## 1. Category Systems


### 1.1 Aristotelian Realism



Philosophical interest in categories may be traced back to Aristotle
who, in his treatise *Categories,* attempts to enumerate the
most general kinds into which entities in the world divide. He does
not begin from a single highest kind, but rather lists the following
as the ten highest categories of things "said without any
combination" (*Categories* 1b25):


* Substance (e.g., man, horse)
* Quantity (e.g., four-foot, five-foot)
* Quality (e.g., white, grammatical)
* Relation (e.g., double, half)
* Place (e.g., in the Lyceum, in the market-place)
* Date (e.g., yesterday, last year)
* Posture (e.g., is lying, is sitting)
* State (e.g., has shoes on, has armor on)
* Action (e.g., cutting, burning)
* Passion (e.g., being cut, being burned)



There are two sorts of substance: a primary substance is, e.g., an
*individual* man or horse; the species (and genera) of these
individuals (e.g., *man*, *animal*) are secondary
substances. While the ten categories are all equally highest kinds,
primary substances nonetheless have a certain sort of priority, since
"all the other things are either said of the primary substances
as subjects or in them as subjects. So if the primary substances did
not exist it would be impossible for any of the other things to
exist" (*Categories* 2b4).



Elsewhere, in *Metaphysics* (998b22), Aristotle argues
explicitly that there cannot be a highest genus (e.g., of
*being* or *unity*) shared by entities of different
categories (cf. Ackrill 1963, 81). For a species is defined in terms
of its subsuming genus and differentia (e.g., man is definable as an
animal that is rational), and while the genus (animal) may be
predicated of the species (man), it may not be predicated of the
differentia (rational). As a result, if *being* (or unity) were
a genus, no differentiae could be said to have being (or to be one);
but "the differentiae of any genus must each of them both have
being and be one" (*Metaphysics* 998b22-3).



The ancient Greek term 'kategoria' described what could be
said against someone in a court of law, and indeed Aristotle uses what
can be said *of* or *in* a subject as a route to
distinguishing categories. There is controversy in the literature,
however, about precisely how he arrived at his categories (Studtmann
2007). On one prominent interpretation, put forward by J. L. Ackrill,
Aristotle arrived at his list of categories by way of distinguishing
"different questions which may be asked about something"
and noting "that only a limited range of answers can be
appropriately given to any particular question" (Ackrill 1963,
78-9), e.g., the question 'what is it' can only be
asked of a substance, and only answers describing substances are
appropriate. The question 'how much', by contrast,
requires a quantity for an answer, and so on.



But although on this interpretation Aristotle seems to have arrived at
his categories by considering different sorts of question and answer,
the categories he was offering were supposed to be categories of
entities, not of language; language was just a clue to truths about
*the world*. As J. L. Ackrill writes, Aristotle's
*Categories* "is not primarily or explicitly about names,
but about the things that names signify...Aristotle relies
greatly on linguistic facts and tests, but his aim is to discover
truths about non-linguistic items" (1963, 71).



Other interpretations have also been suggested about how
Aristotle's categories were derived. Some hold that
Aristotle's list was arrived at by reflecting on grammatical
categories, and assuming a parallelism between structures of language
and structures of the world (Baumer 1993). But others have developed
interpretations that do not consider Aristotle to have arrived at his
categories by considering linguistic matters such as grammatical
structure or the questions we may ask. Instead, they take them to
arise from more worldly considerations such as which types of entity
any sensible particular must be related to (Moravcsik 1967). For an
overview of the interpretive options, see Studtmann (2007).



In any case, regardless of how the categories were derived,
Aristotle's approach to categories is generally taken to be in
the spirit of what Brian Carr calls "categorial realism"
- an approach conceiving of a system of categories as a list of
the highest genera of beings (not merely of language or thought
- even if those may be used in deriving the metaphysical
categories). As Studtmann (2007) puts it, Aristotle " assumes
rather than defends a posture of realism with respect to the
metaphysical structures of the world". Given this approach, a
complete system of categories would offer a systematic inventory of
what there is, considered at the most abstract level (although it is
not clear whether Aristotle intended his categories to be exhaustive).
Thus on a categorial realist approach, providing a system of
categories can be seen as one, or even *the* central task of
metaphysics (cf. Grossman 1983, 3). Such a system of categories may
also play a central role in answering individual questions of nature,
providing the most general sort of answer to questions of the form
"What is this?", and providing the basis for definitions
of narrower sorts of things by specifying the most general category
(genus) under which things of this sort fall, and the differentia that
distinguishes them from other things of the same category. This has
endured as the paradigmatic approach to categories, and several recent
authors have offered new theories of categories in this spirit of
Aristotelian realism (see SS1.4 below).


### 1.2 Kantian Conceptualism



Others, however, have shied away from this robustly realist approach
to categories, generally on grounds of skepticism about our ability to
discern intrinsic divisions in 'reality itself', and have
instead treated the project of categories as a matter of laying out
the highest categories governing our conceptual scheme. This shift in
approach to what Carr (1987, 6) calls "categorial
conceptualism" was made famous by Immanuel Kant. While Kant
famously denied that we have access to intrinsic divisions (if any) of
the thing in itself that lies behind appearances or phenomena, he held
that we *can* discover the essential categories that govern
human understanding, which are the basis for any possible cognition of
phenomena. Thus, as H. J. Paton puts it, for Kant "We can have
*a priori* knowledge by means of the categories, only if the
categories are due to the nature of the mind and are imposed by the
mind on the objects which it knows" (1936, 258).



In his *Critique of Pure Reason*, Kant arrives at his list of
categories by first enumerating the forms of possible
*judgment* (A70/B95-A93/B109). On this view, objective
empirical judgments (i.e., empirical judgments which purport to refer
to objects rather than merely subjective seemings or connections of
sense impressions, and which purport to be universally valid for all
judging subjects) are endowed with their objectivity and generality in
virtue of the a priori concepts embodied in the relevant forms of
judgment. If we can identify all of the possible forms of objective
empirical judgment, we can then hope to use them as the basis to
discover all of the most general concepts or categories that are
employed in making such judgments, and thus that are employed in any
cognition of objects (Korner 1955, 48-49).



Thus in distinguishing his categories, Kant begins from Aristotelian
logic in outlining four respects in which one can classify any
judgment: according to its quantity, quality, relation, or modality.
In each of these respects or 'moments' of judgment, there
are three alternative classifications; thus, e.g., in respect of
quantity, a judgment may be universal, particular, or singular; in
respect of its relation, a judgment may be categorical, hypothetical,
or disjunctive, and so on. These Aristotelian ways of classifying
judgments are the clue to discerning the twelve correlated concepts of
the understanding. So, e.g., from noting that all judgments are either
universal (e.g., All swans are white), particular (e.g., Some swans
are white) or singular (e.g., Cygmund is white), we can arrive at the
three corresponding categories of quantity: unity, plurality, and
totality. Via this route, Kant ultimately distinguishes twelve pure
concepts of the understanding (A80/B106), divided into four classes of
three:


* Quantity
	+ Unity
	+ Plurality
	+ Totality
* Quality
	+ Reality
	+ Negation
	+ Limitation
* Relation
	+ Inherence and Subsistence (substance and accident)
	+ Causality and Dependence (cause and effect)
	+ Community (reciprocity)
* Modality
	+ Possibility
	+ Existence
	+ Necessity



The categories are presented as forming a single exhaustive list, with
the four classes of categories imposing four different forms of unity
on the object known (Paton 1936, 295-9). Thus, one may
separately inquire after an object's quantity, quality,
relation, and modality, receiving one of the three sub-answers in each
case on the way to a more complete characterization of the object.



Although these are categories of the understanding, they nonetheless
retain a certain sort of ontological import, as it is *a
priori* that they apply universally to all objects of possible
cognition (A79/B105). In this way, by delineating the concepts that
are *a priori* necessary for the *cognition* of objects,
we can acquire knowledge of categories governing any possible
*object* of cognition, and so acquire a sort of descriptive set
of ontological categories, though these must be understood explicitly
as categories *of objects of possible cognition*, not of the
thing in itself. Thus Kant was able to treat his system of concepts as
a system of categories in something like the Aristotelian sense,
"for our primary purpose is the same as his [Aristotle's],
although widely diverging from it in manner of execution"
(A80/B105). Nonetheless, it is clear that for Kant the categories find
their original source in principles of human understanding, not in
intrinsic divisions in mind-independent reality, and are discoverable
by paying attention to possible forms of human judgment, not by study
of the world itself, nor by study of our contingent manners of
speaking.

 An approach like Kant's has been defended more
recently by P. F. Strawson and others following him, who undertake the
project of "descriptive metaphysics", which is concerned
with describing "the most general features of our conceptual
structure" (1959 [1963], xiii), thus providing more general and
durable results than we might expect analyses of language to give us.

### 1.3 Husserlian Descriptivism



Edmund Husserl introduced two sorts of innovation to the study of
categories. First, while Aristotle used language as a clue to
ontological categories, and Kant treated concepts as the route to
categories of objects of possible cognition, Husserl explicitly
distinguished categories of *meanings* from categories of
*objects,* and attempted to draw out the law-like correlations
between categories of each sort (Smith 2007, 139ff.). Secondly,
whereas Aristotle and Kant each lay out a single system of categories,
Husserl distinguishes two ways of arriving at top-level ontological
classifications: by *formalization* and by
*generalization,* yielding two separate, orthogonal, systems of
categories, in two different dimensions (cf. Smith 2004, chapter
8).



Husserl is careful to distinguish categories of *meanings* (by
way of which we can think *about* the highest kinds or
'essences' of objects) from the categories *meant*
- the latter are the categories *of objects*, or
*ontological* categories, considered as the highest essences
that entities might have: "by 'categories' we can
understand, on the one hand, concepts in the sense of meanings, but on
the other also, and to better effect, the formal essences themselves
which find their expression in these meanings" (1913 [1962],
61-2). But although the two sorts of categories must be
distinguished, according to Husserl categories of the two sorts are
essentially correlated (see below), so we can learn about one by way
of the other.



Regardless of whether we are studying categories of meanings or of
objects, Husserl is quite clear that the study of categories, for him,
is an entirely *a priori* matter; the categories of meanings
and objects alike "arise ... solely in relation to our
varying thought-functions: their concrete basis is solely to be found
in possible acts of thought, as such, or in the correlates which can
be grasped in these" (1913 [2000], 237). As he puts it later, in
the *Ideas*, the study of categories is a study of essences,
based in *essential insights* about the types of meanings and
correlative types of things. Such studies of essence may be conducted
by way of imaginative variation of cases, independently of any matter
of fact, including whether or not there actually is anything
*of* the ontological kinds distinguished (1913 [1962], 51).
Thus Husserl's ontological categories, in this sense, are
*descriptive* categories of highest essences of possible things
(that might fall under those essences), and do not purport to provide
an inventory of what things *actually* exist (as a matter of
empirical fact).



Husserl provides an extensive discussion of categories of meaning in
the *Logical Investigations,* arguing that differences in
categories of meaning (which seem to be more like syntactic than
semantic categories) can be distinguished by noting where nonsense
results from substituting one term for another. E.g., in the sentence
"This tree is green" we may substitute "chair"
- but not "careless" - for "tree"
without turning sense into nonsense, marking the difference between
the meaning categories of *nominative material* and
*adjectival material* (1913 [2000], 511-512).
Husserl's understanding of 'nonsense' is rather
strict: he counts only those strings of words that are
*syntactically* incorrect (so that they form a mere 'heap
of words' and cannot be combined into any unified meaning
(Husserl 1913 [2000], 522)) as strictly nonsensical, and thus as signs
of differences in categories of meaning. (Husserl repeatedly
distinguishes the nonsense of verbal formations like "a round
or" (in which no unified meaning emerges) from cases of mere
*absurdity* such as "a round square", in which the
expression has a unified meaning, although it is *a priori*
that no object can correspond to the expression (1913 [2000],
516-17)).



Correlated with the categories of meanings are *ontological*
categories; e.g., object, state of affairs, unit, plurality, number,
and relation are (formal) categories that categorize objects, not
meanings (Husserl 1913 [2000], 237). Categories of the two sorts are,
according to Husserl, connected by 'ideal laws'. Thus, for
example, presumably objects are the ontological correlates of the
meaning category of nominative expressions, properties are the
ontological correlates of adjectival expressions, and states of
affairs are the ontological correlates of propositions. So while
Husserl does not (to my knowledge) explicitly lay out a method of
discerning *ontological* categories, it may be that we can
derive them by beginning from the above nonsense test for
distinguishing meaning categories, and then shifting attention to the
correlative ontological categories, since "pure truths
concerning meaning can be transformed into pure truths concerning the
object" (1913 [1962], 61).



As well as explicitly distinguishing categories of meanings from
categories of the correlated objects that could possibly be
'meant', Husserl introduced a second innovation to the
study of categories by distinguishing highest *formal* essences
(which Husserl calls 'categories') from highest
*material* essences (which he calls 'regions')
(1913 [1962], SS10; cf. Smith 1995, 329-330 and Smith 2007,
142-148). Thus far I have been describing the *formal*
ontological categories, the correlates of the different meaning
categories distinguishable by the nonsense test. In fact, Husserl
reserves the term 'category' for the highest formal
genera, which are distinguished by a process of *formalization*
- a removal of content. These 'categorial essences'
begin with 'object in general' at the top of the tree,
which is then divided at the next level into categories including (as
examples) object, state of affairs, property, relation, number, etc.
(compare lists 1913 [2000], 237 and 1913 [1962], 61). Much as
Aristotle distinguished (independent) primary substances from
(dependent) things of other kinds, within his formal categories
Husserl distinguishes the 'substrative' category of
individuals (or, more properly, the mere *this-there*) from the
dependent 'syntactic objectivities' - the correlates
of nominative terms that are derived from ways in which we speak about
the primary substances (1913 [1962], 62-3 and 67) (as, e.g., the
nominative term 'this relation of brightness' may be
derived from claims that 'A is brighter than B' (1913
[2000], 797-8)).



Husserl's material categories, by contrast, classify entities
according to their nature or essence, with the highest material genera
to be arrived at by a process of *generalization* to the most
general kind of content involved, rather than by the
*formalization* that involves an emptying of all content (1913
[1962], 65). The highest material categories are the three regions:
nature (including physical objects and events), culture (including
artifacts, social entities, and values), and consciousness (cf. Smith
2004). While formal and material category systems each form a
hierarchy (1913 [1962], 64), considered jointly their categories are
not mutually exclusive, since one and the same entity may be
categorized either in terms of its material nature or its form. For
further discussion of Husserl's categories, see Smith (2007,
135-161).



Husserl is nowhere explicit about the proper method for distinguishing
material ontological categories, but he does distinguish material
absurdity from formal absurdity, and from the formal nonsense that
marks the difference in meaning categories (1913 [2000], 523).
Expressions are *formally* absurd if it is *a priori*
that no object could correspond to them, based purely on formal,
logical laws, without regard to which particular material concepts are
employed, e.g., "a round not-round thing" is formally
absurd; its absurdity would remain regardless of which adjective we
substituted for 'round' or which noun for
'thing'. On the other hand, expressions are materially
absurd if the impossibility of there being any corresponding object is
based in the particular material concepts employed, e.g., 'a
round square' is a materially absurd expression based in the
particular meanings of 'round' and 'square'.
Thus presumably one could attempt to distinguish material ontological
categories by the material absurdity that results from substituting
expressions for objects of different *material* kinds; 'a
round table', for example, makes perfect sense, but if we
substitute for 'table' a term for a geometric figure such
as 'square' or for a day of the week such as
'Thursday', we get a materially absurd statement (to which
it is *a priori* that nothing corresponds). As we will see in
SS2.2 below, Gilbert Ryle developed Husserl's nonsense test
for category differences in something like this way.



Roman Ingarden (1960 [1964], 22ff) took Husserl's
multi-dimensional ontology one step further. Like Husserl, he
distinguished formal categories from material categories, but he also
distinguished categories in a third dimension: existential categories
(those describing an entity's mode of being). The highest
existential categories on Ingarden's list are the real
(spatio-temporal being), the ideal (abstract), the absolute
(completely independent, atemporal), and purely intentional
(consciousness-dependent). While any conceivable entity should be
uniquely locatable in a single category of each dimension, the three
sorts of ontology are mutually orthogonal, providing different most
abstract ways of considering the putative entity in question. Thus,
e.g., a sculpture might be categorized formally as an object,
materially as a work of art, and existentially as purely
intentional.


### 1.4 Recent Category Systems



In the twentieth century, systems of ontological categories fell
somewhat out of fashion (for reasons I will discuss in SS1.5
below), with most discussion of categories shifting to merely
articulating category *differences* rather than aiming to
outline a comprehensive system of categories.



One important exception to this comes in the work of Samuel Alexander,
who, in his 1920 work *Space, Time and Deity* develops a theory
of categories in the realist spirit. Alexander defends a monist
ontology in which he posits Space-Time as "the one monistic
entity that encompasses every entity and every feature in
reality" (Fisher 2015, 246). He sees the categories as grounded
on the intrinsic nature of Space-Time, and posits as categorial
features only those which are 'pervasive', that is,
instantiated by every entity. The categories he identifies come in
three 'grades' (or ranks of increasing complexity, in
which the latter grades presuppose the former), giving us the
following system:


* Grade 1
	+ Existence
	+ Universality
	+ Relation
	+ Order
* Grade 2
	+ Substance
	+ Causality
	+ Quantity
	+ Number
* Grade 3
	+ Motion



In recent years there have also been several notable attempts to offer
new systems of categories in either the realist or descriptivist
spirit, although little agreement exists about what the categories are
or how one could decide among competing systems.



Ingvar Johansson (1989) and Roderick Chisholm (1996) both take a
neo-Aristotelian realist approach to categories, attempting to lay out
a complete system of the categories, where this is understood as
providing a list of categories of real entities in the world. Ingvar
Johansson explicitly insists that his interest is in the world:
"This book is a book about the world. I am concerned with
ontology, not merely with language" (1989, 1), and attempts to
offer "a realist theory of categories regarded as real aspects
of being" (1989, 2). His list (1989, 20) includes nine main
categories (some of which subdivide further):


* Space-time
* State of affairs
* Quality
	+ Substance
	+ Property
* External Relation
* Grounded Relation
* Inertia
* Spontaneity
* Tendency
* Intentionality
	+ Real
		- Presentational
		- Representational
	+ Fictional



Unlike Aristotle, Johansson makes no explicit use of language in
discerning ontological categories, instead appealing to the method of
successive abstraction (Johansson 1989, 1-2). Thus, e.g., we
arrive at the category 'quality' by moving up in
abstraction from a particular shade of dark red, to red, color, and
finally quality. Similarly (to use an example of Sellars') one
might try to arrive at the category of 'substance' by
considering an individual entity, say, Fido, and moving by successive
abstraction from "Fido is a dachshund" through "Fido
is a dog" and "Fido is an animal", ultimately
reaching "Fido is a substance" (1970 [1974], 321). Like
Aristotle's categories, Johansson's categories top out
with a number of distinctions without a highest single category
subsuming them all.



Like Aristotle and Johansson, Roderick Chisholm presents his work on
categories as being "about the ultimate categories of
reality" (1996, 3). Unlike them, however, Chisholm (1996, 3)
lays out categories in the form of a Porphyrian tree starting from a
single most general category comprising everything, but divided into
successively narrower genera at lower levels of branching. (For an
interesting discussion of whether such general terms as
'entity' or 'thing' could be seen as naming a
highest category, see Thompson 1957, cf. SS2.3 below).
Chisholm's system of categories thus reads:


* Entia
	+ Contingent
		- States
			* Events
		- Individuals
			* Boundaries
			* Substances
	+ Necessary
		- States
		- Nonstates
			* Attributes
			* Substance



Other contemporary authors have approached the issue of categories in
a purely descriptive spirit. Reinhardt Grossmann, for example,
distinguishes eight highest categories (1983, xvi):


* Individuals
* Properties
* Relations
* Classes
* Structures
* Quantifiers
* Facts
* Negation



But although Grossmann characterizes his book as an attempt to
"bring Aristotle's *Categories* up-to-date"
(1983, xv), he is explicit in denying that he is making any claims
about whether or not there are things belonging to any of the eight
categories he distinguishes, taking this as beyond the scope of
ontology (1983, 10-12).



Joshua Hoffman and Gary Rosenkrantz (1994) lay out a tree-form system
of categories, with *entity* the *summum genus*,
subdivided into *abstract* and *concrete* (rather than
Chisholm's *contingent* and *necessary*), each of
which is further subdivided:


* Entity
	+ Abstract
		- Property
		- Relation
		- Proposition
	+ Concrete
		- Event
		- Time
		- Place
		- Substance
			* Material Object
			* Spirit
		- Limit
		- Collection
		- Privation
		- Trope



They, too, explicitly offer their system of categories in the spirit
of categorial description, as offering an analysis of the various
possible categories of being, rather than making any claims about
which of these categories is non-empty (1994, 7-8).



E. J. Lowe takes categories to be categories of "what kinds of
things can exist and coexist" (2006, 5). Such categories, he
argues, are to be individuated according to the existence and/or
identity conditions of their members; fundamental categories are those
regarding which the existence and identity conditions for their
members cannot be exhaustively specified in terms of ontological
dependence relations they bear to entities of other categories (2006,
8). Accordingly, he argues that there are four fundamental ontological
categories: objects (individual substances such as Fido), modes
(property or relation instances such as Fido's four-leggedness),
kinds (substantial universals such as the kind *dog*), and
attributes (property or relation universals, such as *being
four-legged*). But although he argues that there are exactly four
*fundamental* categories, Lowe nonetheless takes a hierarchical
approach to arranging categories. The four fundamental categories
appear at the third level of his hierarchical chart; the categories
that appear at the higher levels (particulars and universals at the
second level; entity at the top) are "mere abstractions and do
no serious ontological work on their own account" (2006, 39).
His fuller chart of categories appears as follows:


* Entities
	+ Particulars
		- Objects
			* Substances
			* Non-substances
		- Modes (monadic and relational)
	+ Universals
		- Kinds
		- Attributes (properties and relations)



Others, taking the project of developing categories in an explicitly
realist spirit and driven by the goal of offering a parsimonious
ontology, have aimed to offer a more minimal system of fundamental
ontological categories. For example, Laurie Paul (2016) has recently
defended a 'one-category' ontology that accepts (at the
fundamental level) only the existence of intrinsic characters or
qualities. Against those who accept more categories, she argues that
we have no need (for example) for a fundamental division between
individuals and their properties, and that a one-category ontology is
both more parsimonious and has a better claim to carve the world at
its 'ontological joints'.


### 1.5 Skepticism about Category Systems



Both realist and descriptivist category systems, at least as
traditionally presented, seem to presuppose that there is a unique
true answer to the question of what categories of entity there are
- indeed the discovery of this answer is the goal of most such
inquiries into ontological categories. Grossman, for example, argues
that a list of categories must be complete, contain everything, with
everything in its *proper* place (1983, 4). Johansson similarly
takes his project as to "develop a coherent system of
*all* the most abstract categories needed to give a true
description of the world" (1989, 1). Arguments about which of
the many systems of categories offered is correct likewise seem to
presuppose that there is a uniquely correct list of categories.



But actual category systems offered vary so much that even a short
survey of past category systems like that above can undermine the
belief that such a unique, true and complete system of categories may
be found. Given such a diversity of answers to the question of what
the ontological categories are, by what criteria could we possibly
choose among them to determine which is uniquely correct?



Some minimal standards of adequacy immediately suggest themselves
(Butchvarov 1995, 75). Whether one takes a realist or descriptivist
approach to providing a system of categories, if that system is
supposed to be comprehensive, it clearly must meet at least the
standard of being exhaustive - providing a category for
everything there is (on the realist approach) or might be (on the
descriptive approach). Nonetheless, one may, as Hoffman and
Rosenkrantz (1994, 140) do, present a system of *some*
fundamental categories without taking it to be exhaustive.



Another minimal criterion of adequacy is generally taken to be that
the highest categories (or, for tree systems, the categories at each
level of branching) be mutually exclusive, ensuring that whatever
there is (or might be) finds its place in exactly one highest
category, or one category at each level (Chisholm 1989, 162). (This
still allows for nested categories, so that something may belong to
both a more specific category like substance and a more general
category like individual.)



But these criteria are not enough to provide the needed reassurance.
First, we lack assurance that most proposed category systems meet even
these minimal conditions. As mentioned above, Aristotle drew out his
categories largely by considering the types of question that could be
asked (and the types of answer appropriate to them). It is difficult
to know, however, how one can be assured that all kinds of questions
have been surveyed, and so difficult to know that an exhaustive list
of categories has been offered - a point Aristotle does not
attempt to demonstrate (Ackrill 1963, 80-81). Indeed, the fact
that Aristotle provides different lists of categories in different
places suggests that he did not consider his list final and
exhaustive. Similarly, Kant's system of categories can be
thought to be exhaustive only as long as the list of forms of judgment
from which he derives them exhausts the possible forms of judgment
- but we have reason to think this is not so (Korner 1955,
50). Johansson, as we have seen, instead uses the method of successive
abstraction. But it is not clear how following such a method could
ensure either that the categories thereby distinguished are exhaustive
(how do we know we have considered something of each highest kind if
we do not yet know what the highest kinds are?) or even mutually
exclusive.



Secondly, even if we can verify that the standards of mutual
exclusivity and exhaustiveness are met, these conditions alone are far
too weak to uniquely pick out a system of categories. Provided one
accepts the law of the excluded middle, an endless supply of mutually
exclusive and exhaustive classifications can be generated at will: we
can divide things into the spatio-temporally located and the
not-spatio-temporally-located, the intentional and the
non-intentional, the extended and the non-extended, to name but a few
of the more relevant ways in which things could be divided. Indeed one
of the sources of puzzlement about categories comes from the fact that
philosophers have selected so many different sorts of divide as
*the* fundamental category difference - for Descartes,
the extended and the thinking (unextended), for Chisholm the
contingent and the necessary, for Hoffman and Rosenkrantz the concrete
and the abstract, and so on. Thus another reason for skepticism about
the existence of a *unique* set of categories comes from the
fact that categories are supposed to be the most abstract genera under
which things (may) fall. But from any given entity, abstraction may
apparently be done in a variety of ways - even if we are careful
to do so in ways that ensure mutual exclusivity and
exhaustiveness.



Doubts about possibilities for discovering the one true category
system have led many to eschew talk of category systems altogether,
and others to adopt some kind of relativism about category systems
that ceases to take systems of categories seriously as candidate lists
of *the* single set of highest genera under which anything
falls (or could fall). Jan Westerhoff (2005), for example, argues that
there is no unique, absolute set of ontological categories. On his
view categories in metaphysics turn out to be analogous to axioms in
mathematical theories; in each case, there may be more than one way to
systematize our knowledge from a relatively simple basis. The result
is a kind of relativity about systems of categories: "which set
of ontological categories we choose is primarily a matter of
convenience, in the same way as specific axiomatizations of
propositional logic or Newtonian mechanics are more convenient to use
than others" (2005, 218). As a result, Westerhoff argues, we
must reassess the importance of ontological categories in metaphysics
- these should not be considered "the most fundamental
parts of the world, but... the most fundamental parts of *our
systematization of the world*" (2005, 135).



Others have taken the variety of category systems explicitly offered
or presupposed by philosophers as mere evidence of the particular
presuppositions of their thought, or prejudices of their age -
not as evidence about anything to do with the world and its divisions.
Thus, e.g., Stephan Korner's discussion of categorial
frameworks is designed to make explicit how a thinker's
framework categorizes objects, making use of certain individuative
principles, and making clear his reasons for holding that framework
(1970, 10). R. G. Collingwood, in similar vein, treats the task of
metaphysics generally as merely uncovering the "presuppositions
underlying ordinary science" (1940 [1998]).



The specific worries about (1) guaranteeing the mutual exclusiveness
and joint exhaustiveness of the categories, and (2) whether or not any
single system of categories could purport to be uniquely correct, can,
however, be met by certain ways of formulating ontological categories.
The first sort of worry can be met by ensuring that categories (of the
same level) are defined in ways that guarantee mutual exclusiveness
and exhaustiveness. Thus, e.g., Thomasson (1999, chapter 8)
distinguishes categories in terms of what relations of dependence a
purported entity has *or lacks* on mental states (and a second
dimension is distinguished in terms of what relations of dependence a
purported entity has or lacks on spatio-temporally located objects),
so that the law of the excluded middle alone ensures mutual
exclusiveness and exhaustiveness of the categories distinguished.
(Dummett's method of distinguishing categories provides another
route for guaranteeing mutual exclusivity - see SS2.3
below).



Multi-dimensional systems (Husserl 1913 [1962], SS10; Ingarden
1960 [1964], Chapter 2; Thomasson 1999, Chapter 8; Smith 2007, Chapter
8) address the second worry to some extent by acknowledging that the
different dimensions of categorization are possible, and that no
one-dimensional list can purport to completeness. In principle,
multi-dimensionalists may even accept that there is no fixed number or
limit on how many one-dimensional lists of categories there may be,
though each such list may purport to provide a unique, correct,
exhaustive categorization of entities considered in the chosen
respect.



In any case, given the great potential uses of a system of categories
(many of which do not depend on claims that that category system is
uniquely 'correct'), we should not prematurely abandon
attempts at developing and evaluating systems of categorization. Even
if we do not think of a category system as providing a realist
inventory of all that exists or a description of the fundamental
intrinsic 'divisions of reality', a system of categories
laid out in the descriptivist spirit provides a framework within which
existence questions can be answered in a systematic and wholesale way,
by enumerating categories so that we can then undertake further
investigations into whether or not there really is anything of each
kind. Working from within a categorial framework can help ensure that
whatever ontology we provide is principled and unified, avoiding
*ad hoc* and piecemeal decisions. The descriptivist's
categories also provide a tool that may be used elsewhere in ontology,
e.g., in helping to ensure that comparisons of parsimony are
legitimately made (by examining which categories of entity are
accepted and which denied by various theories), and in checking that
potential solutions to metaphysical problems are not overlooked by
tacit use of a category system that is not exhaustive (Thomasson 1999,
Chapters 8 and 9). Another important use of systems of categories is
that, with a proposed set of categories laid out, we can, as Daniel
Nolan (2011) suggests, go on to investigate questions about the
relationships among entities that are placed in different categories:
for example, questions about whether events depend on or are grounded
in things, or (as Nolan suggests) whether things and events may
ultimately be identified as belonging to a single category.
Assumptions about categorizations play such a strong role in
philosophical discussions (e.g., discussions of the Cartesian theory
of mind, Platonist theories of mathematics, etc.), that doing the work
on categories necessary to make these categorial assumptions explicit
and open them for examination must remain a highly useful exercise
regardless of doubts about the prospects for discovering a uniquely
correct system of categories.


### 1.6 Categories in Other Disciplines



For those who approach categories in a descriptive spirit, as a matter
of determining the categories of our language or thought, it is
natural to turn to linguistics or cognitive science for
assistance.



A prominent approach to determining the ontological categories that
are implicit in the use of natural language is via Natural Language
Ontology, which provides one way of undertaking a descriptivist
approach to categories. As Friederike Moltmann (2017) makes clear,
however, the methodology for doing natural language ontology is
importantly different from attempts to determine a common sense
ontology by simply asking what ontological claims or categories people
explicitly accept or would accept on reflection. So, for example,
Natural Language Ontology doesn't determine the ontological
categories of a natural language by looking to explicit assertions
speakers make (or would assent to) about categories, such as
"objects are not events". Instead, natural language
category distinctions are revealed by uncovering the
*presuppositions* of sentences used by ordinary speakers. For
example, the fact that one can acceptably say "The building
existed last year" but not "The building took place last
year", and "John's arrival took place last
week" but not "John's arrival existed last
week", it has been argued, presupposes a difference of category
between material objects and events, since the conditions of
applicability for these predicates apparently presuppose that they
apply to entities of different categories (Moltmann 2017, Section
3.1). Since Natural Language Ontology "concerns itself with the
ontological categories and structures a speaker accepts when using a
language, not those a speaker accepts when engaging in some form of
philosophical reflection", its results may differ widely from
the ontological categories many philosophers would reflectively
accept, and even from those commonly attributed to natural language
(Moltmann 2017, Section 1). We have reason to engage in natural
language ontology, Moltmann argues, since it may give us "the
best indication of how we, implicitly, conceive of things"
(2017, Section 7). A question that remains is whether there will be a
uniform ontology found across all natural languages, perhaps one fixed
by our cognitive structure.



One might, of course, turn to cognitive science to attempt to address
the question of whether there is a fixed system of categories
determined by our cognitive structure. And indeed, discussions of
categories also play an important role in cognitive science, where the
goal is not to discover the fundamental categories of being, but
rather the means by which experiencers come to categorize their world.
Here, debates have centered on how humans in fact come to group things
into categories - whether this involves lists of definitional
(observable or hidden) features, resemblance to prototypes, prominent
features weighted probabilistically, etc. Debates also concern the
relation between conceptual and linguistic categories, which levels of
category are more basic, whether there is a most basic set of
categories, whether or to what extent categorizations are consistent
across cultural groups, and whether or not some fundamental categories
are innate. The psychologist Susan Carey (2011) has engaged in a
number of studies on infants and primates which, she argues, suggest
that there are a number of concepts of 'core cognition'
that are innate, designed to represent certain classes of entity in
the world, and that are shared across pre-linguistic human infants,
adults, and other primates. These include the concept of object (taken
as a sortal concept that makes use of boundedness and spatio-temporal
continuity in individuation), quantity, intentional agency, and
causation. For further discussion of the debates about categorization
in cognitive science see Lakoff (1987) and Rakison and Oakes
(2003).



Recently, work on ontological categories has attracted interest not
only among philosophers, but also in information science and the
biomedical sciences, where ontologies are used to organize the
knowledge represented in information systems (Smith 2003). In some
cases, the ontologies developed are domain-specific (e.g. specific to
medical information, geographic information, etc.), but there has also
been a great deal of interest in developing a 'top-level'
ontology of maximally general categories applicable to all specific
domains and enabling data sharing across systems. It is such top-level
ontologies that draw upon philosophical work on ontological categories
most directly, although categorial distinctions also play a crucial
role in domain-specific ontologies. Both sorts of philosophical work
on categorization promise to have a wide variety of practical
applications to information management that are just beginning to be
explored (see Sowa 1995, Munn & Smith (eds.) 2008)


## 2. Category Differences



Much recent work on categories has been influenced by skepticism about
the possibility of offering a system of ontological categories.
Difficulties like those mentioned above have undermined the idea that
a uniquely true and comprehensive system of ontological categories can
be found. The skepticism that comes from noting the proliferation of
category systems is compounded by general skepticism about
metaphysics. In some cases this has come from imputations of logical
positivists that all metaphysical talk is nonsense. More recently, the
skepticism has arisen from general doubts about the epistemology of
metaphysics (Bennett 2009, Kriegel 2013, Thomasson 2015), as well as
more specific doubts that we can make sense of the idea that the world
has a distinctly 'ontological structure', or that we could
discover what that structure is.



As a result, while categories have continued to play a central role in
analytic philosophy in the past century, and while some have continued
to pursue work on categories in the realist spirit, others have
shifted their focus to identifying *differences* in
*semantic* categories rather than drawing out *systems*
of *ontological* categories. Thus when Gilbert Ryle (1949, 1938
[1971]) talks of categories, he does not speak directly of categories
of *entities*, but rather of differing logical types of
concepts, where such type differences are detectable by the
absurdities that result from substituting in terms of one sort for
terms of another in sentences of certain kinds (see SS2.2 below).
Wilfrid Sellars, developing a strategy of Ockham's, argues
explicitly that we may construe category statements as disguised
metalinguistic statements about the conceptual role of certain
expressions (and their functional counterparts in other languages).
According to Sellars, "Socrates is a substance", for
example, has the sense of "The *Socrates* is a
basic mental singular term", and "Yellow is a
quality" has the sense of "The *yellow* is a
(one-place) predicate (in mentalese)" (1970 [1974], 328) (where
the "*\_\_\_*" notation has the function of
enabling us to speak about linguistic roles without being tied to a
particular natural language). As a result, we can replicate the work
done by traditional category distinctions between, e.g., substance and
quality, without committing ourselves ontologically to the existence
of qualities or other abstracta (1970 [1974], 329). On Sellars'
view, the categories are "metaconceptual, second-order
functional classifications of the most fundamental types of
first-order conceptual roles within a norm-governed linguistic
practice or conceptual framework" (O'Shea forthcoming,
section 1). An interesting upshot of this Sellarsian way of looking at
the categories is that are not fixed, but may change over time as the
conceptual roles in our norm-governed practices change (O'Shea
forthcoming, section 2).


### 2.1 The Uses of Category Distinctions



Those who focus on articulating category distinctions rather than on
laying out complete systems of categories generally invoke categories
not in hopes of providing answers to such basic metaphysical questions
as 'what exists', but rather as a way of exposing,
avoiding, or dissolving various presumed philosophical mistakes,
confusions, and paradoxes.



Thus, e.g., Russell and Whitehead introduced type theory (which might
in some sense be considered a theory of categories) to avoid a certain
form of paradox found in Fregean set theory (where we must consider
the putative set of all non-self membered sets, which is a member of
itself if and only if it is not a member of itself), liar's
paradoxes ("This sentence is false", which is true if and
only if it is false), etc. On their analysis, paradoxes like these
arise from the attempt to form an illegitimate totality by trying to
collect into a single totality a collection that has members that
presuppose the existence of the totality. To avoid such paradoxes, we
must accept that "Whatever involves *all* of a collection
must not be one of the collection" (1913 [1962], 37) and thus
that such totalities (involving all of a collection) must be of a
higher type, making, e.g., classes of sets of a higher type than are
sets of individuals, and so on, leading to an infinite hierarchy of
types. The type-mixing paradox-generating claims are rejected as
ill-formed and meaningless (1913 [1962]).



Most famously, Ryle (1949) introduced the idea of the category mistake
as a way of dispelling the confusions he thought to be rampant in the
Cartesian theory of the mind, and thus of dissolving many apparent
problems in philosophy of mind. According to Ryle, one makes a
category mistake when one mistakes the logical type or category of a
certain expression (1949, 16-17). Thus, e.g., a foreigner would
make a category mistake if he observed the various colleges,
libraries, and administrative offices of Oxford, and then asked to be
shown the university. The foreigner mistakes the university for
another institution like those he has seen, when in fact it is
something of another category altogether: "the way in which all
that he has already seen is organized" (1949, 16). The category
mistake behind the Cartesian theory of mind, on Ryle's view, is
based in representing mental concepts such as believing, knowing,
aspiring, or detesting as acts or processes (and concluding they must
be covert, unobservable acts or processes), when the concepts of
believing, knowing, and the like are actually dispositional (1949,
33). Properly noting category distinctions may help alleviate a
variety of philosophical problems and perplexities, and the idea of
the category mistake was widely wielded (by Ryle and others) with this
aim. Ofra Magidor suggests that it is "far from clear what Ryle
took the central mistake in the dualistic position to be" (2013,
10). Jonah Goldwater (forthcoming), however, argues that, in *The
Concept of Mind*, the category mistakes Ryle identifies all have
the form of mistakenly conjoining entities that belong in two
different categories - implicitly assigning their conjuncts to a
shared category. But on Ryle's view (Goldwater argues) there is
often no single highest category ('existent') under which
we can subsume the conjoined entities, and so we cannot sensibly
conjoin, count, or quantify over them together. This, Goldwater
argues, not only clarifies the basis for Ryle's critique of both
Cartesian and physicalist theories of mind, but also has the potential
to dissolve various current debates in metaphysics, such as arguments
against co-location that are based on denying (for example) that there
is a statue *and* a lump *both* on the pedestal. Huw
Price (2009, 330-335) argues that the category differences Ryle
identifies can be seen as underlain by *functional* differences
in the sorts of language used. The thought that category mistakes are
symptoms of underlying functional differences, as Price puts it,
suggests that to evaluate claims of category mistakes, and their
relevance to traditional 'metaphysical' problems, we need
"first-order scientific inquiries into the underlying functions
of language in human life" (Price 2009, 335). Thomasson (2022,
23-29) picks up this idea, and begins to suggest how work in
systemic functional linguistics can provide a way of understanding
differences in linguistic function that may underlie a range of
philosophical problems.



Work on category distinctions also has other applications in assessing
traditional debates in metaphysics. Thomasson (2007) argues that
various mistakes and puzzlements in ontology can be traced to the
mistaken use of category-neutral existential and quantificational
claims. A great many arguments in ontology rely on claims about
whether, in various situations, there is some *object* present
(or how many objects there are), where the term 'object'
must be used in a category-neutral way for the argument to go through
(Thomasson 2007, 112-118). But if existential and
quantificational claims must tacitly presuppose some category or
categories of entity over which we are quantifying, then such
arguments go astray. Thomasson (2007) gives independent grounds for
thinking that all quantification must at least tacitly presuppose a
category or categories of entity over which we are quantifying, and
argues that adopting that view provides the uniform basis for
dissolving a number of problems supposed to arise with accepting an
ontology of ordinary objects. Jonah Goldwater (2021) argues that the
arguments standardly given for being eliminativist about a certain
kind of entity all rely on mistaken principles or judgments about
ontological categories, to which the right response is typically to
rectify these mistakes about categories, not to eliminate the
entities. This analysis, Goldwater argues, provides a kind of indirect
support for 'permissivist' ontologies, that would accept
the existence of numbers, properties, holes, or other entities to
which metaphysicians have often sought to avoid commitment.



Work on category distinctions is also relevant to debates in
linguistics and philosophy of language about what, exactly, a category
mistake is, and why category mistakes are infelicitous. Analyzing
category mistakes and why they are infelicitous (as Magidor 2019 makes
clear) has further relevance for linguistic theories of syntax,
semantics, pragmatics, and of metaphorical and fictional discourse.
Magidor (2013, 2019) surveys past answers to the question of what
makes a category mistake infelicitous, including: that they are
syntactically ill-formed, that they are meaningless, that they are
meaningful but lacking in truth-value, and that they are (despite
being well formed, meaningful and having truth-value) pragmatically
inappropriate. Magidor argues against the first three options, and
defends instead a presuppositional account of why sentences that seem
to contain category mistakes are infelicitous. Roughly, on her view, a
sentence like 'Two is green' triggers the presupposition
that two is colored - a presupposition that is difficult to
accommodate (2013, 132). Thus, on her analysis, the sentence is
infelicitous, but still has a truth-value (it is false).


### 2.2 The Ryle/Husserl Method of Distinguishing Categories



While those who only make use of the idea of category differences
(rather than purporting to offer a category system) have no need to
worry about how to provide an exhaustive list of categories, they
nonetheless owe an account of the conditions under which we can
legitimately claim that two entities, concepts, or terms are of
different categories, so that we know when a category mistake is (and
is not) being made. Otherwise, they would face the charge of
arbitrariness or *ad hocery* in views about which categories
there are or where category differences lie. Yet there is little more
agreement about the proper criteria for distinguishing categories than
there is about what categories there are.



Ryle famously considered absurdities to be the key to detecting
category differences. But although Ryle made the method famous, he
apparently derived the idea from Husserl's method of
distinguishing categories of meaning (cf. Ryle 1970, 8; Simons 1995,
120; Thomasson 2002, 124-8, and SS1.3 above). But while
Husserl used *syntactic* nonsense as a way of detecting
differences in categories of *meaning* (yielding different
grammatical categories), Ryle broadened the idea, taking
*absurdities* more widely conceived to be symptoms of
differences in *logical* or *conceptual* categories
(1938 [1971], 180). Thus, e.g., the statement "She came home in
a flood of tears and a sedan-chair" (Ryle 1949, 22) is perfectly
well-formed syntactically, but nonetheless Ryle classifies it as a
sentence that is absurd, where the absurdity is a symptom of the fact
that the sentence conjoins terms of different categories.



In his earlier paper "Categories", Ryle describes the test
for category differences as follows: "Two proposition-factors
are of different categories or types, if there are sentence-frames
such that when the expressions for those factors are imported as
alternative complements to the same gap-signs, the resultant sentences
are significant in the one case and absurd in the other" (1938
[1971], 181) - in other words, two expressions (or rather: what
they signify) differ in category if there are contexts in which
substituting one expression for the other results in absurdity. This
test, of course, provides no way of establishing that two expressions
*are* of the same category (but only that they are not), since
there is an infinite number of sentence-frames, and one may always yet
be found that does not permit the substitution to be made without
absurdity. It also leaves open and merely intuitive the notion of
'absurdity' itself; in fact, Ryle concludes his paper
"Categories" with the question "But what are the
tests of absurdity?" (1938 [1971], 184). Ryle's approach
was further developed, in a more formal fashion, by Fred Sommers
(1959, 1971).



J. J. C. Smart (1953) criticized Ryle's criterion for drawing
category distinctions on grounds that it could apparently be used to
establish a category difference between any two expressions
whatsoever. "Thus 'the seat of the - is hard'
works if 'chair' or 'bench' is put into the
blank, but not if 'table' or 'bed' is. And if
furniture words do not form a category, we may well ask what do"
(1953, 227). Without a test for absurdity apart from a certain kind of
intuitive unacceptability to native speakers, we seem to be left
without a means of declaring 'Saturday is in bed' to be a
category violation but 'The seat of the bed is hard' not
to be. Bernard Harrison attempts to meet this challenge by
distinguishing the sorts of inappropriateness that result from
violations of category facts (such as the former) from those that
result from mere violations of facts of usage (the latter) (1965,
315-16). The use of the term 'bed' could conceivably
be extended in ways that would make 'The seat of the bed is
hard' acceptable (e.g., if beds came to be made with seats),
whereas 'Saturday' could not conceivably be extended in a
way that would make 'Saturday is in bed' acceptable
- any such attempted 'extension' would just involve
using 'Saturday' homonymously (e.g., as the name for a day
of the week and for a person) (1965, 316-18). For further
discussion of intersubstitutability approaches to drawing category
distinctions, see Westerhoff (2005, 40-59 and 2002,
338-339). Westerhoff (2004) develops a method of distinguishing
categories based on substitutability in worldly states of affairs
rather than language.


### 2.3 Fregean Approaches to Distinguishing Categories



Frege treats distinctions in categories as correlates of distinctions
in types of linguistic expression. The category of *object*,
for example, is distinguished by reference to the linguistic category
of *proper name* (Dummett 1973 [1981], 55-56; cf. Wright
1983, 13 and Hale 1987, 3-4) - i.e., an object just is the
correlate of a proper name, where proper names are held to include all
singular terms (including singular substantival phrases preceded by
the definite article). Broadly Fregean approaches have been more
recently developed and defended by Michael Dummett (1973 [1981]) and
Bob Hale (2010).



Hale develops and defends the Fregean idea that "the division of
non-linguistic entities into different types or categories [is]
dependent upon a prior categorization of the types of expressions by
means of which we refer to them" (2010, 403). As he develops the
idea, to be an object is "to be the referent of a
*possible* singular term, to be a property is to be the
referent of a *possible* (first-level) predicate, and so on for
other cases" (2010, 411). He also argues that this encourages a
deflationary approach to existence questions according to which we may
argue for the existence of entities of a certain kind by simply
arguing "that there are true statements involving expressions of
the relevant kind" (2010, 406).



Dummett (1973 [1981]) also aims to develop and precisify a broadly
Fregean approach to category distinctions. Frege leaves the
distinction between so-called 'proper names' and other
parts of speech merely intuitively understood, but Dummett argues
that, e.g., one could make a start at criteria for distinguishing
proper names by requiring substitutability of terms while preserving
the well-formedness of a sentence (which, as we have seen in
SS1.3, also plays a role in Husserl's distinction of meaning
categories), and while preserving the validity of various patterns of
inference (where the latter requirement is needed to distinguish
proper names from other substantival terms such as
'someone' and 'nobody') (1973 [1981], 58 ff.).
(For further refinements of these criteria, see Dummett (1973 [1981],
61-73) and Hale (1987, Chapter 2).)



In line with Frege's requirement (1884 [1968], SS62) that
names must be associated with a criterion of identity, Dummett argues
that an additional test (beyond these formal tests) is needed to
distinguish genuine proper names (to which objects correspond) from
other sorts of expression: "Even though an expression passes the
more formal tests we devised, it is not to be classified as a proper
name, or thought of as standing for an object, unless we can speak of
a criterion of identity, determined by the sense of the expression,
which applies to the object for which it stands" (1973 [1981],
79).



Thus once grammatical categories are distinguished, enabling us to
thereby distinguish the logical category *object* by reference
to the linguistic category of *proper name*, we can go on to
draw out category distinctions among objects. To avoid confusion,
Dummett calls the first range of distinctions (among logical
categories of objects, properties, relations, etc.) distinctions among
'types' and the second range of distinctions (within the
type *object*) distinctions among 'categories'
(1973 [1981], 76).



Since, as Dummett argues (in a point further developed in Lowe 1989
and Wiggins 2001), proper names and sortal terms must be associated
with a criterion of identity that determines the conditions under
which the term may be correctly applied again to one and the same
thing (1973 [1981], 73-75), we may use the associated criteria
of identity in order to distinguish categories of objects referred to.
All of those names and general sortal terms (usable in forming complex
names) that share a criterion of identity are said to be terms of the
same *category*, even if the criteria of application for the
associated sortals vary (1973 [1981], 546). Thus, e.g., the sortal
terms 'horse' and 'cow' (similarly, names of
horses and cows) are terms of the same category, since they share the
identity criteria suitable for animals.



As Lowe (1989, 108-118) notes, this approach to categories
blocks certain reductivist moves in metaphysics. For, e.g., if sortal
terms such as 'person' and 'organism' are
associated with different identity conditions, then those who seek to
reductively identify persons with biological organisms are involved in
a category mistake.



The idea that category distinctions among objects may be drawn out in
terms of the identity and/or existence conditions associated with
terms of each category has recently gained popularity. Though they
differ in details, versions of the approach have been utilized not
only by Frege, Dummett and Hale but also by Lowe (2006, 6) and
Thomasson (2007).



This approach to drawing category distinctions among objects can avoid
various potential problems and sources of skepticism. It is not
subject to problems like those Smart raised for Ryle's
criterion, for days of the week clearly have different identity
conditions than do persons, whereas beds and chairs seem to share
identity conditions (those suitable for artifacts). Such a method of
drawing out categories also is not subject to the sorts of skepticism
raised above for category systems. Here there is no claim to provide
an exhaustive list of categories, and for a principled reason:
different categories may come into discussion as long as nominative
terms or concepts associated with distinct identity conditions may be
invented.



Following this method also guarantees that the categories
distinguished are mutually exclusive, for it is a corollary of this
position that entities may be identified only if they are governed by
the same identity conditions (and meet those), so that it is ruled out
*a priori* that one and the same entity could belong to two or
more distinct categories, in violation of the mutual exclusivity
requirement.



This method of distinguishing categories also provides a principled
way of answering some of the central questions for theories of
categories, including whether or not there is a single *summum
genus*, and what the relationship is between linguistic/conceptual
and ontological categories. Such completely general terms as
"thing" "entity" or "object", on
Dummett's view, are not genuine sortal terms, since they fail to
provide any criteria of identity. Thus clearly on this view (as on
Aristotle's) there is no *summum genus* under which
categories such as *artifact*, *animal*, etc. could be
arranged as species, since (lacking criteria of identity) such
candidate catch-all terms as 'object',
'being', 'entity' and the like are not even
sortal terms and so cannot be categorial terms.



Views that, like the Rylean and Fregean approaches, distinguish
categories by way of language, are sometimes criticized as capable
only of noting differences in category of certain linguistic
expressions. For why, it might be asked, should that have anything to
tell us about differences in the categories of *real
things*?



Hale argues that there is no serious alternative to using types of
expression that aim at referring to entities of different types if we
hope to characterize such basic logical categories (or types) as
object and property (2010, 408). For what it is to be an object or
property evidently cannot be conveyed merely by ostension, nor by more
substantive criteria, without being restrictive in ways that beg the
question against various views of what objects or properties there
are. Moreover, he argues that we can avoid making our (logical)
categories unduly dependent on what language we actually have by
treating objects and properties as correlates of *possible*,
not merely actual, expressions of the relevant sorts (2010, 411).



Dummett's way of understanding categories of objects also opens
the way for a reply to this objection. For Dummett argues that,
without some associated categorial concept, we cannot single out
objects (even using names or demonstratives) (1973 [1981], 571).
Categorial concepts are necessary for us to single out
'things' at all, and cannot be derived from considering
'things' preidentified without regard to categories. (It
would thus follow from this that Johansson's idea that we could
arrive at categories by abstraction from considering individual
*things* would be wrong-headed.) On this view, then, categories
not only *may* but *must* be distinguished primarily by
way of distinguishing the identity conditions criterially associated
with the proper use of different sortal terms and names. If we cannot
refer to, discover, or single out objects at all except by way of a
certain categorial conception (providing application and identity
conditions), then the categorial differences in our sortal terms or
names (marked by their differences in identity conditions) are
*ipso facto*, and automatically, category differences in the
things singled out by these terms - the possibility of a
'mistake' here just does not arise, and the connection
between the category of an expression used to refer to a given entity
and the category of the entity referred to is ensured.