Open Journal of Philosophy
2012. Vol.2, No.4, 228-234
Published Online November 2012 in SciRes (http://www.SciRP.org/journal/ojpp) http://dx.doi.org/10.4236/ojpp.2012.24034
228
Locke’s Solid Souls
D. Kenneth Br ow n
Department of Philosophy, California Polytechnic State University, San Luis Obispo, USA
Received July 20th, 2012; revis ed Au gust 24th, 2012; accepted September 5th, 2012
John Locke holds that matter is solid, the soul thinks, and for all we know the soul may be a material sub-
stance divinely endowed with a power to think. Though he openly admits to nothing stronger than the
bare possibility of thinking matter, Locke grants that what thinks in us occupies a definite spatial location
to the exclusion of other souls. Solidity is the quality that prevents other things from occupying a spatial
location. Locke’s general criterion for identity is spatiotemporal exclusion of other things of the same
kind. To meet these conditions for identity, souls must be solid. Although Locke refuses to declare that
souls really are material things, taking the solidity of souls to be a condition for their identity is consistent
with the following of Locke’s other important commitments: 1) nominalism about the essences by which
substances are classified; 2) agnosticism about the underlying reality of what supports such “nominal es-
sences”; and 3) the identity of persons is distinct from the identity of any substance. Locke ignores the
implication that souls are solid because the solidity of souls is irrelevant to those three aims. Nevertheless
he could allow for the solidity of souls without giving up on any of his other important and explicitly held
commitments. There is therefore no need for Locke’s commentators to refrain from employing solidity in
their accounts of Locke’s general criterion for identity from fear of attributing to Locke the position that
souls would be solid.
Keywords: John Locke; Identity; Individuation; Soul; Solidity; Materialism; Substance; Essence
Introduction
In An Essay Concerning Human Understanding, John Locke
speculates that for all we can know, the soul may very well be a
material substance, or perhaps a qualitative effect or power of
systems of material substances:
We have the Ideas of Matter and Thinking, but possibly shall
never be able to know, whether an y mere ma teri al Bei ng thi nks ,
or no; it being impossible for us, by the contemplation of our
own Ideas, without revelation, to discover, whether Omnipo-
tency has not given to some Systems of Matter fitly disposed, a
power to perceive and think (IV.iii.6).1
Locke holds that we reach beyond our own capacities to
know for sure whether or not God saw fit to make matter think.
Nevertheless, that matter thinks remains a distinctly conceiv-
able possibility. Though Locke denied that he thought the soul
ultimately is a material substance, I shall argue that Locke’s
discussion of the identity of substances, along with other posi-
tions he holds in the Essay, implies that the identity conditions
for souls as substances requires souls to be solid. Sure enough,
if we accept this conclusion, then souls would have determinate
sizes, shapes, and locations—consequences Locke’s contem-
poraries at least, if not Locke himself, would have found re-
pugnant. Even if we accept that Locke’s Essay allows for, or
even contains, such an argument, we still would have to explain
Locke’s own insistence that souls are at the very least, most
likely immaterial. Yet in spite of protests even from Locke
himself, I shall argue that the solidity of souls can find its place
in the larger project of the Essay and even shed further light on
some of Locke’s most central theses concerning the essences of
substances and their identity conditions. We will also reach a
better understanding of the point of Locke’s own apparent de-
nials of this hitherto repugnant conclusion. I have organized
this project as follows. I shall first present the textual case for
attributing solidity to souls. I shall then present and address
other textually based objections to the solidity of souls. In my
response to these objections I shall argue that the solidity of
souls is consistent with Locke’s nominalism about the essences
by which substances are classified, as well as his agnosticism
about the underlying reality of what supports such “nominal
essences.” I shall also argue that Locke’s account of the identity
of persons renders irrelevant the criteria for the identity of any
substance including souls. I will conclude by examining how
my proposal of the solidity of souls relates to prominent lines of
commentary in the relevant secondary literature which mainly
ignore or evade attributing solidity to souls.
The Argument for the Solidity of Souls
The argument for the solidity of souls arises from Locke’s
account of the relation of identity in general, wherein he states
that
never finding, nor conceiving it possible, that two things of
the same kind should exist in the same place at the same time,
we rightly conclude, that whatever exists any where at any time,
excludes all of the same kind, and is there itself alone
(II.xxvii.1).
Here Locke takes spatiotemporal exclusion of all else of the
same kind to be a central doctrine in our idea of the identity of
anything whatsoever. With respect to substances, Locke thinks
that we have ideas of three kinds: God, souls (i.e. “finite spir-
its”), and bodies (i.e. “matter”) (II.xxvii.2). For any of these
1Citations from Locke’s Essay are from (1979) and in the standard form o
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D. K. BROWN
three kinds of substances, it follows from Locke’s general ac-
count of identity
that one thing cannot have two beginnings of existence, nor
two things one beginning; it being impossible for two things of
the same kind to be or exist in the same instant, in the very
same place, or one and the same thing in different places
(II.xxvii.1).
Locke takes sameness of beginning to follow from the spati-
otemporal exclusion of things of the same kind, including souls,
since
finite spirits having had each its determinate time and place
of beginning to exist, the relation to that time and place will
always determine to each of them its identity, as long as it ex-
ists (II.xxvii.2).
But Locke also thinks “the same will hold of every particle
of matter” (II.xxvii.2). So whether the identity conditions of
souls or of matter are in question, the same general criterion for
identity applies: the present relation to a determinate spatio-
temporal origin determ ines it s identity.
Locke has earlier in the Essay discussed spirits as capable of
occupying definite locations given that they can move around:
There is no reason why it should be thought strange, that I
make mobility belong to spirit: For having no other idea of
motion, but change of distance with other beings that are con-
sidered as at rest; and finding, that spirits, as well as bodies,
cannot operate but where they are, and that spirits do operate
at several times in several places; I cannot but attribute change
of place to all finite spirits; (for of the infinite spirit I speak not
here.) For my soul being a real being, as well as my body, is
certainly as capable of changing distance with any other body,
or being, as body itself; and so is capable of motion (II.xxiii.19).
Souls thus have determinate spatial locations for Locke in
virtue of their mobility whereby they change their location. By
occupying a location in space, souls prevent other souls from
also occupying that very same location at the same time. And,
these locations can be determined in reference to the locations
of bodies.
Earlier in the Essay Locke discusses our idea of that which
excludes all else from a given place: solidity. Locke relates
solidity to the filling of space when he claims that
[t]he idea of which filling of space is, that, where we imagine
any space taken up by a solid substance, we conceive it so to
possess it, that it excludes all other solid substances; (II.iv.2).
Solidity is thus the idea by means of which we distinguish
empty space from space occupied by something (II.iv.3). And it
is just such an occupation of space at a time that a determina-
tion of identity requires. This statement about solidity and spa-
tial exclusion would apply to any solid substance. Here the
point concerns what we attribute to something that does ex-
clude others from its space: by virtue of that spatial exclusion, it
is solid.
The following argument for the solidity of souls results from
drawing a plain inference from Locke’s general account of
identity and the function of the idea of solidity:
1) The identity conditions of substances require them to oc-
cupy some determinate place at some time to the exclusion of
all other substances of the same kind.
2) To occupy a determinate place to the exclusion of other
things of the same kind requires it to be solid.
3) Thus, the identity conditions of substances require sub-
stances to be solid.
4) Souls are substances.
5) Thus, the identity conditions of souls require souls to be
solid.
The logic of this argument is plain. Locke is committed to 1)
and 2) about identity and solidity for substances, so Locke must
be committed to 3). Locke asserts 4), so he must accept 5). The
conclusion of this argument relates solidity to souls by means
of their identity conditions, so to clarify the meaning of the
conclusion some further remarks about the relation of identity
are in order.
Locke’s principle of identity is “existence it self” (II.xxvii.3).
So, if this argument is sound, to deny that souls are subject to
the relation of identity is to deny souls their existence. To speak
of the identity of a soul is just to relate a soul to itself and dis-
tinguish it from things other than itself;2 and that just is what it
is for a soul to exist. Locke considers there to be one sort of
substance which cannot be subsumed within a spatiotemporal
framework for its identity, and that is God. But, other sub-
stances, having discrete particular instances, need criteria for
their diversity in order to fix on them as something whose iden-
tity can then be considered. So, we can understand the argu-
ment under consideration as pertaining only to “finite” sub-
stances. That is, the argument applies to substances that admit
of multiple token instances of a common type. The spatiotem-
poral framework constitutes the conditions for distinguishing
amongst such token instances of the same type. God can thus be
diverse from other substances by means of the spatiotemporal
framework the other substances require for determinations of
their identity.3 In this way, relating a soul to itself employs a
spatiotemporal framework, and it is that framework that sets
that soul apart from all other things diverse from it. As I have
interpreted it, solidity is just the universal criterion for spatio-
temporal exclusion. Solidity would then be the property a sub-
stance must have to be set into a relation of identity, given that
there are discrete instances of the same type of substance. It is
in this sense that we should understand the identity of a sub-
stance to be its existence, and to require a property, viz. solidity,
to determine that existence.
Some Objections
One might object to the argument for the solidity of souls on
grounds that it requires one to apply very selective attention to
Locke’s account of the idea of solidity. This line of objection
would be advantaged were it to be endorsed by Locke himself,
and there might be some reason to think it is. Locke’s account
of solidity is framed by his attempt to explain how we find our
idea of solidity in sensation. Sensation supplies our ideas of
external objects, while reflection provides us with ideas about
that which concerns thinking (II.i.5). The idea of solidity,
Locke claims, arises exclusively from touch, and is the idea
most constantly received from sensation (II.iv.1). He further
proclaims that of all our other ideas, solidity “seems the idea
2Identity is the relation by which things are individuated. Though Locke
himself does n ot for mula te his poi nts in term s of indi vidua tion, o ftentim es he
expresses his positions about identity by means of discussions of the appli-
cation of the relation of identity to the individuation of things.
3In his proof of the existence of God, Locke distinguishes God from matter
without invoking solidity or spatiotemporal exclusion as criteria. Rather, he
concerns himself with distinguishing between “cogitative” and “incogita-
tive” beings (IV.x.9), and assigns eternity and activity only to cogitative
beings (IV.x.10-11). As we will see below, when Locke directs his attention
to the presence or absence of “cogitation”, he is neutral about what other
properties a cogitative being may possess.

D. K. BROWN
most intimately connected with, and essential to body” (II.iv.1),
and continuing with this remark, he strengthens his commit-
ment to the cla im that solidity is
no-where else to be found or imagined, but only in matter.
And though our senses take no notice of it, but in masses of
matter, of a bulk sufficient to cause a sensation in us; yet the
mind, having once got this idea from such grosser sensible
bodies, traces it farther; and considers it, as well as figure, in
the minutest particle of matter that can exist: And finds it in-
separably inherent in body, wherever or however modified
(II.iv.1).
Even in the passage that most clearly supports the notion that
solidity is the idea of something occupying space, Locke ex-
plicitly speaks only of body as having this feature:
This is the idea which belongs to body, whereby we conceive
it to fill space. The idea of which filling of space is, that, where
we imagine any space taken up by a solid substance, we con-
ceive it so to possess it, that it excludes all other solid sub-
stances; […] This idea of it the bodies which we ordinarily
handle sufficiently furnish us with (II.xxvii.2).
The whole chapter on solidity is part of his longer discussion
of the ideas we receive from sensation, and Locke repeatedly
informs us that what we sense are bodies. Locke’s commitment
to the association of solidity with material substances runs quite
deep since, according to Locke, solidity is a primary quality of
body. As such, our idea of solidity is inseparable from our idea
of any body no matter what state that body may be in (II.viii.9).
Further, our idea of solidity is a resemblance of what actually is
in a body itself. Given Locke’s own steadfast insistence that
solidity is an idea whose proper association is with bodies, it
seems that he should somehow deny the solidity of souls.
Locke seems to do so when he explains how, when forming our
idea of the soul, we take care not to ascribe solidity to the soul:
For putting together the ideas of thinking and willing, or the
power of moving or quieting corporeal motion, joined to sub-
stance of which we have no distinct idea, we have the idea of an
immaterial spirit; and by putting together the ideas of coherent
solid parts, and a power of being moved, joined with substance,
of which likewise we have no positive idea, we have the idea of
matter (II.xxiii.15).
Locke later says that this collection of ideas in our complex
idea of spirit is peculiar to immaterial souls. He says the same
of the ideas he has here ascribed to material bodies. But, Locke
does explicitly ascribe existence, duration, and mobility both to
souls and to bodies (II.xxiii.18).4 Although our ideas of souls
and bodies do have some features in common, solidity is not
one of them. The issue at stake here is whether bodies are solid
things that occupy an expanse of space, while souls might be
more like a concentration of various powers into a point with
no spatial volume.5 The ideas of space and solidity are different
ideas. The objection at stake here asserts that the collection of
ideas in the complex idea of spirit can include existence, dura-
tion, and mobility and spatial location, but need not and does
not include solidity.
As it stands, this is a fairly devastating objection to the ar-
gument that souls are solid. For if this objection succeeds, we
would attribute solidity to souls quite in defiance of Locke’s
insistence that solidity applies only to material bodies. This line
of objection would restrict premise 2) only to bodies:
2* For a body to occupy a determinate place to the exclusion
of other bodies requires the body to be solid.
Of course, this would allow a body to be solid while allow-
ing a non-solid soul to occupy the place delimited by the body’s
solidity. This modification of 2) into 2*) thereby allows for the
cohabitation of souls and bodies, but with the metaphysical
price of eliminating criteria for distinguishing souls from each
other. If Locke is committed to 2), he is committed to the solid-
ity of souls. If he rejects 2) in favor of 2*), Locke can reject the
solidity of souls. To determine whether Locke would prefer 2)
or 2*), that is, to determine whether Locke accepts or rejects the
argument for the solidity of souls, we need to consider more
carefully Locke’s account of the distinction between solid bod-
ies and immaterial souls.
Solidity, Thinking, and the Nominal
Essence of Spirit
Locke’s published disputes with Edward Stillingfleet, the
Bishop of Worcester, offer a go od source of developme nts of h is
positions on the status of our knowledge of the immateriality of
the soul. Amongst other attacks on Locke’s Essay, Stillingfleet
accuses him of, at the very least, providing a basis for the doc-
trine that the soul is a material substance. Locke’s replies to
Stillingfleet5 are duly rife with Locke’s views on this topic. He
regularly states that the idea of spirit simply is the idea of that
substance which thinks regardless of any other qualities it may
have. Stillingfleet objects insisting that a thinking substance
cannot under any circumstances be solid. Locke replies:
Against this your lordship will argue, that by what I have
said [in IV.iii.6 of the Essay] of the possibility that God may, if
he pleases, superadd to matter a faculty of thinking, it can
never be proved that there is a spiritual substance in us, be-
cause upon that supposition it is possible it may be a material
substance that thinks in us. I grant it; but add, that the general
idea of substance being the same every where, the modification
of thinking, or the power of thinking joined to it, makes it a
spirit, without considering what other modifications it has, as
whether it has the modification of solidity or no. […] [S]ub-
stance, that has the modification of solidity, is matter, whether
it has the modification of thinking or no (Letter, 33).
When clarifying the position he held on this issue in the Es-
say, Locke plainly states that the idea of spirit does not neces-
sarily exclude solidity. Nevertheless, any thinking substance is
as such a spiritual substance. Likewise, any solid substance is
as such a material substance. Locke thus grants the possibility
that the soul is solid in the same sense that matter is solid since
he is granting the possibility that the soul simply is a material
substance. So, as we noted earlier, the conclusion to the argu-
ment for the solidity of souls implies that souls are material
substances. And, thus far, we see that Locke denies neither the
possibility that this conclusion is true nor that it is entailed by
his other commitments.
4As noted above, Locke considers their sharing of mobility notable enough
to discuss, while taking their sharing of duration and existence to be uncon-
troversial (II.xxiii.19-21).
5Locke emplo ys an analogy wit h geometrical poi nts to motivate th e attribu-
tion of change of place to spirits: “if a mathematician can consider a certain
distan ce, or a ch ange of that d istance b etween two po ints, one may cer tainl y
conceive a distance, and a change of distance between two spirits: And so
conceive their motion, their approach or removal, one from another”
(II.xxiii.19). This passage does not commit Locke to the position that souls
exist at un extend ed sp atial po ints . Instead , the co ncept ion of ch ange of place
requires only the ideas that are included in the notion of a spatial point. And,
those ideas are also in the complex idea of spirit. So, spirits can change
place. 6All references to Locke ’s r eplies to Stillingfleet are to Locke (1823).
230

D. K. BROWN
Locke’s reason for refusing to deny the possibility that souls
are solid (or rather that matter thinks) is simple: there is no
contradiction in such a supposition.7 Again in reply to Stilling-
fleet Locke says:
For I only say, that it is possible, i.e. involves no contradic-
tion, that God, the omnipotent immaterial spirit, should, if he
pleases, give to some parcels of matter, disposed as he thinks fit,
a power of thinking and moving; which parcels of matter, so
endued with a power of thinking and motion, might properly be
called spirits, in contradistinction to unthinking matter. In all
which, I presume, there is no manner of contradiction (Second
Reply, 482).
Though thought and solidity are very different ideas, Locke
finds n o contradic tion betwe en them. Bu t, he insists t hat he has
gone only so far as to allow the mere possibility that souls are
solid. Locke admits this possibility by refusing to deny that
God has the ability to grant the power of thought to any sub-
stance whatsoever. After yet again asserting that God could
give to matter a power of thought, he adds that his own capacity
to conceive of God as having this ability “is the utmost I have
said concerning the faculty of thinking in matter” (294).
Echoing this last remark much later in his Second Reply,
Locke provides us with a thought experiment to show how
conceiving of a solid substance as thinking would not “con-
found the idea of matter with the idea of spirit” (460). We are to
imagine that
God creates an extended solid substance, without the super-
adding any thing else to it, and so we may consider it at rest: to
some parts of it he superadds motion, but it has still the essence
of matter: other parts of it he frames into plants, with all the
excellencies of vegetation, life, and beauty, which are to be
found in a rose or a peach-tree, &c. above the essence of mat-
ter in general, but it is still but matter: to other parts he adds
sense and spontaneous motion, and those other properties that
are to be found in an elephant. Hitherto it is not doubted but
the power of God may go, and that the properties of a rose, a
peach, or an elephant, superadded to matter, change not the
properties of matter; but matter is in these things matter still.
But if one venture to go on one step further, and say, God may
give to matter thought, reason, and volition, as well as sense
and spontaneous motion, there are men ready presently to limit
the power of the omnipotent Creator, and tell us he cannot do it;
because it destroys the essence… (460).
For Locke, to “supera dd” a quality to a substance is to add to
a substance something not contained in its essence. In this
thought experiment, motion, vegetative life, sense, spontaneous
motion, thought, reason, and volition are each successively
superadded to what was originally merely an extended solid
substance conceived as being at rest. All of these superadded
qualities are distinct from extension, solidity, and rest so that to
conceive of such bare matter as having any of these qualities,
we must conceive of them as other than or not entailed by ex-
tension, solidity and rest. Thus the superadded qualities are
distinct from other qualities inconceivable apart from extension,
solidity or rest. Shape is a quality that is not superadded to bare
matter, as the absence of shape would contradict the bare notion
of matter contained in the essence of solid extension. Motion,
however, goes beyond solid extension such that conceiving a
solid extended substance to be moving is to conceive of some-
thing more than a mere solid extended substance. Solid exten-
sion and solid moving extension are different in that the latter
has a quality (viz. motion) superadded to what’s in the former
(viz. only solid extension), but this difference in no way renders
the conception of the one incompatible with the conception of
the other. Instead, what we have here are two ways of conceiv-
ing of solid extension. One way is only in virtue of solid exten-
sion itself, and the other way is of solid extension modified in
some other additional way.
We are perfectly able to conceive of substances as having
additional qualities incomprehensibly, yet undeniably, associ-
ated with them. For Locke, gravity serves as just such an exam-
ple of a quality superadded to matter. Gravitational attraction is
thereby quite different from the transfer of motion by impact.
Solidity implies impenetrability, and, Locke thinks, thus affords
us an easy conception of interaction of bodies by impact. How-
ever, a primitive attractive force in no way is implied by solid-
ity and extension alone. Yet, he acknowledges that given New-
ton’s physics, a force of gravity is undeniably associated with
matter. Thus gravitational at traction is yet another instance of a
superadded power not otherwise included in the mere notion of
a solid extended substance (Second Reply, 463, 467-46 8). Loc ke
asks whether any of the subsequent items on the list of super-
added qualities from his thought experiment confound the no-
tion of extended solid substance. Our inability to conceive of
the basis for the power of a more primitive notion of extended
solid substance to move, live, sense, or think is completely re-
mote from our comprehension, but this ignorance in no way
implies a contradiction between the ideas of the superadded
qualities and of the more primitive substance that receives the
power to sustain the superadded qualities.
In the correspondence with Stillingfleet Locke has greatly
elaborated on his commitment to the possibility that souls are
solid substances. The argument for the solidity of souls goes
farther than just the mere possibility that souls are solid. Since
for Locke the identity of souls is the same as the existence of
souls, (Essay, II.xxvii.3), for souls to be solid, they would be
material substances. And, according to Locke, “from thinking
experimented in us, we have a proof of a thinking substance in
us, which in my sense is a spirit” (Letter, 32). So there are souls
insofar as there are thinkers. Despite the certainty of the exis-
tence of thinking substance, the controversy with Stillingfleet is
over whether or not thinking substance is immaterial. Locke
further claimed
if your lordship means by a spiritual an immaterial sub-
stance, I grant I have not proved, nor upon my principles can it
be proved, […] that there is an immaterial substance in us that
thinks” (Letter, 33).
The flip side of this claim is that Locke’s principles also can-
not prove that the thinking substance in us is material. Given
his caution about drawing unwarranted conclusions about the
necessity of the immateriality of the soul, has Locke in the Es-
say provided for a conclusion that his replies to Stillingfleet do
not allow us to draw?
7Of course Lo cke also holds that the ideas of s ubstances are in herently con-
fused in virtue of including in them the confused notion of substratum or
unknown support for the qualities of the substance. However, what is at
stake here concerning the contradiction is not anything that invokes the
confused notion of support for qualities, but instead whether the collection
of qualities includes some that contradict others. For instance, a moving
thing at rest would be a contradiction regardless of what else might be ob-
scure about the idea of the thing.
In Book III of the Essay, Locke draws some crucial distinc-
tions concerning substances and essences. A nominal essence
consists of ideas grouped together by a supposition that some-
thing independent of them unites them into one thing. That is,

D. K. BROWN
when we form an idea of the soul as that which thinks, we con-
join ideas of various ways of thinking by supposing that some-
thing we call a soul really does unite the various thoughts into a
single thing. In doing so, we have given the names “soul” and
“spirit” to that unified conjunction of ideas. Those ideas thus
form the nominal essence applicable to the soul/spirit. But,
these names refer not only to the ideas that form a nominal
essence, but also to a supposed but unknown thing that we take
to be the real bearer of the power of thinking. If this supposed
bearer of the powers contained in the nominal essence really
exists, its real nature is its real essence. So, whatever really
thinks in us has a real essence that is responsible for thinking.
Locke thinks that our ideas of substances are nominal essences
(III.vi.11). His resistance to Stillingfleet’s concerns about the
real nature of spiritual substance rests largely on the conviction
that the real essences of substances are unknown (III.vi.9). So,
Locke’s denial that his principles imply anything about the
materiality or immateriality of spirit is genuine since Locke
takes Stillingfleet’s concerns to be about the real essence of
spirit. What’s at stake in the argument for the solidity of souls
is whether to include solidity in addition to the power of think-
ing in the nominal essence of spiritual substance. It is important
to see that Locke’s discussion of our ideas of spiritual and ma-
terial substance should be subsumed under his account of such
nominal essences, and not real essences.
As we saw in his replies to Stillingfleet, Locke claims that it
is indisputable that we have an idea of that in us which thinks.
But nevertheless we lack an idea of what it is in us that thinks
apart from that it thinks. So we therefore lack any basis for any
claims about the real essence of what thinks apart from that it
thinks. On these grounds any exclusion of any quality from the
essence of what thinks besides that which directly contradicts
the nature of thought would be unwarranted. We also saw that
Locke could find no contradiction between thought and solid-
ity—they are different ideas, but in no way exclude each other.
Thus, strictly on the grounds of avoiding contradiction, any
exclusion of thought from the nominal essence of a solid sub-
stance, or any exclusion of solidity from the nominal essence of
a thinking substance, would be unwarranted. But, we may also
think of a thinking material being as being composed of two
different nominal essences, if we so wish. In neither case do we
peer beneath our constructed nominal essences into the under-
lying real essences, since real essences of substances are utterly
beyond our comprehension. Locke has carefully composed his
response to Stillingfleet in reference to this position on the lim-
its of our knowledge of essences. Thus, the claim that the iden-
tity of souls requires souls to be solid takes the solidity of souls
to render them as material substances. And since this claim is
not about the real essence of souls, but rather about their nomi-
nal essence, Locke’s explicit commitment to the possibility that
matter thinks is consistent with his assertion that we do have an
idea of souls as immateri a l .
But, since the conclusion of the argument for the solidity of
souls requires that the nominal essence of souls (or spiritual
substances) include the idea of solidity for souls to have iden-
tity, why does Locke think we have, and are justified in having,
an idea of immaterial spirit? When Locke discusses our ideas of
substances as comprising only their nominal essences, he states
that the idea of spirit is derived from reflection on the opera-
tions of the mind “without consideration of matter” (III.vi.11).
In his chapter on our ideas of substances, when Locke charac-
terized our ideas of body and spirit as being distinct, he finished
his account by noting that he has presented “our complex ideas
of Soul and Body, as contra-distinguished” (II.xxiii.22). These
are the ideas of body and soul we happen to have. The ideas we
form of thought and solidity exclude each other for no better
reason than that we simply don’t know how adequately to relate
them causally (IV.iii.28-29). Careful thinking will keep us from
concluding too much from our ignorance such as that thought
and solidity cannot be subsumed in a single nominal essence, or
are not in fact related in an underlying real essence beyond our
comprehensions. Neither of these claims is within the scope of
our knowledge.
The Irrelevance of Solid Souls to Locke’s
Concerns about Identity
So what does this mean for the choice between (2) and (2*)?
The argument we’ve been considering says that thought and
solidity ought to be related—that is, if souls have identity.
Locke’s exchange with Stillingfleet offers another significant
perspective on the argument for the solidity of souls that bears
directly on this point: Locke simply does not care whether
souls meet any general conditions for identity. Stillingfleet
senses this and argues that Locke’s implication that spirit is a
material substance eliminates the possibility of resurrection.
But, Locke doesn’t think that he needs an account of the iden-
tity of souls to allow for the possibility of resurrection. Locke
has three accounts of identity: of substances, of organisms, and
of persons. Locke’s general criteria of identity underlies each
kind of identity, but the specific criteria for identity of one sort
does not account for the identity of things of another sort. Each
particle of matter has its identity as a material substance. The
identity of a person is determined by sameness of conscious-
ness, such as in the relation one’s present consciousness bears
to some set of memories or to expectations concerning the fu-
ture (II.xxvii.9). According to Locke, the continuous history of
self-related consciousness need not belong to a single substance,
spiritual or otherwise, and may belong to several (II.xxvii.10).
Thus, the identity conditions for souls as substances do not
underwrite the identity of a person.8 Soli dity is therefore irrele -
vant to the identity of persons. Since the consequences of the
general idea of identity and solidity on our idea of souls as sub-
stances neither help nor hinder Locke’s account of personal
identity, Locke has no pressing reason to draw explicitly the
inference that souls are solid in his account of personal identity,
and neither has he any reason to work out its implications.
Given Locke’s account of personal identity, the identity of souls
is simply irrelevant to Locke’s purposes. And, this is why
Locke could reasonably remain ambivalent about developing
what would give souls their proper identity conditions.9
Locke’s treatment of the identity of living organisms is sug-
gestive of the sort of place thinking matter would play in
Locke’s account of identity. The identity of an organism is, on
Locke’s account, distinct from the identity of each of its con-
8Only Locke’s commitment to this view, rather than his defense of it, is
relevant here.
9Of course, the remarks here about personal identity are sketchy and cer-
tainly merit fuller treatment elsewhere. Suffice it to say that an account o
f
Lockean personal identity following up on these suggestions would restrict
persons to being “modes” and not substances. This would put such an ac-
count at odds with the one advanced by Jonathan Bennett (2001: ch. 39)
wherein Lockean persons are quasi-substantial. But it might be friendly to
the account developed by Gideon Yaffe (2007), even though he refrains
from invoking solidity as a universal criterion for the spatiotemporal exclu-
sion of substances.
232

D. K. BROWN
stituent parts. Organisms are systems of particles of matter
arranged so as to form the parts of the organism. Life is the
continued functioning of those parts. Organisms constantly
swap particles of matter with their surroundings without losing
their identity as the same organism. So long as the parts of an
organism serve their function within the organism, it matters
not which particles compose those parts (II.xxvii.4-8). Thus the
identity of an organism when considered as an organism is
distinct from the identity of an organism as a lump of matter.
Yet, the organism still is composed entirely of matter such that
were the material parts of an organism to be annihilated, so
would the organism. Thus organisms require that their parts be
composed of material substances for the organism to have its
identity as an organism.
Following Locke’s thinking matter hypothesis, suppose
thought, as well as the other powers of the soul, are powers
exclusively of systems of matter fitly disposed to think. On this
hypothesis the identity of that in us which thinks would, just as
in the case of organisms, require there to be a parcel of solid
material substance arranged in such a manner as to support
God’s superaddition of the power of thought. Thus on this hy-
pothesis, the identity of a soul as a thing that thinks requires the
soul to be a substance whose parts are solid. And just as in the
case of organisms, the conditions for the identity of the thing
that thinks, when considered as a thing that thinks, would not
be the same as the conditions for the identity of its constituent
solid parts as a mere collection of matter. According to the
argument that souls are solid, the thinking matter hypothesis
follows from our ideas of solidity and of the general conditions
for identity, even if Locke didn’t openly want or care to connect
the two. Had he acknowledged this hypothesis as a conse-
quence of his project, he wouldn’t have to give up any of his
other positions. Locke wouldn’t even have had to abandon his
claim that the immateriality of the soul is the more probable
opinion, so long as such a claim is restricted to the unverifiable
realm of real essences. Locke stakes his claim to the high
probability of souls being immaterial as a consequence of his
argument for the existence of God, and as a certainty expressed
in scripture.10 It is at least a plausible suggestion that Locke has
no independent philosophical support for his expressed confi-
dence in the likelihood that souls are immaterial. As far as the
philosophical implications of granting the hypothesis that mat-
ter thinks, Locke need only have restricted its scope to the
nominal essence of thinking substance to ensure that his com-
mitments could find support, or escape conflict, with his project
in the Essay, and that is precisely what he did. It is in this sense
that the solidity of souls is not at all a threat to Locke’s further
purposes concerning personal identity, while it conforms to his
general account of the identity of substances, given his nomi-
nalism about essences.
Evading the Solidity of Souls
Simply attributing solidity to souls to account for their iden-
tity conditions is a novel suggestion. This can be seen better by
examining some of the leading commentary on related topics in
Locke scholarship concerning problems surrounding the iden-
tity of souls.
That Locke found it possible for thinking to be a power of
solid substance is beyond controversy. The importance of this
hypothesis to Locke’s account of the limits of knowledge con-
cerning substance is non-negotiable for Edwin McCann. He
examines the notion that thought could be superadded to matter
as expressing Locke’s commitment to mechanical corpuscu-
lareanism. McCann takes Locke’s commitment to thinking
matter as rising no higher than a possibility, albeit a vitally
important possibility to be maintained in any tenable interpreta-
tion of his thought. For instance, McCann rejects an attempt to
solve a conundrum about the nature of substance in general
partially on the grounds that “it conflicts with one of Locke’s
central doctrines, that of the possibility of thinking matter”
(1994: p. 80). For McCann, though, this remains just a possibil-
ity.11
Nicholas Jolley takes a stronger stance than McCann on
Locke’s commitment to thinking matter by suggesting that
Locke had a basis for a deeper commitment to materialism than
he openly admits:
Locke agrees of course that materialism itself has its difficul-
ties, but once these are recognized, there is no further puzzle
posed by the supposition that body is intermittently the subject
of mental states or properties. Locke of course does not explic-
itly draw this moral, but there are enough hints in the Essay to
suggest that he wishes his readers to draw the moral for them-
selves (1999: p. 93).
Jolley verges on attributing the solidity of souls to Locke
since he presents an inference to materialism from Locke’s
published works that Locke refrains from drawing himself.12
However, Jolley does not connect this implicit materialism to
Locke’s general criteria for the identity of substances. Neither
does Jolley address at all Locke’s claims that souls be spatio-
temporally distinguished from other substances.
The motivation for moving beyond the attenuated position
ascribed to Locke by McCann and Jolley arises from some
difficulties in Locke’s treatment of identity. McCann urges a
general account of identity that takes spatiotemporal continuity
as comprising the basis for the identity of any simple substance
including souls (1987: p. 59). McCann develops at length the
meaning of this principle as it applies to atoms—making much
use of solidity (and indivisibility) as the basis for determining
the continued spatio temporal existence of particles of matte r :
As bodies, or extended solid substances, atoms by their so-
lidity exclude all other bodies or material things from the vol-
ume of space which, thanks to their extension, they at any time
occupy (60).
While he subjects the identity of atoms to extensive inquiry,
McCann abandons the question of what determines the spatio-
11See also McCann (2001: pp. 92-93) and McCann (2007: pp. 177-185).
Michael Ayers (1991) opposes many of McCann’s positions, but agrees that
the possibility of thinking matter remains merely a possibility (Vol. 2, p. 149).
12Jolley also presents Locke’s position as suffering from tensions arising
from the formulation of his hypothesis of thinking matter. Jolley wonders
why matter would have to be “fitly disposed” to receive a quality that God
would superadd to it (1999: pp. 95-99). He thinks a fit disposition for su-
p
eraddition would render superfluous the need for God to superadd a quality
since a fit disposition would itself be the underlying sufficient condition for
the quality to be superadded. But contrary to Jolley, if we take “fitly dis-
posed” simply to mean “determined by God to have some quality super-
added,” it is difficult to find a conflict here. Fit disposition could require the
absence of a quality that, given all the other qualities a thing has, is flatly
inconsistent with the other quality God superadds. Given the absence o
f
some quality, God’s superaddition of some other quality to those qualities
would yield a total set of qualities including no contradictory qualities.
N
evertheless, while Jolley admits that Locke’s position appears to be con-
sistent, he is reticent to say that Locke is not confused in allowing the possi-
bility that thought be superadded t o matter (98).
10Letter, 36.

D. K. BROWN
234
temporal identity of souls.13 McCann thus holds that though the
same general principle determines the identity of atoms as well
as finite intelligences, Locke has the apparatus to explain the
application of the principle to simple material substances, but
neglects to do so for spirits. McCann refrains from calling that
common principle “solidity”, but nor does he say why solidity
is not that principle.14
One commentator unafraid of explicitly invoking solidity to
account for the identity of souls is Thomas Lennon. Inquiring
into Locke’s atomism, Lennon lands on the problem concerning
the application of Locke’s general criteria for identity to both
bodies and souls:
What excludes every other body from the place-time of a
body is its solidity, or the impenetrability resulting from its
solidity. But what is it that excludes another spirit from the
place of a spirit? Nothing from what I can make out in Locke.
Only if there is but one sort of being, namely, solid being, can
Locke’s principle of individuation be made to work. Creation of
any sort, then, is the creation of solidity (1992: p. 310).
This passage seems to be a powerful endorsement of the ar-
gument for the solidity of souls. Like Jolley after him, Lennon
argued that Locke’s rejection of the Cartesian thesis that the
mind always thinks bears a strong affinity with the thinking
matter hypothesis. Lennon doesn’t carry this argument nearly
as far as Jolley since Lennon holds the connection between the
two positions to be a mere affinity. And though Lennon pre-
sents an interesting and convincing case for Locke’s materialist
tendencies, if not his outright materialism, the bulk of his tex-
tual evidence from Locke’s own hand concerns the books
Locke read and commented on, as well as other unpublished
errata. As far as Locke’s published commitments go, Lennon
maintains the softened position, largel y in accord with McCann,
that Locke only allowed for the possibility of thinking matter.
The argument for the solidity of souls nonetheless very strongly
accords with Lennon’s thesis that the conditions for the use of
the relation of identity to individuate simple substances apply
univocally to finite bodies and finite spirits.
Conclusion
We have seen agreement among leading commentators that
Locke has endorsed the possibility of holding one substance
both to be solid and to bear the power of thought. Those who
focus on the general conditions of identity note that Locke has
not provided any account of the identity of finite spirits, while
he has provided solidity as a means of doing so for finite bodies.
Yet, they also acknowledge that Locke takes exclusion from
place to be the principle of identity for both body and soul.
Those who delve into Locke’s mechanism further discover that
Locke treats solidity as the filling of space to exclude others
from occupying it. Locke also hold that solidity and thought
can coexist as qualities in one substance. So why be coy about
the very notion in Locke’s suite of ideas that would quite effec-
tively account for the identity of souls? The identity conditions
for souls thus ought to be regarded as relying upon the idea of
solidity. It unifies his discussions of substances and essences in
his replies to Stillingfleet. And, it serves a vital interpretive role
in unifying Locke’s general account of identity, a central notion
in perhaps his most novel and interesting lasting contribution to
philosophy, his account of personal identity.
Acknowledgements
Portions of this paper were presented at conferences at
Claremont Graduate University, University of California, Irvine,
and University of Turku, Finland. I owe special thanks to Kurt
Smith, Patricia Easton, Olli Koistinen, Thomas Lennon, Edwin
McCann, Alan Nelson and Nicholas Jolley, and to the anony-
mous reviewers for this journal for their helpful comments and
suggestions in the preparation of this paper.
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13M. R. Ayers (1991) agrees with McCann that spatiotemporal continuity is
the basis for the principle of individuation for both matter and spirit, but
adds that “Locke did not explain what it is for a finite intelligence to occupy
a place” (Vol. 2, p. 209).
14Martha Bolton (1994) agrees with McCann and Ayers that Locke allows a
possibility of thinking matter and considers souls to have spatial locations,
while also refraining from attributing solidity to souls as the principle al-
lowing them their spatial location. Her analysis of the identity of body does
not explicitly refer to solidity, but she does use the same texts I considered
above in my determination that solidity is the idea required for spatiotempo-
ral exclusion. Yet Bolton remarks that “[f]inite intelligences are also simple
substances but nothing is said about their identity” (114; s ee also 124 -125).