all respects, which we may call 'Twin Earth'.
However, where on Earth we find H2O, on Twin Earth we find
*XYZ*, a substance with a fundamentally different chemical
structure from H2O, but which shares all its manifest
properties (boiling and freezing points, taste, odour, ability to
support life, being the predominant constituent of precipitation,
seas, lakes etc.) with H2O. Every inhabitant of Earth has a
twin equivalent on Twin Earth. Twin Earthlings (at least those in the
parts bearing a geographical similarity to Earth's United
States, Australia, Jamaica, Pitcairn etc.) speak a language rather
like English and use the term 'water' in circumstances
precisely analogous to our uses of 'water' on Earth, e.g.,
they say that their lakes and reservoirs are full of
'water'.



The semantic Twin Earth argument now proceeds by asking what the
extension of 'water' is, as used by some Earthling, and
likewise what the extension of 'water' is, as used by some
Twin Earthling. Putnam's appeal to our linguistic intuitions is
supposed to yield the verdict that the extension of the
Earthling's use of 'water' includes all and only
H2O whereas the Twin Earthling's use of
'water' includes all and only XYZ. In support of this we
may consider that this is the judgment we must come to if we
Earthlings are not to deny what Earth chemists tell us, viz. that
water is H2O, nor, symmetrically, deny the Twin Earth
chemists' assertion, 'water is XYZ'. Nor will it do
to gainsay our chemists' judgment that water is H2O,
given the assumption of the existence of XYZ. For that is tantamount
to the suggestion that (actual) Berzelius (the discoverer of the
correct chemical formula for water) was not entitled to assert that
water is H2O until he had ruled out the possibility of some
substance other than H2O sharing water's manifest
qualities.



The thought experiment is now almost in a position to claim the
foregoing as a counterexample to descriptivism, for XYZ will satisfy
any set of manifest properties that could be constitutive of the
meaning of 'water' as used on Earth. Yet the extension of
that term excludes XYZ. However, the reference to chemists'
knowledge made above reminds us that there is a difference between not
only the physical constitution of Earth and Twin Earth, but also a
difference between the psychological states of their inhabitants. For
(some of) those on Earth believe 'water is H2O'
whereas their Twin Earth counterparts believe 'water is
XYZ'. Perhaps, at least for those speakers, those beliefs are
also constitutive of the meanings of the two terms 'water'
(Earth) and 'water' (Twin Earth). In order to exclude this
response, Putnam asks us to consider the same comparison made in 1750,
before any beliefs of this form were held. We now assume that there
has been no major change in the (cross-temporal) extension of
'water' (Earth) since 1750. (That is to say, when David
Hume asked for a glass of what he called 'water', he was
requesting the same stuff as we are when we make a request using the
same term.) Since, as argued above, the extensions of
'water' (Earth) and 'water' (Twin Earth)
differ today, their extensions differed in 1750. But in 1750 the
descriptions associated with the word 'water' cannot
differ between Earth and Twin Earth. This is because the molecular
structure of water was unknown in 1750, prior to the chemical
revolution (when phlogiston theory was replaced by Lavoisier's
new oxygen theory). Given that the molecular structure was unknown
even to the experts, the descriptive resources available to Earthians
and their Twin Earthian counterparts were identical. Nonetheless,
their references using 'water' differed. Hence,
descriptions do not suffice to fix reference.


### 4.3 Direct Reference and Externalism



The conclusions of the arguments given above include not only the
falsity of descriptivism as a thesis about the reference of natural
kind terms, but also the more general truth of *semantic
externalism*. The Earthling in 1750 and their Twin Earth
counterpart are supposed to be intrinsic physical duplicates. (There
is a problem in that one would expect the twin to have XYZ in its
blood stream, etc., not H2O, so the two are not perfect
duplicates; but we can take it that this physical difference should
not affect the argument.) Although they are duplicates, the terms they
use have different extensions; likewise their thoughts have different
truth conditions. Consequently, the reference, extension, and truth
conditions of a person's utterances do not supervene on their
physical state. These things are fixed, at least in part, by external
considerations. We might retain a conception of the psychological such
that the psychological still supervenes on the physical; but then one
would have to deny that 'thinking about water'
characterizes a psychological state. Or, we must deny that the
psychological state of a subject supervenes upon the physical state of
the subject.



If descriptions do not fix extensions, and some external factor fixes
extensions, then by what process are extensions fixed? Kripke does not
intend to propose a new theory of reference fixing, although the
outlines of such a theory are fairly clearly implied by what he says.
His principal aim is to make the case for the Millian
denotation-without-connotation view of singular proper names, and then
extend this (contrary to Mill's own opinion) to certain general
terms also, including natural kind terms.



The Millian view may be called *direct reference* (Kaplan
1977), since the reference of singular proper name is not mediated by
a connotation/sense/intension. The semantic value of such a name just
is the individual to which it refers. Such terms are thus *rigid
designators*--they denote the same object with respect to all
possible worlds (Kripke 1980, 48). How does this view extend to
natural kind terms? The answer is that the view does not transfer if
we take the analogue of the denotation of a singular term to be the
extension of a kind term. This is because presumably the extension of
a kind term differs from world to world, since the individual members
of a kind, such as individual tigers and lumps of gold, are generally
contingent entities.



The obvious route to an analogy between kind terms and singular direct
reference is to focus not on natural kind predicates but on singular
natural kind terms (see first paragraph of Section 4 above). We should
regard terms such as 'water', 'gold',
'*Rattus rattus*', etc. as denoting abstract
individuals and objects (or some other kinds of entity that may be the
constant recipient of reference across worlds). Thus the direct
reference view applied to such terms says that the semantic value of
those terms just is those abstract individuals, and the claim that the
terms are rigid designators is the claim that they denote the same
abstract individual, the same natural kind, with respect to all
possible worlds. Rigid designation is a consequence of direct
reference (but not vice versa). (See Marti 2003 for a
discussion of the relationship between Millianism, direct reference,
and rigid designation. See also Sullivan 2005.)



Treating kinds as abstract individuals permits an analogue for natural
kind terms to the natural story concerning the reference of singular
proper names that emerges from Kripke's discussion. In the
simplest case, an individual receives a name in a naming ceremony, and
the use of that name is transferred from speaker to speaker,
preserving its reference through each transfer. Thus the reference of
a later use is determined by tracing back the route of transmission to
the original naming ceremony. In the natural kind case, which is
rather more of an idealization than the case of a concrete particular,
the original naming ceremony picks out a natural kind via an ostension
of one of its samples. This is the so-called 'causal'
theory of reference. Note that 'causal' refers to the
causal transmission of the use of the name. (Although an ostensive
reference fixing will be causal, the link between the kind (or its
sample) and the first use of the kind term could be secured by a
reference-fixing description. In such as case the reference-fixing
description does merely that; it does not supply a meaning (sense or
intension).



Thus far Kripke's account of the semantics of natural kind terms
treats them very much on a par with proper names, suggesting that
kinds are much like abstract individuals, being potential recipients
of singular reference--Millianism has been extended from proper
names of individuals to natural kind terms. (Mill's own view of
'general terms' was not Millian in this sense, but closer
to a Fregean view.) Putnam gives a more elaborate account of the
reference of natural kind terms, which apparently does not depend on
treating kinds as abstract individuals. According to Putnam, the
extension of a natural kind term is fixed by ostending or otherwise
identifying a sample of the kind and appealing, explicitly or
implicitly, to a 'same kind relation'. Thus the extension
of 'water' includes all and only those things that bear
the same kind relation to some sample, which may be specified loosely,
for instance as 'the watery stuff around here' (where
'watery' is defined in terms of the manifest
characteristics of water, not in terms of 'water'). (Note
that Putnam says 'same liquid relation', but that would
not provide the extension of a natural kind term, since it would
exclude the gaseous and solid phases of water.) Putnam (1975a) thereby
extends the notion of rigid designation to general terms, since (a)
the same kind relation is a cross-world relation, and (b) it is
anchored to a specific, actual world sample. Consequently all samples
of water in all possible worlds will bear the same kind relation to
one another. That this excludes XYZ depends upon its being the case
that a sample that does not share the 'important physical
properties' of our sample cannot be of the same kind.



The view that natural kind terms are rigid designators has
consequences for identity statements involving natural kind terms
(e.g., "brontosaurus is apatosaurus"). The reason is that
any identity statement relating two rigid designators is necessarily
true. Hence identity statements involving natural kind terms express
necessary truths, even if knowable only *a posteriori*. Kripke
regards theoretical identities such as 'water =
H2O', where one of the terms is a description, as
instances of such *a posteriori* necessary identities. Such a
claim requires an argument to show that 'H2O'
is a rigid designator, since it is not a directly referring
expression. We may take 'H' and 'O' to
designate hydrogen and oxygen rigidly. However, if the formula
'H2O' just means 'hydrogen and oxygen
compounded in the ratio 2:1' one might wonder whether, if the
laws of nature are contingent, there might be worlds where hydrogen
and oxygen are compounded in that ratio in such a way that the
resulting compound does not belong to the same kind as H2O,
and is not water. If that is possible, then
'H2O' is not a rigid designator, and
'water = H2O' is not necessary. (As noted
above, Putnam's Twin Earth thought experiment does not establish
the identity of water and H2O--all it establishes is
that being constituted of H2O is a necessary condition of
being water. In which case it is consistent with some instances
(actual and possible) of H2O not being water.) Most
discussions accept that to count H2O as a substance it must
be compounded of hydrogen and oxygen in the way that they are actually
compounded. The rigid designation of simple, directly referring names
and kind terms is often explained in terms of their having an
indexical character. In ideal cases where the name is introduced via
an ostension of its intended referent, the designation is literal. In
other cases indexicality is less apparent, as we see in the case of
'water' where there is something like an implicit
ostension of 'watery stuff around here' (though
'water' could not *mean* 'watery stuff around
here', since then the referent of 'water' would then
be different in an XYZ environment). A similar implicit indexicality
is found in the theoretical specification of kind terms, which
explains their rigid character also.



The Millian view applied to simple natural kind terms faces various
problems. One of these problems concerns empty terms.
'Phlogiston', 'caloric', 'ether',
'humour', are all natural kind terms that fail to refer to
anything. Millianism implies that there is no semantic difference
between these terms at all. Surely 'phlogiston does not
exist' and 'caloric does not exist' mean different
things; Lavoisier asserted the first and denied the second with
apparent consistency. The most common response to this problem is a
retreat from pure Millianism for such terms. *Some* descriptive
element is included in their semantics.



This need not be a full retreat to a Fregean view for two reasons. The
descriptive element may not suffice to pick out the kind (in the
non-empty cases)--the implicit historical ostension of a sample
may be necessary to fix reference. And the description itself may have
non-Fregean features--it may have an indexical component, for
example, a reference to the actual world, that a Fregean would deny.
The retreat from pure Millianism (and a partial vindication of
Mill's own view of general terms, which Kripke rejected) need
not undermine the claim that kind terms are rigid designators. Rigid
designation does not require direct reference; complex theoretical
expressions, such as 'H2O', may, as we saw,
also be rigid designators.



Several philosophers have argued that rigid designation for kind and
other general terms faces a serious objection: rigid designation for
kind terms is trivial (Soames 2002; Schwartz 2002; Marti 2004).
Rigid designation requires that the object designated by a rigid
designator be the same in all worlds. That requirement rules out the
extension of a kind term as its designation, since the same kind may
have different extensions in different worlds. On the other hand,
regarding the extension as an abstract individual also faces
difficulties. One might take 'Mary's favourite
colour' to designate different colours in different worlds, and
thus understood it is not a rigid designator. But we could take
'Mary's favourite colour' to designate (rigidly) an
unusual abstract individual *Mary's-favourite-colour*,
which has violet things as its extension in the actual world, but
turquoise things in another possible world and so on. This thought
experiment suggests that it is always possible to regard a general
term as a rigid designator. One option for resisting this move is to
deny that in every case there is some entity for the term to refer to.
There is no thing abstract or concrete that is
*Mary's-favourite-colour*. Property terms may denote a
natural property, in Lewis's (1986) terms a *sparse*
property or they do not denote at all. Similarly, natural kind terms
denote genuinely natural entities that are likewise sparse or they
fail to denote.


### 4.4 From Reference to Essence?



Above we saw that an identity statement conjoining two rigid
designators expresses a necessary truth, one that may be knowable only
*a posteriori*. That follows simply from the definition of
'rigid designator'. We have also seen that natural kind
terms are rigid designators. That seems simply to be a fact about the
semantics of natural kind terms. Consequently it looks as if a
non-trivial metaphysical fact--that there are necessary truths
knowable only *a posteriori*--is a consequence of
Kripke's semantics for natural kind terms. Furthermore, when we
consider theoretical identities, such as 'water is
H2O' and 'gold is the element with atomic
number 79' etc., it appears that we are being given a special
kind of necessary truth, a truth that it necessary because it
specifies the essence of a kind.



The conclusion that we can derive metaphysical facts from semantic
ones has been regarded as highly questionable by many commentators.
How could facts about our *use of terms* lead to conclusions
about the *essences of things and kinds*? Furthermore, the
anti-descriptivist semantics involved is closely related to semantic
externalism, but the latter need not imply essentialism, as Tyler
Burge's social externalism shows (Burge 1979, 1986). Nathan
Salmon (1982) argues that no non-trivial essentialist consequences can
be inferred from a theory of reference.



One might take the view that the apparent derivation of a metaphysical
conclusion from semantic premises is a sleight of hand, with hidden
essentialist premises being smuggled into the argument. If one were to
assume that 'H2O' is a rigid designator, one
might be accused of tacitly assuming metaphysical essentialism.
However, one might alternatively take the view that semantics and
metaphysics are more intimately related (Bird 2020). For example, one
might hold that the semantics itself makes certain metaphysical
assumptions, in which case the metaphysics would follow from the
semantics, but innocently and for fairly trivial reasons. Note that a
thoroughgoing Fregean descriptivism also has metaphysical
consequences, since it rules out essentialism. According to the view
that the correct semantics implies an essentialist metaphysics, the
semantic claim that 'water' is a rigid designator is
itself metaphysically loaded. But this is not to say that Putnam,
Kripke, and other philosophers are smuggling in metaphysical
assumptions. Rather, it is to say that competent users of the term
tacitly make those assumptions. Putnam's Twin Earth thought
experiments make those assumptions explicit. Note that, as the above
discussion has shown, while the thought experiments are normally
expressed in the formal mode--we ask whether XYZ would fall
within the extension of the term 'water' (Earth)--the
essentialist conclusion can be reached directly by phrasing the same
question in the material mode: we ask whether XYZ would *be*
water.