## 1. Essences of Substances: Real and Nominal



The canonical definition of the real essences is given in III.iii.15,
where Locke contrasts his 'real essences' with what he
calls 'nominal essences':



> 
> 
> *First, Essence* may be taken for the very being of any thing,
> whereby it is, what it is. And thus the real internal, but generally
> in Substances, unknown Constitution of Things, whereon their
> discoverable Qualities depend, may be called their
> *Essence*...*Secondly*,...it being evident,
> that Things are ranked under Names into sorts or *Species*,
> only as they agree to certain abstract *Ideas*, to which we
> have annexed those Names, the *Essence* of each *Genus*,
> or Sort, comes to be nothing but that abstract *Idea*, which
> the General, or *Sortal* (if I may have leave so to call it
> from *Sort*, as I do *General* from *Genus*,)
> Name stands for. And this we shall find to be that, which the Word
> *Essence* imports, in its most familiar use. These two sorts of
> *Essences*, I suppose, may not unfitly be termed, the one the
> *Real*, the other the *Nominal Essence*.
> 
> 
> 



Real essences are the unknown constitutions which cause the
observable qualities of substances, and the nominal essences are the
abstract ideas that constitute the definitions of species or genera.
To flesh this out a little more clearly we must define a few more
terms.



Locke defines the 'quality' of any substance as "the
Power to produce any *Idea* in our mind" (II.viii.8).
There are three categories of qualities according to the mechanical
natural philosophy. *Primary qualities* are the inseparable
features of a body, e.g., its solidity, extension,
figure, motion or rest, number, situation, etc. (II.viii.9 &
23). *Secondary qualities* are the colors, sounds, smells,
tastes, etc., of any object. These are *secondary* because
these features of objects do not exist in the objects themselves, but
rather are nothing but the power of the primary qualities of the
object to produce an idea in us of a certain kind (II.viii.10). So,
for example, the color of the table is not in the table, rather, it is
part of how the matter and texture of the table (i.e., its primary
qualities), as it reflects, absorbs and refracts light, has the power
to produce ideas in us of that color. Finally, the *tertiary
qualities* of a body are those powers in it that, by virtue of its
primary qualities, give it the power to produce observable changes in
the primary qualities of other bodies, e.g., the power of the sun to
melt wax is a tertiary quality of the sun (II.viii.23).



When one creates a nominal essence, one is cobbling together a
collection of particular qualities--from among these three kinds
of qualities--that one observes together in a substance and
making that collection definitive of a species or genus. Having said
that, I should point out that when Locke thinks of a *species*
or *genus*, he is not thinking of these terms as applying
exclusively in the biological realm, but rather as applying generally
to any possible classification scheme we create when we organize our
world through naming. So, for example, *gold* is as much a
species of the genus *metal* as *human* is a species of
the genus *animal*.



The term 'nominal' refers to names or naming, and is used
by Locke to indicate that classifying substances based on their
similar qualities is an exercise in naming. For example, the nominal
essence of gold is the abstract idea that constitutes our definition
of 'gold', i.e., yellowish, heavy, malleable metal that
can be dissolved in *Aqua Regia*, but not in sulfuric acid,
etc. According to Locke, the nominal essence contains the properties
that give meaning to the name 'gold' and allow us to know
what gold is wherever we find it. What makes the nominal essence an
*essence* is that it contains both the necessary and sufficient
conditions for something to belong to its species or genus. Because of
how gold is defined, every yellowish, heavy, malleable metal that
dissolves in water and hydrochloric acid and not in sulfuric acid, is
gold.



In this way, nominal essences are created by human choice: we decide
what we want to include in our definitions of species or genera. To
borrow from Locke, species are "the workmanship of the
understanding", not of nature.



> 
> 
> I would not here be thought to forget, much less to deny, that Nature
> in the Production of Things, makes several of them alike: there is
> nothing more obvious, especially in the Races of Animals, and all
> Things propagated by Seed. But yet, I think, we may say, the
> *sorting* of them under Names, *is the Workmanship of the
> Understanding, taking occasion from the similitude* it observes
> amongst them, to make abstract general *Ideas*, and set them up
> in the mind, with Names annexed to them, as Patterns, or Forms, (for
> in that sense the word Form has a very proper signification,) to
> which, as particular Things existing are found to agree, so they come
> to be of that Species, have that Denomination, or are put into that
> *Classis*. (III.iii.13)
> 
> 
> 



Nature provides us with similarities and we create definitions of
species and genera. However, no matter how dependent general names are
on our definitions, we are answerable to nature and must keep our
definitions as close as we can to what nature provides. It is in this
sense that species and genera are the workmanship of the human
understanding; we take what nature gives us and we create our own
definitions and taxonomical categories.



On the other hand, a *real essence* is an *essence*
because it makes the object to be what it is. It is *real* in
that it does not include human choice for it to be what it is, rather,
it is precisely as nature made it. In the case of a piece of gold, the
real essence for the corpuscularian is that collection of
particles that make up that particular piece of gold and give it its
qualities of color, weight, electrical conductivity, malleability,
etc.


## 2. Polemics: Real and Nominal Essences vs. Scholastic Substantial Forms



Natural philosophy (as contrasted with metaphysics and mathematics) in
the Aristotelian tradition is the science of physical substances in
the sublunary world. The aim of natural philosophy is knowledge of
causes, and for the Aristotelian tradition, this includes knowledge
of essences. Locke's above definition of real and
nominal essences is given as a critical response to the Aristotelian
influenced Scholasticism of the late medieval and early renaissance
natural philosophy. Locke's distinction between real and nominal
essences appears to be in reference to the Aristotelian distinction
between real and nominal definitions. A real definition, according to
the Aristotelian tradition, is one that accords with the natural
hierarchy and identifies the essence of the species or genus being
studied, e.g., 'man is the rational animal'
(*reason* being the essence of the species, *animal*
being the genus). A nominal definition, by contrast, is a
pseudo-definition that identifies the members of the species or genus,
but fails to capture the essence, e.g., 'man is the featherless
biped'. A science of nature, for Aristotle and the Scholastics,
consists of real definitions and not mere nominal ones. While Locke
agrees that a science requires knowledge of essences, and thus of
species, he differs from the Aristotelians by arguing that there are
no real definitions of substances, but there are of moral and
mathematical concepts (see Ayers 1996, *Locke*, vol. II, pp.
18-30). This account of real essences, then, is crafted to
be capacious enough to accommodate both the Aristotelian and
corpuscularian versions on offer. 



According to the Scholastic way of understanding the term
'essence', it refers to what makes a thing what it is:



> 
> 
> Concerning the real Essences of corporeal Substances, (to mention
> those only,) there are, if I mistake not, two Opinions. The one is of
> those, who using the Word *Essence*, for they know not what,
> suppose a certain number of those Essences, according to which, all
> natural things are made, and wherein they do exactly every one of them
> partake, and so become of this or that *Species*.
> (III.iii.17)
> 
> 
> 



The Scholastic concept of essence, or what is more properly called a
'substantial form', is of an imperceptible, immaterial
essence that imposes order onto the matter that it is joined with and
makes it a member of a natural species or genus, imbuing it with all
of its relevant characteristics, i.e., its essential properties.
Essential properties are those features that tell us to what species
or genus an object belongs, e.g., being warm blooded is an essential
property of mammals, being rational is an essential property of
humans, being yellowish and heavy are essential properties of gold.
Each of these essential properties is caused by the substantial form
that is conjoined with the matter of the substance to make it what it
is.



In this way, then, the substantial form of any object in the
Scholastic theory plays two roles: it (a) tells us to what species the
substance belongs; and (b) it causes the observable qualities and
properties of the substance. For example, according to the Scholastic
theory, a lump of gold is an instance of *gold* because its
matter possesses the substantial form of gold which causes all of the
properties of gold, e.g., malleability, yellowish color, heaviness,
etc..



By contrast, however, Locke does not think of the essence of a
physical object as an immaterial substantial form, nor does he think
that a single entity can play both roles of determining the species or
genus membership of a substance *and* be the unobservable
underlying cause of the qualities of the substance. Instead, Locke
divides up the concept of essence into two different concepts playing
two distinct roles. First, he defines the real essence of physical
objects as the internal constitution of the substance that is the
cause of the observable qualities of the object. He then argues that
this internal constitution is not what answers the 'what is
it?' question because the species membership of physical beings
is only determined by their outward appearance, i.e., by their nominal
essences. Second, he defines the nominal essence as that collection of
observable qualities that are included in the definition of the
species or genus. This process has three stages; we create an abstract
idea based on similarities among objects, we then give a name to that
abstract idea, and finally we classify objects that fit our abstract
idea into the same species or genus. So, for Locke, the real essence
plays role (b), that is, the causal and explanatory role of being the
physical cause of the properties and qualities of each substance; but
only the nominal essence plays role (a), that is, the nominal essence
alone determines its species or genus membership. The chart below
illustrates this division of labor:




|  |  |  |
| --- | --- | --- |
|  | (a) *Determines Species/Genus

Membership* | (b) *Causes/Explains Observable

Qualities* |
| *Scholastics* | Substantial Form | Substantial Form |
| *Locke* | Nominal Essence | Real Essence |



One of Locke's main purposes in making this distinction is to
deploy it in his criticisms of the Scholastic account of (a), the
species of natural substances. However, when criticizing the
Scholastic account of (b), the explanation of the qualities of bodies,
Locke typically refrains from invoking the concept of 'real
essence' and instead talks about the primary, secondary and
tertiary qualities of bodies (see, for example II.viii.10-26).
So, at a minimum, it seems that Locke invoked this distinction between
real and nominal essences in order to prove a negative point: real
essences are not relevant to role (a).



As noted above, according to the Scholastics, species and genera are
created by nature; every natural substance is a combination of matter
and substantial form. The substantial form of any substance is its
species essence, which is an immaterial entity that makes the object
what it is, makes it a member of a particular species and genus and
causes its characteristic properties. So, for the Scholastics, all
natural substances belong to a determinate species by nature,
independently of any human naming activities. On this view then, if we
know what qualities a substance possesses, and we know that all
qualities are produced by a substantial form, then it is a simple
matter to determine its species simply by noting the qualities the
substance exhibits. For example, if what makes a particular object a
piece of gold is the possession of the substantial form of gold, then
whatever exhibits the properties of gold has the substantial form of
gold, and so we know the species to which it belongs is *gold*.
Creating a taxonomy for the Scholastics is attempting to discover how
nature has already classified substances into species and genera by
looking for similarities among their persistent qualities (qualities
that tend to remain even when the circumstances or conditions of the
object change) that reveal the deeper similarity of sharing
substantial forms.



By distinguishing between real and nominal essences, Locke can deploy
these new concepts in a sustained critique of the Scholastic theory of
species. Locke offers three main arguments against the Scholastic
view.



First, he deploys the corpuscularian hypothesis against the
Scholastics to argue that 'borderline cases' create a
serious challenge to their doctrine of substantial forms, but which
are allowed for, and even explained by, corpuscularian principles. By
his lights, if we assume that nature produces substances by imposing a
substantial form (what Locke mockingly calls 'moulds and
forms'), or species essence onto them, then the existence of
entities that either fit into more than one species, or do not fit
into any single species, shows us at least that not all beings are
produced according to natural moulds and forms:



> 
> 
> [The opinion] which supposes these *Essences*, as a certain
> number of Forms or Molds, wherein all natural Things, that exist, are
> cast, and do equally partake, has, I imagine, very much perplexed the
> Knowledge of natural Things. The frequent Productions of Monsters, in
> all the Species of Animals, and of Changelings, and other strange
> Issues of humane Birth, carry with them difficulties, not possible to
> consist with this *Hypothesis*: Since it is as impossible, that
> two Things, partaking exactly of the same real *Essence*,
> should have different Properties, as that two Figures partaking in the
> same real *Essence* of a Circle, should have different
> Properties. (III.iii.17)
> 
> 
> 



If nature made species or genera by giving all members of the same
kind the same form or mould (or, he seems to suggest above, real
essence), then there would be no way to explain generative variation
in nature (he reiterates this kind of argument in III.vi.15-17)
or the differences between samples of the same species of chemicals or
minerals. On the other hand, if nature makes real essences, which are
material structures that cause the observable qualities of each
substance, then these kinds of variations can be explained by
variations among the physical structures of the bodies (see Jolley
1999, pp. 144-50).



Secondly, as we saw above, Locke adduces the semantic point that our
general terms (and their corresponding ideas), being human constructs
*via* the process of abstraction, constitute the (nominal)
essences of things, not some immaterial substantial form:
"Whereby it is evident, that the *Essences of* the
*sorts, or* (if the Latin word pleases better) *Species*
of Things, are nothing else but these abstract *Ideas*"
(III.iii.12). Abstract ideas are formed when we take a set of objects
and mentally strip away enough particular ideas to reveal what
properties they all have in common. The resulting ideas are purely
mental constructions; the putative substantial forms of the objects
are irrelevant to this process. Therefore, essences and species are
human constructs, not the products of nature, i.e., they are purely
*nominal*.



Locke's third argument rests on the Scholastic's
assumption that nature produces many things alike. The natural
production of physically similar entities, however, is a double-edged
sword. Nature not only produces similarities among objects, but it
produces far too many similarities. That is, among the vast array of
similar qualities that objects share we must select those few which we
take to be important enough to constitute the meaning of the general
term and thereby constitute the species or genus. This selection
process is not something that nature can do, only we can decide which
qualities are important enough to count as part of the definition of a
species or genus.



Locke has a further argument up his sleeve: generality does not exist
in nature. For the Aristotelian tradition, perception occurs when the
mind receives the form (i.e., sensible species) without the
matter of the substance. So, the human mind has access
to forms (which are general) through
perception. However, in the lead up to III.iii.15, Locke
stresses that generality occurs only in words and ideas, not in nature
(III.iii.1, 10, 13), and that we do not perceive substantial
forms (II.xxiii.3; II.xxxi.6). Nominal essences provide the only
universality we possess in natural philosophy (III.iii.9, 10, 11).



By introducing the distinction between real and nominal essences and
thereby dividing their roles, Locke has provided himself with a number
of resources to criticize the Scholastic theory of species. There are,
however, reasons to worry about the viability of these criticisms.
After all, these arguments seem to beg the question against the
Aristotelian tradition. The existence of substantial forms need not
entail the existence of hard-edged natural kinds, and so Locke's
'moulds and forms' metaphor does not damage the Aristotelian
position. In the borderline cases argument, the Aristotelian tradition
can appeal to their matter/form distinction and remind Locke that even
though the form guides the changes in material substances, not all
matter is suitable for all changes. That is, a form is
teleological and is what guides the matter as it
achieves its final cause, but not all matter is suitable to
achieve that end. For example, wood might be highly suitable to
construct a ship, but stone would not be. The matter, if
suitable, may indeed achieve the end of the formal cause, however,
when the matter is not suitable it can result in
monsters and deformities. With regard to abstract ideas, i.e., nominal
essences, being the foundation of species, the Aristotelian tradition
can surely reject the claim that the form plays no role in the
formation of our abstract ideas. So long as our abstract ideas must be
answerable to what exists in nature, and so long as nature presents us
with the effects of the substantial form, what we have
available to include in our nominal essences will be very
much rooted in the substantial form. This same argument can be applied
to the third criticism as well; substantial forms guide our
selection of nominally essential properties because they carve nature
at its joints and help some appear more natural than others. Finally,
the claim that generality does not exist in nature
is question-begging: if there are substantial forms, then there
is generality in nature.



It is likely that Locke was aware of the limits of these criticisms,
after all, he was both educated with and taught Scholastic texts.
Nevertheless, as Jolley argues (1999, 146), Locke could reply
by arguing that the properties of bodies are
causally dependent on their essences, just as all the properties
of every triangle flow from the essences of the triangles,
but then why do mathematical and geometrical essences
produce hard-edged kinds while the essences of material bodies do
not? More to the point, if substantial forms are causal and
explanatory posits in Aristotelian *scientia*, and they are
held to be knowable *via* perception, but the evidence
from perception yields a host of borderline cases, then it
is plausible to conclude that either substantial forms are not doing
their job or there are no such forms. However, this reply will
not cut much ice with the Aristotelians, who have yet to be shown
why their appeal to the weakness of matter is insufficient to explain
generative variation.



Perhaps a more powerful interpretation of Locke's arguments would be
to see them as an empirical challenge to (rather than as a refutation
of) the idea that nature comes pre-structured by substantial forms. In
reference to borderline cases, Locke could be arguing that, without
assuming that there are substantial forms, nature does not give us
convincing evidence of such order. The semantic argument can be seen
as a similar empirical challenge: there is no evidence that nature is
narrowly constraining the creation of our nominal essences (we have
broad latitude in their creation), but if there were substantial forms
we should expect much less latitude. It is this challenge that seems
to undergird Locke's third argument: even though nature provides us
with the qualities we ultimately include in our nominal essences, it
is up to us to determine what goes into each nominal essence, and
there is no place for a substantial form to be involved in that
process. Although this might be a stretch, even Locke's fourth
criticism may still function as an empirical challenge. Perception is
a highly contentious subject that seems beyond our ability to explain,
even by appeal to the existence of substantial forms. So, without
evidence that there are such forms, there is no reason to take nature
to be the locus of generality. Whether these replies would be
effective, or even whether these are what Locke had in mind, it is
clear that Locke was convinced that substantial forms are not a viable
component of natural philosophy.


## 3. Historical Antecedents: Bacon and Boyle



While the concept and definition of 'real essence' is
original with Locke, at least a generation before him there were
Englishmen who defended non-Scholastic taxonomical theories that
employed material structures, either atomic or corpuscularian
structures, to play both roles (a) and (b), and therefore serve as the
essences of physical substances. One notable example is from Sir
Francis Bacon.



In Book II SSSSi-vii of *The New Organon* (1620),
Bacon seems to give a version of corpuscular natural kinds by
redefining the concept of 'formal cause' as both the
material structure of bodies and the laws that govern the natural
effects of these structures that accounts for their species and genus
membership. For example, in II.ii he says,



> 
> 
> For though nothing exists in nature except individual bodies which
> exhibit pure individual acts [powers] in accordance with law...It
> is this law and its clauses which we understand by the term
> Forms.... (Bacon 1620, 103)
> 
> 
> 



And later in II.iii: "But he who knows forms comprehends the
unity of nature in very different materials" (Bacon 1620, 103).
These Baconian forms, as they constitute the powers of material
bodies, divide nature into two classes of material structures: the
elements (what he calls 'major associations') and the
genera and species of natural bodies (his 'minor
associations'):



> 
> 
> We want elements to be understood in the sense not of the prime
> qualities of things, but of the major constituents of natural bodies.
> For the nature of things is so distributed that the quantity or mass
> of certain bodies is very great, because their structure requires the
> texture of an easy and common material...But the quantity of
> certain other bodies in the universe is small and occurs rarely,
> because the texture of their matter is very different, very subtle and
> for the most part delimited and organic; such are the species of
> natural things, metals, plants, animals. (Bacon 1620, 226)
> 
> 
> 



According to Bacon, more general material structures convene to create
larger--and more sparse--bodies, and given the laws of
nature that correlate powers to these structures, these bodies have
their natures and belong to a species or genus due to the structures
and causal powers of their constituent parts. Baconian forms exhibit
the two features of what me might call 'corpuscular
forms': material structures plus the laws of nature combine to
cause and explain the properties of bodies and determine their species
 membership.[2]



Similarly, in the *Origine of Formes and Qualities*, Robert
Boyle employs the concept of a material essence when he argues for the
existence of corpuscular forms that do all the same philosophical and
explanatory work as both the real and nominal essences:



> 
> 
> ...why may we not say, that the Form of a Body, being made up of
> those Qualities united in one Subject, doth likewise consist in such a
> Convention of those newly nam'd Mechanical Affections of Matter,
> as is necessary to constitute a Body of that Determinate kind. And so,
> though I shall for brevities sake retain the word *Forme*, yet
> I would be understood to mean by it, not a Real *Substance*
> distinct from Matter, but onely the Matter it self of a Natural Body,
> consider'd with its peculiar manner of Existence [corpuscular
> structure], which I think may not inconveniently be call'd
> either its *Specifical* or its *Denominating State*, or
> its *Essential Modification*, or, if you would have me express
> it in one word, its *Stamp*: for such a Convention of Accidents
> is sufficient to perform the Offices that are necessarily
> requir'd in what Men call a *Forme*, since it makes the
> body such as it is, making it appertain to this or that Determinate
> Species of Bodies, and discriminating it from all other Species of
> Bodies whatsoever.... (Boyle 1666, p. 324.)
> 
> 
> 



That is, Boylean corpuscular forms both cause and explain the
qualities of bodies and sort natural bodies into species based on the
structure of the matter and the laws binding qualities to the specific
 structures.[3]



So, we see that there are corpuscular versions of natural kind realism
afoot in early seventeenth-century England where both roles of
determining the species or genus membership and of causing and
explaining the observable qualities are played by the same entity:
corpuscular structure. The degree to which Locke agrees with, rejects
or even engages with these views is a matter of scholarly
controversy.



Nevertheless, we can say that, just as with the Scholastic natural
philosophy, there were adherents to the mechanical natural philosophy
who postulated a single entity to play both the causal and
classificatory roles; we can call these philosophers
'corpuscular realists' and we may say that they argue for
the existence of what we might call 'corpuscular species',
i.e., species created by nature by corpuscular forms. By contrast,
again, Locke objects to this general approach for the creation of a
scientific taxonomy:



> 
> 
> Nor indeed *can we* rank, and *sort Things*, and
> consequently (which is the end of sorting) denominate them *by
> their real Essences*, because we know them not. Our Faculties
> carry us no farther towards the knowledge and distinction of
> Substances, than a Collection of those sensible *Ideas*, which
> we observe in them; which however made with the greatest diligence and
> exactness, we are capable of, yet is more remote from the true
> internal Constitution, from which those Qualities flow, than, as I
> said, a Countryman's *Idea* is from the inward
> contrivance of that famous Clock at *Strasburg*, whereof he
> only sees the outward Figure and Motions. (III.vi.9)
> 
> 
> 



If we have no knowledge of real essences, then there is no way to use
real essences in our sorting practice. Moreover, as he alludes at the
end of the quotation, in order for a general inference about the
internal constitutions of substances to be justified, we would have to
know a lot more about the causal connections between the internal
constitution and what we observe than we do:



> 
> 
> ...it is impossible for us to know, that this or that quality or
> *Idea* has a necessary connexion with a real Essence, of which
> we have no *Idea* at all, whatever Species that supposed real
> Essence may be imagined to constitute. (IV.vi.5)
> 
> 
> 



The differences between Locke and the natural kind realists from among
the ranks of the corpuscularians can be illustrated by the following
chart:




|  |  |  |
| --- | --- | --- |
|  | (a) *Determines Species/Genus

Membership* | (b) *Causes/Explains Observable

Qualities* |
| *Scholastics* | Substantial Form | Substantial Form |
| *Corpuscular Realists* | Corpuscular Form | Corpuscular Form |
| *Locke* | Nominal Essence | Real Essence |



There are Locke scholars who (though this is controversial) argue that
Locke uses this distinction between real and nominal essences to
criticize the taxonomical theories of both the Scholastics and those
mechanistic natural philosophers who invoke corpuscular species forms
(see Jones 2010, pp. 147-172).



It is universally agreed that Locke thinks that we sort things into
species and genera based on nominal essences and species and genera
are a kind of human linguistic categorization process. On the other
hand, one might think, Locke has no reason to reject natural kinds
(which concern the metaphysics of nature, and not the meanings of
names) simply because our sorting process is arbitrary. In other
words, there are those who argue that Locke was a natural kind
realist, even though he was convinced that taxonomies are the
workmanship of the understanding. Whether Locke thinks that there are
natural kinds, i.e., types of real essences independently of human
sorting activities, is an interpretive issue we shall address in
 4.3 below.


## 4. Interpretive Issues



In this section, I discuss four main areas of interpretive dispute
concerning real essences: (i) relativized real essences; (ii)
individual real essences; (iii) real essences and natural kinds; and
(iv) real essences, internal constitutions and substance.


### 4.1 Relativized Real Essences



There are two main interpretations of real essences in the literature:
the relativized real essence thesis, according to which a real essence
just is whatever microstructure causes the nominally essential
qualities of the object; and (for lack of a better name) the
unrelativized real essence thesis, whereby the real essences consist
of the total microstructure of the substance, not just the features
causally responsible for the nominal essence. Textual evidence for the
relativized real essence thesis comes from III.vi.6:



> 
> 
> By this *real Essence*, I mean, that real constitution of any
> Thing, which is the foundation of all those Properties, that are
> combined in, and are constantly found to co-exist with the *nominal
> Essence*; that particular constitution, which every Thing has
> within it self, without any relation to any thing without it. But
> *Essence*, even in this sense, *relates to a Sort*, and
> supposes a *Species*: For being that real Constitution, on
> which the Properties depend, it necessarily supposes a sort of Things,
> Properties belonging only to *Species*, and not to
> Individuals...Here are *Essences* and *Properties*,
> but all upon supposition of a Sort, or general abstract *Idea*,
> which is considered as immutable: but there is no individual parcel of
> Matter, to which any of these Qualities are so annexed, as to be
> *essential* to it, or inseparable from it. That which is
> *essential*, belongs to it as a Condition, whereby it is of
> this or that Sort: But take away the consideration of its being ranked
> under the name of some abstract *Idea*, and then there is
> nothing necessary to it, nothing inseparable from it.
>  [4]
> 
> 
> 



Some Locke scholars take this to imply that the real essence is always
relativized to the nominal essence for Locke. If properties belong
only to species--not to individuals--and real essences are
defined as the real constitution of a thing that is the foundation of
its properties, then it seems to follow that the real essence of a
thing is defined in relation to the species to which the thing
belongs; essences relate to a sort (III.vi.4).



Critics of the relativized real essence interpretation of the
*Essay* will point out that if real essences are only for
sorted substances, then Locke ought to consistently use the term
'real essence' whenever referring to the submicroscopic
structure of sorted individuals. However, in II.xxiii.37 Locke uses
'internal constitution,' not 'real essence' to
refer to the causal ground of the nominally essential qualities of
gold:



> 
> 
> ...the greatest part of the *Ideas*, that make our complex
> *Idea* of *Gold*, are Yellowness, great Weight,
> Ductility, Fusibility, and Solubility, in *Aqua Regia*,
> *etc.* all united together in an unknown *Substratum*;
> all which *Ideas*, are nothing else, but so many relations to
> other Substances; and are not really in the Gold, considered barely in
> it self, though they depend on those real, and primary Qualities of
> its internal constitution, whereby it has a fitness, differently to
> operate, and be operated on by several other Substances.
> 
> 
> 



These powers he lists are nominally essential properties of gold, but
since Locke refers to their causal ground as an 'internal
constitution' and not as a 'real essence,' one might
argue, it seems that 'real essence' is not uniquely
employed to designate the submicroscopic foundation of the powers
included within the nominal essence.



Moreover, critics of the relativized real essence interpretation will
likely focus on the part of (III.vi.6) that seems to bracket off all
nominally essential qualities from a real essence: "By this
*real Essence*, I mean, that real constitution of any
Thing...which every Thing has within it self, without any
relation to any thing without it." That is, one might argue that
if we can consider a real essence all by itself, independently of any
relations to other objects or observers, then real essences are not
relativized to nominal essences because we can consider a real essence
without reference to "any relation to anything without it"
and nominally essential qualities require relations to observers.



There are important issues here that a consistent and plausible
interpretation of Locke on real essences must take into account. How
one does so will impact what one may say with respect to the
possibility of real essences for individual objects in Locke.


### 4.2 Individual Real Essences



A related, but importantly distinct issue, concerns the possibility of
real essences for individuals. On the one hand, if every object has an
underlying physical constitution, then there seems to be no problem
with thinking of individuals has possessing real essences. On the
other hand, if real essences are only for sorted individuals, then
there are no real essences for individuals until we have classified
them by a nominal essence; until an individual is sorted, it only has
an internal constitution, not a real essence. If one adopts the
relativized real essence interpretation of the *Essay*, then
there are no real essences for unsorted individuals. If, however, one
adopts the unrelativized real essence interpretation, then it appears
possible that Locke could think that unsorted individuals have a real
essence and not just an internal constitution.



Of course, in III.vi.4 Locke famously denies that anything is
essential to a particular:



> 
> 
> 'Tis necessary for me to be as I am; GOD and Nature has made me
> so: But there is nothing I have, is essential to me. An Accident, or
> Disease, may very much alter my Colour, or Shape; a Fever, or Fall,
> may take away my Reason, or Memory, or both; and an Apoplexy leave
> neither Sense, nor Understanding, no nor Life...None of these are
> essential to the one, or the other, or to any Individual whatsoever,
> till the Mind refers it to some Sort or *Species* of things;
> and then presently, according to the abstract *Idea* of that
> sort, something is found *essential*. Let any one examine his
> own Thoughts, and he will find, that as soon as he supposes or speaks
> of *Essential*, the consideration of some *Species*, or
> the complex *Idea*, signified by some general name, comes into
> his Mind: And 'tis in reference to that, that this or that
> Quality is said to be *essential*. So that if it be asked,
> whether it be *essential* to me, or any other particular
> corporeal Being to have Reason? I say no; no more than it is
> *essential* to this white thing I write on, to have words in
> it. But if that particular Being, be to be counted of the sort
> *Man*, and to have the name *Man* given it, then Reason
> is *essential* to it, supposing Reason to be a part of the
> complex *Idea* the name *Man* stands for....
> 
> 
> 



However, one might argue that in this section Locke is only
considering nominally essential qualities and saying that none of the
candidates for nominally essential qualities are essential to a being,
*qua* species or genus member, until it has been sorted.



Moreover, the second sentence of III.vi.6, where he inserts the
clarification that a real essence is "...that particular
constitution, which every Thing has within it self, without any
relation to any thing without it," appears to say that there can
be real essences of unsorted individuals. If a real essence is
definable as a corpuscular structure *cum* causal powers that a
body has *independently of anything else*, then it seems to
follow that there are individual real essences independently of how we
sort them. That is, if every natural substance has some submicroscopic
corpuscular constitution on which its observable qualities depend,
then it seems to follow that unsorted individuals have a real essence
(see Owen 1991 and Vienne
 1993).[5]



In II.xxxi.6, Locke refers to individual real essences: "The like
ignorance I have of the real Essence of this particular Substance, I
have also of the real Essence of all other natural ones". In
III.iii.15, as he is defining the concept of a real essence, he says:
"*Essentia*, in its primary notation signifying properly
*Being*. And in this sense it is still used, when we
speak of the *Essence* of particular things, without
giving them any Name". One might argue that, if a substance can have
an essence prior to being named--prior to having a nominal
essence--then the only other kind of essence it could have is a real
essence.



It is possible that Locke was not always clear and consistent in his
use of the term 'real essence' and that these puzzles are
impossible to resolve. However, it is also possible that when Locke
was focusing on the precise meanings of terms that he was indeed
careful, and there is a consistent interpretation of his views on this
issue. Either way, any interpretation of Locke on real essences must
determine whether there are real essences for only sorted individuals
or not.


### 4.3 Real Essences & Natural Kinds



Another important exegetical issue that arises in the *Essay*
is whether Locke thought that there are natural kinds based on real
essences. Scientific disciplines categorize the objects they study
into *kinds*. To say that a kind is *natural* is to say
that a grouping of similar individuals exists independently of human
decisions. The hope of each scientific discipline is to identify as
many natural kinds as possible within their domain and to determine
their properties accurately. To this end, we create scientific
taxonomies, like the periodic table of elements, the phylogenetic
tree, etc., to organize the kinds we have identified and to study
their properties. On the face of it, the question of whether there are
natural kinds in Locke seems to be the same as the question of whether
there are species, and as we saw above (in SSSS1-3)
Locke denies that there are natural species. However, it is possible
to distinguish between a species (a classification humans make based
on observed similarities) and a natural kind (a grouping that occurs
naturally and independently of human naming practices). Locke is
clearly aware of this distinction, so it is possible that what he has
said about sorting and nominal essences only reveals what he thinks
about nominal essences and our practice of sorting, which is a
semantic issue, not about the metaphysics of nature.



To be sure, there are passages in the *Essay* that seem to
indicate that Locke was a natural kind realist even if he was a
conventionalist about taxonomical practices. For example:



> 
> 
> The other, and more rational Opinion, is of those, who look on all
> natural Things to have a real, but unknown Constitution of their
> insensible Parts, from which flow those sensible Qualities, which
> serve us to distinguish them one from another, according as we have
> Occasion to rank them into sorts, under common Denominations.
> (III.iii.17)
> 
> 
> 



In other words, one might argue, if real essences cause the observable
qualities of bodies, and we sort things into species based on
observable similarities, then if we assume that similarities among the
real essences always results in similar observable qualities, and
similar observable qualities are always caused by similar real
essences, there is reason to think that Locke assumed that our nominal
essences track, at least to some degree, real kinds in nature.



There are places in the *Essay* where Locke seems to think
along these lines. In III.vi.36 he says "*Nature makes many
particular Things, which do agree* one with another, in many
sensible Qualities, and probably too, in their internal frame and
Constitution." In a letter to Molyneux, Locke says:



> 
> 
> I hope I have no where said, there is no such sort of creatures in
> nature as birds; if I have, it is both contrary to the truth and to my
> opinion. This I do say, that there are real constitutions in things
> from whence these simple ideas flow, which we observ'd combined
> in them. And this I farther say, that there are real distinctions and
> differences in those real constitutions one from another; whereby they
> are distinguished one from another, whether we think of them or name
> them or no. (Locke, *Works*, vol. IV, pp. 305-6)
> 
> 
> 



Indeed, there are passages where Locke speaks of the
"imperfection" of our species ideas and the need to make
them better conform to nature. For example, in III.xi.24 he says
that:



> 
> 
> And therefore in Substances, we are not always to rest in the ordinary
> complex *Idea*, commonly received as the signification of that
> Word, but must go a little farther, and enquire into the Nature and
> Properties of the Things themselves, and thereby perfect, as much as
> we can, our *Ideas* of their distinct Species...
> 
> 
> 



There is no reason, so one may argue, to suspect that our species
ideas could be either perfected or imperfect if there is no natural
kind archetype. On the other hand, there is some evidence that Locke
anticipated this kind of account and rejected it. For example, he
says:



> 
> 
> ...That we find many of the Individuals that are ranked into one
> Sort, called by one common Name, and so received as being of one
> *Species*, have yet Qualities depending on their real
> Constitutions, as far different one from another, as from others, from
> which they are accounted to differ *specifically*. This, as it
> is easy to be observed by all, who have to do with natural Bodies; so
> Chymists especially are often, by sad Experience, convinced of it,
> when they, sometimes in vain, seek for the same Qualities in one
> parcel of Sulphur, Antimony, or Vitriol, which they have found in
> others. For though they are Bodies of the same *Species*,
> having the same nominal *Essence*, under the same Name; yet do
> they often, upon severe ways of examination, betray Qualities so
> different one from another, as to frustrate the Expectation and Labour
> of very wary Chymists. But if Things were distinguished into
> *Species*, according to their real Essences, it would be as
> impossible to find different Properties in any two individual
> Substances of the same *Species*, as it is to find different
> Properties in two Circles, or two equilateral Triangles.
> (III.vi.8)
> 
> 
> 



This argument is a *modus tollens*: if the members of a
nominally defined species had the same type of real essence, then they
should all exhibit the same qualities (including those not included in
the nominal essence), but they do not exhibit all of the same
qualities, therefore they do not all have the same type of real
essence.



This question of whether real essences could be the foundation of
natural kinds for Locke is as old as the *Essay* itself. Both
Gottfried Wilhelm Leibniz and Edward Stillingfleet, Lord Bishop of
Worcester, point out that Locke's position on real essences
seems to entail that there are natural kinds based on similarities
among the real essences. As Leibniz put it in his critique of the
*Essay* entitled *New Essays on Human Understanding*
(1704),



> 
> 
> [i]t is true that we cannot define a species in terms of something
> which is unknown to us [e.g., real essences]; but the outer features
> serve in place of it.... (Leibniz 1704, p. 311)
> 
> 
> 



Later, he reiterates this point:



> 
> 
> [when] I think of a body which is at once yellow, fusible and
> resistant to cupellation, I am thinking of a body whose specific
> essence, though hidden from me within it, gives rise to these
> qualities and reveals itself, at least confusedly, through them.
> (Leibniz 1704, p. 405)
> 
> 
> 



Locke died before Leibniz's *New Essays on Human
Understanding* were published (they were not published until 1765,
sixty-one years after Locke's death and nearly fifty years after
Leibniz's death), so we do not have Locke's response to
Leibniz's arguments, nevertheless, Stillingfleet gave a similar
argument and we have Locke's replies to him.



Like Leibniz, Stillingfleet appeals to the concept of real essence as
a "natural kind" concept, claiming that there are types of
real essences, and belonging to the same natural kind is a function of
possessing the same kind of real essence:



> 
> 
> But in this one sun there is a real essence, and not a mere nominal or
> abstracted essence; but suppose there were more suns; would not each
> of them have the real essence of the sun? For what is it makes the
> second sun to be a true sun, but having the same real essence with the
> first? If it were but a nominal essence, then the second would have
> nothing but the name. (Locke, *Works*, vol. IV, p. 83. Here
> Locke is quoting from Stillingfleet's *Discourse in
> Vindication of the Doctrine of the Trinity*, where the latter
> criticizes Locke's *Essay*.)
> 
> 
> 



Locke's response is to turn the whole discussion away from real
essences and back onto the nominal essences:



> 
> 
> Your lordship's proof here brought out of my *Essay*,
> concerning the sun, I humbly conceive will not reach it; because what
> is said there, does not at all concern the real, but nominal essence;
> as is evident from hence, that the idea I speak of there, is a complex
> idea; but we have no complex idea of the internal constitution, or
> real essence of the sun. Besides, I say expressly, that our
> distinguishing substances into species by names, is not at all founded
> on their real essences. So that the sun being one of these substances,
> I cannot, in the place quoted by your lordship, be supposed to mean by
> essence of the sun, the real essence of the sun, unless I had so
> expressed it. But all this argument will be at an end, when your
> lordship shall have explained what you mean by these words,
> "true sun." In my sense of them, any thing will be a true
> sun, to which the name sun may be truly and properly applied; and to
> that substance or thing, the name sun may be truly and properly
> applied, which has united in it that combination of sensible
> qualities, by which any thing else that is called sun is distinguished
> from other substances, i.e. by the nominal essence.... (Locke,
> *Works*, vol. IV, pp. 83-4)
> 
> 
> 



Although this is controversial exegetical territory, it appears that
Locke is aware of this 'real essence' version of natural
kinds, but it further appears that he is unwilling to discuss it; his
response was to steer clear of the metaphysical claim and return to
the semantic issue of nominal essences.



As natural as it is to interpret Locke as a natural kind realist,
there are problematic passages for that interpretation. With respect
to the similarities among real essences, which was the main concern of
Leibniz and Stillingfleet, in IV.vi.4 Locke argues that the assumption
that species are determined by real essences fails to tell us what
would count as a member of any species. That is, because we have no
knowledge of real essences, no general claims about species would be
justified and no individual would obviously belong to a particular
species. So the assumption that Leibniz and Stillingfleet propose
appears to be theoretically unmotivated. Moreover, as Locke's
argument in III.vi.39 suggests, similarities in structure of the real
essences, even if known, would still require us to decide which
similarities (not to mention what degrees of similarity) would amount
to *similar enough* to belong together in the same class:



> 
> 
> For what is sufficient in the inward Contrivance, to make a new
> *Species?* There are some *Watches*, that are made with
> four Wheels, others with five: Is this a specifick difference to the
> Workman? Some have Strings and Physies, and others none; some have the
> Balance loose, and others regulated by a spiral Spring, and others by
> Hogs Bristles: Are any, or all of these enough to make a specifick
> difference to the Workman, that knows each of these, and several other
> different contrivances, in the internal Constitutions of
> *Watches?* 'Tis certain, each of these hath a real
> difference from the rest: But whether it be an essential, a specifick
> difference or no, relates only to the complex *Idea*, to which
> the name *Watch* is given: as long as they all agree in the
> *Idea* which that name stands for, and that name does not as a
> generical name comprehend different *Species* under it, they
> are not essentially nor specifically different. (III.vi.39; see also
> III.vi.30)
> 
> 
> 



Of course, this view has been challenged recently. Allison Kuklok
(2018) argues that this watch passage does not show that Locke took
our sorting practices (at either the phenomenal or microphysical
level) to be arbitrary. For her, a mistake that interpreters of Locke
make is to claim that the watch passage asserts that there is no fact
of the matter about how things should be classified, and therefore,
our classifications are arbitrary. According to Kuklok, this watch
analogy is intended to show the irrelevance of hidden internal
differences to species whose boundaries are antecedently drawn at the
phenomenal level by our natural sorting practices. By her lights, once
we acknowledge that passages like the watch analogy do not concern
classification, we see that Locke never argued that classification is
arbitrary.



On her reading, Locke thinks that species are made by the vulgar prior
to our theorizing about internal constitutions, and these species
cannot plausibly be understood as the result of arbitrary choice.
According to Kuklok, the naturalness of our classifications stems from
the fact that Locke holds that humans have a natural disposition
toward species realism, and the fact that nature presents us with
clusters of 'leading qualities' that appear as patterns or
standards. A relevant passage from Locke comes from II.xxxii.14: "God
in His wisdom...set [simple ideas] as Marks of Distinction in Things,
whereby we may be able to discern one Thing from
another". Given the vulgar realism and the leading patterns in
nature, the vulgar classify based on non-arbitrary natural
similarities. It is implausible "to say that the similarities and
differences between things leave it undetermined how we are to group
and distinguish them" (2022, p. 318). 



However, those who interpret the watch passage as implying
arbitrariness in classification (see Uzgalis (1988) and Jones (2010))
would have at least two replies. First, that 'arbitrary'
does not mean 'random' or that all qualities are equally
adequate as *differentia*. The vulgar have good empirical
reasons for their taxonomies, however, there are lots of similarly
good and natural reasons for different taxonomies. That does not imply
that every taxonomy is equally as acceptable. Moreover, given the
background of corpuscularians who appeal to corpuscular forms as a
ground for natural kind realism, and given that Locke's focus is
the internal structure and arrangement of objects, it is plausible to
see the watch passage as insisting that real essences are not an
adequate foundation for a theory of natural kinds.



As we can see, whether, in fact, Locke employed real essences as the
foundations of natural kinds is currently one of the hottest debates
among Locke scholars, and there have been a number of good studies
written in recent years.


### 4.4 Real Essences, Internal Constitutions and Substance



Throughout the *Essay*, Locke discusses real essences, internal
constitutions and substratum, and getting clear on each concept is
required in order to produce a consistent and accurate interpretation
of Locke on real essences. While it is not possible to do justice to
any of the subjects here, it is important to give some background
before proceeding. When Locke defines the 'real essence',
he often describes it as "that Constitution, whereon the
Properties of these several things depended" (III.iii.19). On
occasion he even connects internal constitutions with essences as if
they were synonymous: "[collections of simple ideas] are
therefore supposed to flow from the particular internal Constitution,
or unknown Essence of that Substance" (II.xxiii.3). Similarly,
the concept of 'substance' or 'substratum',
i.e., pure substance in general, is conceived as an underlying
'thing' which supports, or upholds, the properties of
bodies:



> 
> 
> The *Idea* then we have, to which we give the general name
> Substance, being nothing, but the supposed, but unknown support of
> those Qualities, we find existing, which we imagine cannot subsist,
> *sine re substante*, without something to support them, we call
> that Support *Substantia*; which, according to the true import
> of the Word, is in plain *English*, *standing under*, or
> *upholding*. (II.xxiii.2)
> 
> 
> 



The fact that the *Essay* uses internal constitutions,
substance, and real essences as the underlying ground of the qualities
of physical objects has led to some debate over the way Locke
understands the relationship between the three concepts.



Regarding the relationship between real essences and substance in
general, Michael Ayers has argued that Locke uses the terms
'substance' and 'real essence' as
extensionally equivalent, but not intensionally equivalent. That is,
both terms refer to the same objects, but convey a very different
meaning. For Ayers, Locke's use of 'substance'
conveys the idea of a thing that supports properties, whereas
'real essence' conveys the idea of a submicroscopic
material structure that causes the powers of the object; these are
different meanings, but both terms refer to only one
 object.[6]



On the other hand, some Locke scholars interpret the concepts of
'substance' (in the sense of pure substance in general, or
'substratum') and 'real essence' as both
intensionally and extensionally distinct because they play very
different philosophical roles. For example, according to Nicholas
Jolley, Locke employs the concept of a real essence when he is
addressing issues of scientific explanation and he appeals to
substratum when he is discussing the general concept of what it is to
be a *thing* (as opposed to a property or mode) (see Jolley
1999, pp. 70-78).



On the relationship between real essences and internal constitutions,
one view is that Locke uses the terms 'real essence' and
'internal constitution' interchangeably. For example, in
II.xxxi.6 he says "its real Essence, or internal Constitution,
on which these Qualities depend, can be nothing but the Figure, Size,
and Connexion of its solid Parts." Again in III.iii.19 he uses
'real essence' as if it were synonymous with
'internal constitution': "In all which, and the like
Changes, 'tis evident, their real *Essence*, i.e. that
Constitution, whereon the Properties of these several things depended,
is destroy'd, and perishes with them."



By contrast, Susanna Goodin (1998) argues that 'real
essence' and 'internal constitution' cannot be
synonymous because, according to Locke, real essences are unknowable
(IV.vi.5 & 12), but internal constitutions are knowable
(II.xxiii.12). One way to distinguish them is, as we saw above, to
adopt the relativized view and interpret the real essence as the
internal constitution that underlies the nominally essential
properties, whereas the internal constitution of a substance is simply
its physical make-up, independently of considerations of its nominal
essence. So, on this view, Locke's uses of 'real
essence' and 'internal constitution' are
non-synonymous technical terms.



P. Kyle Stanford (1998), on the other hand, distinguishes real
essences from internal constitutions by arguing that real essences, by
definition, include the causal powers of the body, whereas the
internal constitution is simply its material components. Thus, for
Stanford, a real essence "consists of those features of an
entity from which all of its observable or experienceable properties
can be logically deduced" (Stanford 1998, p. 80). By contrast,
knowledge of the internal constitution, *qua* mere material
components, entails no such deducibility. For example, in IV.vi.11 he
says that knowledge of the real essence of an object entails knowledge
of all of its powers, but knowledge of mere material structure cannot
entail that kind of knowledge. Moreover, in all the quotations where
Locke talks about real essences, he does not just define them as a
material structure, but also as the causal grounds of the observable
qualities of the body, indicating that the powers are included in the
definition.


## 5. Real Essences of Simple Ideas and Modes



One of Locke's epistemological projects is to distinguish
metaphysics and natural philosophy from ethics and mathematics, and
one way he does that is to distinguish their subject matters.
Metaphysics and natural philosophy are concerned with substances,
whereas mathematics and ethics are concerned with modes. For Locke,
since an essence accounts for the existence and nature of any being,
i.e., "Essence may be taken for the very being of anything,
whereby it is what it is", it follows that the concepts of real
and nominal essences apply outside the realm of metaphysics and
natural philosophy as well. By his lights, both simple ideas and modes
have real and nominal essences. To see this, some definitions are in
order.



By 'simple idea', Locke means a single, uniform concept or
appearance that cannot be analyzed into more basic parts:



> 
> 
> The coldness and hardness, which a Man feels in a piece of
> *Ice*, being as distinct *Ideas* in the Mind, as the
> Smell and Whiteness of a Lily; or as the taste of Sugar, and smell of
> a Rose: And there is nothing can be plainer to a Man, than the clear
> and distinct Perception he has of those simple *Ideas*; which
> being each in it self uncompounded, contains in it nothing but *one
> uniform Appearance*, or Conception in the mind, and is not
> distinguishable into different *Ideas*. (II.ii.1)
> 
> 
> 



Our complex idea of gold might include its color, heaviness, solidity,
etc., but each of these ideas can be set aside and considered all on
its own as a simple idea. One simple idea, e.g., that of the
yellowness of the lump of gold, is simple because it is a single,
uniform appearance of that particular shade and not a collection of
more basic appearances. The same goes for our idea of its hardness and
heaviness. We can mentally separate the color from the hardness or
heaviness and think of it all on its own.



By 'mode', Locke means complex ideas of qualities that
depend upon substances for their existence:



> 
> 
> First, *Modes* I call such complex *Ideas*, which
> however compounded, contain not in them the supposition of subsisting
> by themselves, but are considered as Dependences on, or Affections of
> Substances; such are the *Ideas* signified by the Words
> *Triangle*, *Gratitude, Murther, etc*. (II.xii.4)
> 
> 
> 



So, for example, gratitude does not exist on its own, rather it
requires a *thing* that is grateful in order to exist. Mixed
modes are,



> 
> 
> the Complex *Ideas*, we mark by the names *Obligation,
> Drunkenness*, a *Lye*, etc. which consisting of several
> Combinations of simple *Ideas* of different kinds, I have
> called *Mixed Modes*, to distinguish them from the more simple
> Modes, which consist only of simple *Ideas* of the same kind.
> These mixed Modes being also such Combinations of simple
> *Ideas*, as are not looked upon to be the characteristical
> Marks of any real Beings that have a steady existence, but scattered
> and independent *Ideas*, put together by the Mind, are thereby
> distinguished from the complex *Ideas* of Substances.
> (II.xxii.1)
> 
> 
> 



Given that there are real and nominal essences for modes and that a
general knowledge consists in knowledge of essences, then Locke needs
some way to distinguish the essences of modes from those of
substances.



According to Locke, one of the main differences between substances on
the one hand and simple ideas and modes on the other is that the real
essences of substances are distinct from their nominal essences,
whereas for simple ideas and modes, they are the same.



> 
> 
> *Essences* being thus distinguished into *Nominal and
> Real*, we may farther observe, that *in* the Species of
> *simple* Ideas *and Modes*, they *are always the
> same*: But *in Substances, always quite different*. Thus a
> Figure including a Space between three Lines, is the real, as well as
> nominal *Essence* of a Triangle; it being not only the abstract
> *Idea* to which the general Name is annexed, but the very
> *Essentia*, or Being, of the thing it self, that Foundation
> from which all its Properties flow, and to which they are all
> inseparably annexed. But it is far otherwise concerning that parcel of
> Matter, which makes the Ring on my Finger, wherein these two
> *Essences* are apparently different. For it is the real
> Constitution of its insensible Parts, on which depend all those
> properties of Colour, Weight, Fusibility, Fixedness, *etc.*
> which are to be found in it. Which Constitution we know not; and so
> having no particular *Idea* of, have no Name that is the Sign
> of it. But yet it is its Colour, Weight, Fusibility, and Fixedness,
> *etc.* which makes it to be *Gold*, or gives it a right
> to that Name, which is therefore its nominal *Essence*.
> (III.iii.18)
> 
> 
> 



So the real essences of a triangle, or gratitude, or murder, or
yellow, or sweet, etc., are exactly the same as their nominal
essences. There is nothing more to being a triangle than being a
plane, closed three-sided figure. Similarly, there is nothing more to
being a particular 'species' or shade of yellow than the
nominal
 definition.[7]



While it is clear that Locke envisioned some analogy between the real
and nominal essences of substances with those of modes, it is unclear,
however, just how that analogy should be cashed out. For example,
Roger Woolhouse argues that it is a mistake for Locke to say that mode
ideas have a real essence at all because there is nothing like a
corpuscular structure that is analogous to the definition of a concept
like triangle or lying. Moreover, there is, according to Woolhouse,
nothing that binds the nominally essential qualities of modes
together, whereas, for substances there is an underlying physical
structure (see Woolhouse 1971, pp. 125-128). Indeed it is quite
difficult to see how a mode could have a real essence if the model for
what a real essence is comes from corpuscular structures. If Woolhouse
is right, then Locke has no way to make good on his intended analogy
and he is unable to adequately differentiate modes from
substances.



P. Kyle Stanford (1998), on the other hand, suggests that the analogy
between real and nominal essences was intended to be based in their
entailment relations. Stanford argues that, just as every property of
a substance flows from the real essence with geometrical necessity
(IV.vi.11), so too does every property of a triangle follow from its
definition with geometrical necessity. And this fact is what all real
and nominal essences have in common for Locke. By his way of thinking,
even though the connection between the real essence of a piece of gold
is the *causal* ground of the nominally essential properties of
gold, and the real essence of a triangle is the *logical*
ground for the nominally essential properties of a triangle, they are
alike in that all nominally essential properties can be deduced from
the idea of the real essence; if we knew the real essence of a
triangle, then we could deduce from it any of the properties of a
triangle. To make this analogy work, Stanford argues that the real
essence of a substance is more than a corpuscular structure, it must
also include the causal powers that produce the observable qualities
of the substance; in like fashion, the real essence of modes includes
the logical ground from which the nominally essential properties of
the term flow. If Stanford is correct, then there is a possible
analogy between the real and nominal essences of modes and substances
that yields an interesting interpretation of the real essences of both
modes and substances.



On either reading, however, it appears that Locke is saddled with the
view that in cases of simple ideas, ideas of modes and mixed modes, if
we understand the meanings of the words, we know all that there is to
know regarding those concepts. Moral and mathematical concepts and
ideas are mere trifling propositions, analytic truths and
uninformative. However, it will be difficult for many moral
philosophers to accept that all they are doing is cashing out the
definitions of terms and dealing with uninformative analytic truths;
asserting that lying is wrong seems to be more than a trifling
opinion. Moreover, it is unclear how Locke thinks moral terms can be
normative, i.e., how they can constitute commands we ought to
obey--if his position is correct.



In a similar vein, since many mixed modes, which include all moral
concepts, are arbitrary mental constructs, it follows that we create
moral concepts by producing their definitions. Since the real and
nominal essences of mixed modes are the same, it follows that if we
know the real essence of lying (knowingly and falsely asserting that
'*p*' with the intention to mislead someone whom we
ought not to mislead, etc.), we can deduce from that idea all the
components of lying, and so there is nothing more to know about lying
than the ideas contained within its definition. A problem with
Locke's account here is that, as John Mackie points out, social
scientists seek out the real essences of social phenomena such as
adultery, incest, jealousy, lying and suicide, but understanding these
complex phenomena involves more than simply understanding the meaning
of the word. If we are to understand why people lie, we must know more
than a definition of lying. Of course, it is possible for Locke to
argue that once we understand all that is included in lying, we have
uncovered its real essence, why people lie, and the many ways they do
so, however, are separate questions that we cannot expect to be
decided by a semantic theory. It is unclear how he would solve the
normative problem (see Mackie 1976, pp. 90-91; see also Jolley
1999, pp. 155-161).