## 1. Introduction



Two features of Locke's intellectual landscape are most salient
for understanding his philosophy of science, one of which concerns the
new science's methodology, and the other its content. First,
then, is the new methodological approach to understanding the natural
world. This approach is accompanied by profound shifts in disciplinary
boundaries and in conceptions of induction and scientific knowledge.
Locke's reaction is mostly progressive. Impressed by
experimental methods and cognizant of their poor fit with the
Aristotelian ideal, he defines a distinct kind of knowledge, one
inferior to genuine scientific knowledge but appropriate to human
sensory capacities. In so doing, he develops an epistemological basis
for the new, experimental philosophy. Yet his reaction also has its
conservative aspect, one that some see as having limited him in the
face of the new science's evolving methodology. He retains an
ideal notion of scientific knowledge as demonstrative and certain, and
while the speculative systems of the Aristotelians and the Cartesians.
are the two main targets of his *Essay*, he shares that ideal
with them.



The second salient feature is the dominant scientific theory of
Locke's day: the new science's corpuscular hypothesis. As
defined for the purposes of this article, the corpuscular hypothesis
(i) takes observable bodies to be composed of material particles or
corpuscles, (ii) takes impulse (action by surface impact) to be either
the primary or sole means of communicating motion, and (iii) attempts
to reduce colour and other qualities at the level of observable bodies
to the primary or inherent properties of the particles composing those
bodies. In what may be called its orthodox version ("pure
mechanism," as Ayers (1981, p. 212) calls it) the corpuscular
hypothesis restricts those inherent properties to size, shape, number,
and motion, and holds that all other qualities and operations are
explicable in terms of that restricted set of properties. The orthodox
version thus implies a *proviso of contact action*--that
bodies causally interact only locally, by impact, such that unmediated
action at a distance is deemed impossible. (Although a number of
commentators use the terms 'corpuscular hypothesis' and
'mechanism' interchangeably, distinguishing them has
certain benefits. For instance, it permits us to classify Isaac Newton
among the corpuscularian theoristis without engaging the debate about
whether he adhered to the contact action proviso. The definitions
given here also agree largely with those in the entry on
 John Locke.)
 Plenist and atomist versions of the corpuscular hypothesis may be
distinguished. Plenist theorists deny the void and assert a plenum of
matter, as Descartes does by identifying matter with extension, and
though these theorists speak of particles, their particles are not
atoms, being infinitely or at least indefinitely divisible. Atomist
theorists, by contrast, accept the void and take the particles or
corpuscles comprising compound bodies to be indivisible, or at least
probably so. Since Locke's sympathies clearly lie with the
atomist version, the term 'corpuscular hypothesis' shall
refer to that variant throughout this article unless indicated
otherwise. Locke develops central theses of the *Essay* in
close conjunction with the corpuscular hypothesis. The most notable of
these is the distinction between real and nominal essences, the former
denoting a substance's internal constitution and the latter
denoting the observable properties we use to name or categorize it.
Although Locke develops that distinction in connection with the
primary-secondary quality distinction associated with corpuscular
theorists, including his mentor, Robert Boyle, it does not follow that
the two distinctions are interchangeable. Locke often treats the
hypothesis with skepticism, and its status and purpose are a source of
controversy.



This article examines questions connected with the two salient
features noted, and in connection with the first, it also examines
Locke's relationship to Newton, a figure instrumental to the
changing conceptions of scientific knowledge. Section 2 addresses
questions connected to those conceptions. What does Locke take science
(*scientia*) or scientific knowledge to be generally, why does
he think that *scientia* in natural philosophy is beyond the
reach of human beings, and what characterizes the conception of human
knowledge that he develops for natural philosophy? Section 3 addresses
the question provoked by Locke's apparently conflicting
treatments of the corpuscular hypothesis. Does he accept or defend the
corpuscular hypothesis? If not, what is its role in his thought, and
what explains its close connection to key theses of the
*Essay*? Since a scholarly debate has arisen about the status
of the corpuscular hypothesis for Locke, Section 3 reviews some main
positions in that debate. Section 4 considers the relationship between
Locke's thought and Newton's. All citations of *An
Essay Concerning Human Understanding* are indicated by
'E', followed by the book and section numbers. Page
numbers referring to the Nidditch edition are also provided.


## 2. Locke on knowledge in natural philosophy: *Scientia* and human knowledge



Locke's great epistemological contribution to philosophy is a
conception of human knowledge suitable for the experimental science of
his day, one that in natural philosophy will replace the old,
Aristotelian conception. According to the Aristotelian conception,
scientific knowledge--*scientia*--is certain
knowledge of necessary truths, which can be expressed in syllogistic
form, the conclusion following from self-evident premises. In the
domain of natural philosophy, it is certain knowledge of real
essences. Although Locke does not seriously entertain a radical
skepticism--finding "a very manifest difference between
dreaming of being in the Fire, and being actually in it" (E
IV.ii.14, pp.537-538)--he recognizes that the demands of
*scientia* are too stringent for the new, experimental science.
Nevertheless, the concept of *scientia* plays an important role
as he develops his conception of the probabilistic sort of knowledge
that is possible for humans in the domain of natural philosophy: it
serves as a foil as he explains why humans must settle for
probabilistic knowledge in natural philosophy, where *scientia*
will always remain out of reach. *Scientia* can serve as a foil
because it is attainable for human beings in certain domains; however
Locke thinks that for nobler spirits, notably angels, it is also
attainable within natural philosophy.



This section begins by reviewing the history of the concept of
*scientia* and the factors that prevent human beings from
attaining it in natural philosophy, which impel Locke to develop an
alternative conception of human knowledge there. This section also
explains what Locke thinks *scientia* in natural philosophy
would amount to and the lesser human knowledge that must serve us in
its place.


### 2.1 Historical roots of *scientia*



The conception of genuine scientific knowledge that Locke inherits and
to some extent retains, *scientia*, has its roots in Aristotle,
specifically in the beginning of Book I, SS2 of the *Posterior
 Analytics*.[1]
 For Aristotle, scientific knowledge can have only necessary truths as
its objects and requires a knowledge of causes. Accordingly,
scientific knowledge of a fact requires knowing its necessity by
knowing its necessary relations to its causes. Although there are
manifold causes, the main one at issue here is the formal
cause--the nature or essence--as Aristotle indicates
 elsewhere.[2]



> 
> We suppose ourselves to possess unqualified scientific knowledge of a
> thing, as opposed to knowing it in the accidental way in which the
> sophist knows, when we think that we know the cause on which the fact
> depends, as the cause of that fact and of no other, and, further, that
> the fact could not be other than it is....The proper object of
> unqualified scientific knowledge is something which cannot be other
> than it is (Aristotle, *Posterior Analytics*, I.2).
> 



The knower's epistemic stance toward a necessary truth and its
relation to causes is one of certainty. The certain fact is
demonstrable via a syllogism in which the premises are self-evident,
requiring no demonstration themselves.



> 
> We do know by demonstration. By demonstration I mean a syllogism
> productive of scientific knowledge, a syllogism....The premisses
> must be primary and indemonstrable; otherwise they will require
> demonstration in order to be known, since to have knowledge, if it be
> not accidental knowledge, of things which are demonstrable, means
> precisely to have a demonstration of them. The premisses must be the
> causes of the conclusion, better known than it, and prior to it; its
> causes, since we possess scientific knowledge of a thing only when we
> know its cause; prior, in order to be causes; antecedently known, this
> antecedent knowledge being not our mere understanding of the meaning,
> but knowledge of the fact as well (Aristotle, *Posterior
> Analytics*, I.2).
> 



The stipulation that the premises of a scientific demonstration must
be indemonstrable, that is, self-evident, leads to a *prima
facie* difficulty. This conception of scientific knowledge is
intended to encompass not only conceptual propositions, but also
propositions about the real natures or essences of substances, which
are propositions about the world. As with any other demonstration, a
demonstration in natural philosophy must have premises that are
self-evident, since otherwise a regress would ensue. Propositions
about the world are experience-based, however, and therefore, it is
not clear how a demonstration's premises could be
self-evident.



It would be overstating matters to say that this problem appears only
from a contemporary perspective; there was some recognition of it in
the ancient and medieval
 periods.[3]
 Still, before the advent of the experimental science, the problem was
not strongly felt because experience was understood in a different
way. For one thing, the notion of an experiment--an artificially
constructed, single event or series of events designed to test for a
predicted outcome--did not exist. Moreover, among the scholastic
Aristotelians, a single, naturally occurring event could not by itself
be regarded as revealing of natural processes; such an event could be
a "monster," that is, an event that conflicts with nature
rather than having been produced by
 it.[4]
 Events as generally experienced, however, were regarded as being
revealing of nature and consequently, they could provide the universal
truths needed as premises in the
 syllogism.[5]
 How was the gap between events as usually experienced--which
still amounts to a limited sample of evidence--and the universal
claim derived from them bridged? It is only from the modern and
contemporary perspectives that such a gap exists to be bridged. For
Aristotle and medieval thinkers, human faculties are so constituted as
to be able to apprehend nature, that is, to discern the essences of
 substances.[6]
 In short, then, because the internal essences that form the content
of natural philosophy are real, and because our faculties are
constituted to apprehend those real essences, natural philosophy can
be a science; it is a domain in which certain, demonstrative knowledge
can be had, despite its dependence upon experience.



The exemplars of scientific knowledge, to be sure, are conceptual
disciplines, not only geometry, but also rational theology, the latter
being the quintessential science for the
 medievals.[7]
 Yet for the most part, natural philosophy stands side by side with
conceptual disciplines, even into the modern period. Bacon, though
associated with induction, accepts the demonstrative conception of
scientific knowledge, and so does Galileo, who uses experiments to
reveal fundamental principles (though also using them in other
 ways).[8]
 As empirical methods are refined and more widely applied, however,
the belief that natural philosophy can stand under the umbrella of
*scientia* comes under increasing
 pressure.[9]
 Some thinkers resist the pressure, most notably Descartes, who
derives his laws of nature by an a priori reflection upon God's
nature, and, placing his confidence in these rationalist methods,
denies that his third law of nature is undermined when observations of
colliding bodies conflict with
 it.[10]
 Yet for the experimentalists themselves, including Locke's
mentor, Boyle, observations and experiments are the primary route to
knowledge. (It should be noted, however, that most advocates of
experimentalism did not consider speculation wholly illegitimate,
instead insisting that it be delayed until considerable experimental
and observational evidence had been amassed; see Anstey (2011, pp.
4,5).) It is this approach that puts natural philosophy on the path to
Hume's problem of induction and most influences
 Locke.[11]


### 2.2 *Scientia* in natural philosophy and obstacles to human attainment



As indicated earlier, *scientia* serves as the backdrop against
which Locke develops the conception of the type of knowledge that is
humanly possible in natural philosophy. Having retained the ideal of
*scientia* exemplified by geometry while also having
assimilated the significance of Boyle's experimental method,
Locke is driven to his characteristic pessimism about the kind and
extent of knowledge possible for us in natural philosophy. "The
meanest, and most obvious Things that come in our way, have dark
sides, that the quickest Sight cannot penetrate into" (E
IV.iii.22, p. 553). Due to the weakness of our faculties, Locke
suspects, "natural Philosophy is not capable of being made a
Science" (E IV.xii.10, p. 645). This section considers
Locke's general notion of *scientia*, what would be
required for *scientia* in natural philosophy, and the
obstacles that prevent human beings from attaining *scientia*
in that domain.


#### 2.2.1 *Scientia* in general



Locke takes knowledge generally to consist in "the perception of
the connexion and agreement, or disagreement and repugnancy of any of
our Ideas" (E IV.i.1-2, p. 525), and among the three kinds
of knowledge that he distinguishes--intuitive, demonstrative, and
sensitive--the former two are kinds of certain knowledge.
Intuitive and demonstrative knowledge differ in the number of
intuitions involved, and consequently, differ in their degree of
certainty (E IV.ii.14, pp.537-538). Intuitive knowledge is the
most certain because the truth is grasped immediately. There are no
intermediate steps, and doubt is impossible because the mind can no
more avoid recognizing the truth than the open, functioning eye could
avoid seeing light when turned toward the sun (see E IV.ii.1, p. 531).
Demonstrative knowledge, though also qualifying as certain, is less so
because it involves intermediate steps. We cannot grasp immediately
that the three angles of any triangle are equal to two right triangles
but must instead construct the steps of a proof. Upon doing so and
grasping the connections among the proof's steps, we have
demonstrative knowledge (E IV.ii.2-3, p.531-532).
Intuitive and demonstrative knowledge are forms of *scientia*,
then, which Locke defines as "certain universal Knowledge"
(E IV.iii.29, p. 559). Mere "particular matters of fact"
(E IV.iii.25, pp. 555-56) do not qualify.



To understand Locke's notion of *scientia*, we must
consider its objects: real essences and the necessary connections that
flow from them. According to the Aristotelian view, a single essence
both grounds the properties of a thing, making it what it is, and
provides the basis for classifying it. Repudiating that view, Locke
draws a distinction between real and nominal
 essences.[12]
 Whereas the nominal essence consists of the set of observable
qualities we use to classify a thing (which implies that the nominal
essence could vary across time or communities), the real essence (or
real or internal constitution, as he sometimes writes) is that which
makes a thing what it is.



> 
> Essence may be taken for the 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*. This is the proper original
> signification of the Word, as is evident from the formation of it;
> *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 (E III.iii.15, p. 417).
> 



Discussion of real essence often focuses upon the real essences of
material substances, and in that case we say that the real essence is
the causal ground of the substance's perceivable properties (for
further details and a useful chart, see the entry by Jones,
"Locke on Real Essence"). As will be discussed in a
subsequent section, many commentators interpret Locke as identifying a
material substance's real essence with some subset of its
constituent corpuscles' primary qualities, but that
identification presums Locke's acceptance of the corpuscular
hypothesis (e.g., Osler 1970, p. 12; Mandelbaum 1964, p.1). According
to another interpretation, the real-nominal essence distinction is
metaphysical, and thus more fundamental than the primary-secondary
quality distinction, which is a physical distinction, belonging as it
does to a particular physical theory, the corpuscular hypothesis. Yet
leaving that debate aside for the moment, we may note from the first
sentence of the above-quoted passage that Locke does not restrict the
notion of real essence to substances. This means that we can speak of,
say, the real essence of a triangle, understanding it as that which
grounds the triangle's qualities, making it what it is.



Since a substance's qualities flow from its real essence, having
scientific knowledge of a substance requires knowing both its real
essence and the necessary connections between that and its other
qualities. Geometry serves as an exemplar, as it did for so many of
Locke's predecessors. In knowing what a triangle is, we cannot
conceive things being otherwise than that the sum of its three angles
equals the sum of two right angles.



Such knowledge is so certain that we cannot conceive even of God
having made things otherwise:



> 
> Thus the *Idea* of a right-line Triangle necessarily carries
> with it an equality of its Angles to two right ones. Nor can we
> conceive this Relation, this connexion of these two *Ideas*, to
> be possibly mutable, or to depend on any arbitrary Power, which of
> choice made it thus, or could make it otherwise (E IV.iii.29, pp.
> 559-560).
> 



*Scientia* is possible in another conceptual domain also:
morality. Morality is characterized by discernible necessary
connections, and Locke is adamant that we can have the same level of
certainty there as in geometry.



> 
> *Where there is no Property, there is no Injustice*, is a
> Proposition as certain as any Demonstration in *Euclid*: For
> the *Idea* of *Property*, being a right to any thing;
> and the *Idea* to which the Name *Injustice* is given,
> being the Invasion or Violation of that right; it is evident,
> that...I can as certainly know this Proposition to be true, as
> that a Triangle has three Angles equal to two right ones (E IV.iii.18,
> pp. 549-50).
> 


#### 2.2.2 *Scientia* in natural philosophy



What would be required for *scientia* in natural philosophy?
Since *scientia* generally concerns real essences, and since
natural philosophy for Locke concerns material substances and their
powers, *scientia* in natural philosophy would be knowledge of
material substances' real essences and their necessary
connections to the qualities flowing from them.



If we could have *scientia* in natural philosophy, we could
know a substance's qualities without making observations or
experiments. To take one of Locke's frequent examples, if we
could know gold's real essence, we would then know its
qualities, even if not a single sample of gold existed.



> 
> Had we such *Ideas* of Substances, as to know what real
> Constitutions produce those sensible Qualities we find in them, and
> how those Qualities flowed from thence, we could, by the specifick
> *Ideas* of their real Essences in our own Minds, more certainly
> find out their Properties, and discover what Qualities they had, or
> had not, than we can now by our Senses: and to know the Properties of
> *Gold*, it would be no more necessary, that *Gold*
> should exist, and that we should make Experiments upon it, than it is
> necessary for the knowing the Properties of a Triangle, that a
> Triangle should exist in any Matter, the *Idea* in our Minds
> would serve for the one, as well as the other (E IV.vi.11, p. 585).
> 



Which qualities exactly would we be able to deduce? We would be able
to deduce a substance's tertiary qualities, that is, its powers
to produce certain effects in other
 substances.[13]
 If we knew the real essences of opium and hemlock, then just as if we
were performing a geometric deduction, or just as a locksmith
understands why a given key will open one lock rather than another, we
could deduce that opium produces sleep, that hemlock causes death and
why each substance produces its effects.



> 
> I doubt not but if we could discover the Figure, Size, Texture, and
> Motion of the minute Constituent parts of any two Bodies, we should
> know without Trial several of the Operations one upon another, as we
> do now the Properties of a Square, or a Triangle. Did we know the
> Mechanical affections of the Particles of *Rhubarb, Hemlock,
> Opium*, and a *Man*, as a Watchmaker does those of a Watch,
> whereby it performs its Operations, and of a File which by rubbing on
> them will alter the Figure of any of the Wheels, we should be able to
> tell before Hand, that *Rhubarb* will purge, *Hemlock*
> kill, and *Opium* make a Man sleep....The dissolving of
> Silver in *aqua fortis*, and Gold in *aqua Regia*, and
> not *vice versa*, would be then, perhaps, no more difficult to
> know, that it is to a Smith to understand, why the turning of one Key
> will open a Lock, and not the turning of another (E IV.iii.25, pp.
> 555-56) (cf. Boyle, who had had the same idea, explaining it at
> length in *The Origin of Forms and Qualities*, 1666, pp.
> 16-19).
> 



Knowing real essences would enable us to deduce tertiary qualities,
then. What about secondary qualities, however? Here, matters are
initially less clear; Locke seems to be saying that while having more
acute senses would not eliminate the secondary quality of sound, but
might do away with the secondary quality of color. In a passage where
he imagines our having very acute senses, including
"microscopical eyes," he unquestionably assumes that we
would still experience the secondary quality of sound: "If our
Sense of Hearing were but 1000 times quicker than it is, how would a
perpetual noise distract us" (E II.xxiii.12, pp. 302-303).
Yet, in a preceding passage, he suggested that if our faculties were
designed for detecting real essences, we would not experience color at
all:



> 
> Had we Senses acute enough to discern the minute particles of Bodies,
> and the real Constitution on which their sensible Qualities depend, I
> doubt not but they would produce quite different *Ideas* in us;
> and that which is now the yellow Colour of Gold, would disappear, and
> instead of it we should see an admirable Texture of parts of a certain
> Size and Figure. This Microscopes plainly discover to us: for what to
> our naked Eyes produces a certain Colour, is by thus augmenting the
> acuteness of our Senses, discovered to be quite a different thing; and
> the thus altering, as it were, the proportion of the Bulk of the
> minute parts of a coloured Object to our usual Sight, produces
> different *Ideas*, from what it did before....Blood to the
> naked Eye appears all red; but by a good Microscope, wherein its
> lesser parts appear, shews only some few Globules of Red, swimming in
> a pellucid Liquor; and how these red Globules would appear, if Glasses
> could be found, that yet could magnify them 1000 or 10000 times more,
> is uncertain (E II.xxiii.11, pp. 301-302).
> 



Reflecting upon these examples, however, suggests that he is not, or
not always, imagining that microscopical eyes would do away with color
altogether; rather, it might in some cases enable us to view tinier
particles having different colors than those we perceive in the
aggregate object. In his example, only as an aggregate body does blood
as an aggregate body appears uniformly red; under a microscope, only
the globules appear red, while some other parts of it seem
translucent. Once the microscope is used, color is not eliminated from
the experience of seeing blood, but is instead seen as being
differently distributed. A pixelated painting provides a rough
analogy; a shape seen from afar may appear uniformly green, but up
close is seen as comprising tiny blue and yellow dots.


#### 2.2.3 Obstacles to human attainment



*Scientia* in natural philosophy would require knowledge of
both real essences and their necessary connections among qualities,
yet neither is possible for human beings, Locke concludes. One
obstacle to *scientia*, then, is that real essences escape us.
God has given us sensory capacities that are suitable for such
practical endeavors as finding our way to the "market and
exchange," but as the "microscopical eyes" passage
indicates, they are not useful for detecting the minute parts of
bodies.



Another obstacle is that we are almost entirely unable to discern the
necessary causal connections among the qualities of substances. (And
Locke does take those connections to be necessary--but does he
construe them in terms of nomological or logical necessity? Given his
view that demonstrative knowledge in natural philosophy is possible
for immortals and serves as an ideal for mortals, he seems to be
thinking in terms of logical necessity, much like the Aristotelians he
was reacting against (see Ott, 2009, p. 13). Locke does find two
instances in which we can discern necessary connections between
qualities of bodies: "Some few of the primary Qualities have a
necessary dependence, and visible connexion one with another, as
Figure necessarily presupposes Extension, receiving or communicating
Motion by impulse, supposes Solidity" (E IV.iii.14, p. 546).
Apart from these two exceptions, however, necessary connections escape
us. In part, this is due to the first obstacle, our inability to
discover real essences, due to the minuteness of particles. It is also
due, however, to the remoteness of so many bodies, those which lie
"beyond this our Earth and Atmosphere...even beyond the
Sun, or remotest Star our Eyes have yet discovered" (E IV.vi.11,
p.586-87, and IV.vi.12, p.587). For according to Locke's
speculations, all things might be causally interconnected in complex
ways, in which case knowing one real essence would require knowing all
those with which it is causally connected.



Still, while human beings cannot attain *scientia* in natural
philosophy, there are other epistemic agents who can. God certainly
knows real essences (E III.vi.3, p. 440), and "'tis
possible Angels have" ideas of real essences as well (E
III.vi.3, p. 440).



> 
> It be not to be doubted, that Spirits of a higher rank than those
> immersed in Flesh, may have as clear *Ideas* of the radical
> Constitution of Substances, as we have of a Triangle, and so perceive
> how all their Properties and Operations flow from thence, but the
> manner how they come by that Knowledge, exceeds our Conceptions (E
> III.xi.22, p. 520).
> 



That Locke finds it natural to speak in the same breath of matter and
spirits marks him as belonging to the age of natural philosophy rather
than science. It is because knowledge of necessary connections can be
referred to these higher epistemic agents that *scientia* is so
strongly entrenched as an ideal, even though he recognizes the need
for a quite different conception of knowledge.


### 2.3 Human knowledge in natural philosophy (sensitive knowledge)



The conclusion that no intuitive or demonstrative knowledge of
substances is possible for us because their real essences and
necessary connections remain out of reach leaves Locke at a
crossroads. One path is the skeptical belief that without certainty,
no knowledge of substances is possible at all. He rejects that path,
denying that hyperbolic doubt could be genuine for either the self (E
IV.ix.2, pp. 619-20) or for external objects (E IV.xi.3, p.
631). The other path is the one that he follows. Here, he lowers the
bar by admitting a third kind of knowledge, which lacks certainty:
sensitive knowledge.
 [14]



Sensitive knowledge is knowledge of the "effects [that] come
every day within the notice of our Senses," without an
understanding of their causes; "we must be content to be
ignorant of" those causes (E IV.iii.29, pp. 559-560).
Instead of knowing real essences, the causal basis of the properties
we perceive, we know only those perceived properties, from which we
construct nominal essences. Instead of employing deduction, we are
forced to rely upon "trials"--observations and
induction. Instead of knowing the necessary connections holding
between a substance's real essence and its other qualities,
including its tertiary qualities (which might include, recall, causal
connections with substances beyond the remotest star), we know only
the co-existences of properties. And from the mere, regular
co-existence of properties found in observed cases, Locke observes, we
could not know with certainty that the same set will be found
co-existing in the next case.



> 
> For all the Qualities that are *co-existent* in any Subject,
> without this dependence and evident connexion of their *Ideas*
> one with another, we cannot know certainly any two to
> *co-exist* any farther, than Experience, by our Senses, informs
> us. Thus though we see the yellow Colour, and upon trial find the
> Weight, Malleableness, Fusibility, and Fixedness, that are united in a
> piece of Gold; yet because no one of these *Ideas* has any
> evident *dependence*, or necessary connexion with the other, we
> cannot certainly know, that where any four of these are, the fifth
> will be there also, how highly probable soever it may be (E IV.iii.14,
> p. 546).
> 



Our discoveries about co-existing properties--are merely
contingent particulars, or, insofar as they are applied beyond the
particular cases we have actually observed, are mere probability.
Nevertheless, they can qualify as real knowledge. To qualify, our
ideas must meet certain conditions. The complex idea that we refer to
as a substance must comprise all and only those simple ideas that we
have found to co-exist in nature. With this, Locke aims to show that
sensitive knowledge deserves its appellation, since it can be
distinguished from arbitrary or otherwise poorly grounded claims
(e.g., that fluidity has been found to co-exist with brittleness, in a
single substance and at a single given
 temperature.[15])
 Sensitive knowledge is something far less than *scientia* but
far more than ungrounded opinion.



> 
> Herein therefore is founded the *reality* of our Knowledge
> concerning *Substances*, that all our complex *Ideas* of
> them must be such, and such only, as are made up of such simple ones,
> as have been discovered to co-exist in Nature. And our *Ideas*
> being thus true, though not, perhaps, very exact Copies, are yet the
> Subjects of *real* (as far as we have any) *Knowledge*
> of them (E IV.iv.12, p. 568).
> 



As for general claims about substances based upon observed particular
matters of fact, these too can qualify as real knowledge. Admittedly,
when four of the five properties previously found co-existing together
occur again, it is only probable that the fifth will be present as
well. Yet we can still form an abstract idea of gold, as a substance
having all five properties, and call this general claim knowledge
because "whatever have once had an union in Nature, may be
united again" (E IV.iv.12, p. 568).



Has contemporary science enabled us to go beyond sensitive knowledge?
In particular, have discoveries about compounds, elements, and
subatomic particles provided us with knowledge of real essences? Much
of the force of this question derives, to paraphrase Nicholas Jolley,
from the fact that many of those discoveries about matter's
structure were not conceived empirically, but only confirmed
empirically; they were initially conceived as possibilities through
the hypothetico-deductive model, and the predictions deduced from the
models were then compared to empirical data (Jolley 2002, p. 69). Yet
as Jolley also points out, these commentators may have missed the full
import of Locke's geometric model; in a passage quoted earlier,
Locke tells us explicitly that if we knew the real essence of gold, we
could deduce its qualities even if the metal did not
 exist.[16]
 So while the predictions of any model developed via the
hypothetico-deductive model must survive the test of observations,
observations in Locke's *scientia* are *wholly
unnecessary*. To put the point another way, Locke takes natural
philosophy to be an empirical domain only for human beings, whose
faculties are impoverished. For nobler spirits, it would resemble
geometry.


## 3. Tension in Locke's thought and a consequent debate



The last few decades have seen a lively debate about the role of the
corpuscular hypothesis in Locke's *Essay*. This section
examines the sources of that debate and reviews some of the main
positions figuring in it.


### 3.1 Tension in Locke's thought



As we have seen, Locke develops some central theses of his
*Essay* in connection with the corpuscular hypothesis. In his
theory of ideas, corpuscles provide at least a structural basis for
simple ideas, and depending upon one's interpretation, there may
be a causal relationship as well. Further, and of particular interest
here, Locke often appears to identify a material substance's
real essence with the set or some subset of its component
particles' primary qualities. In the following well-known
passage, for instance, he points to the primary qualities of a
body's parts--their bulk or solidity, motion, and
shape--as the causal ground of the qualities we perceive.



> 
> The particular *Bulk, Number, Figure, and Motion of the parts of
> Fire, or Snow, are really in them* whether any ones Senses
> perceive them or no: and therefore they may be called *real
> Qualities*, because they really exist in those Bodies. But
> *Light, Heat, Whiteness*, or *Coldness, are no more really
> in them, than Sickness or Pain is in* Manna. Take away the
> Sensation of them; let not the Eyes see Light, or Colours, nor the
> Ears hear Sounds; let the Palate not Taste, nor the Nose Smell, and
> all Colours, Tastes, Odors, and Sounds, as they are such particular
> *Ideas*, vanish and cease, and are reduced to their Causes,
> *i.e.* Bulk, Figure, and Motion of Parts (E II.viii.17, pp.
> 137-138).
> 



He similarly seems to identify the real essence of bodies with primary
qualities just prior to the "microscopical eyes" passage.
There, he suggests that instead of seeing colors (or instead of seeing
them as we currently do), we could discover bodies' internal
constitutions, if only we knew the "texture and motion of the
minute Parts of corporeal things" (E II.xxiii.12, pp.
302-303). A commitment to the corpuscular hypothesis is again
suggested when he despairs of understanding the production of
secondary qualities: even if "we could discover the size,
figure, or motion of those invisible parts, which immediately produce
them [secondary qualities]," we still cannot discover any
"undoubted Rules" concerning their production or
connection, nor "conceive how any *size, figure, or
motion* of any Particles, can possibly produce in us the
*Idea* of any *Colour, Taste*, or *Sound*"
(E IV.iii.13, p. 545). Here, he appears to despair of understanding
how secondary qualities are produced by primary ones; he appears to
take the corpuscular hypothesis's reductionist claim to be true,
but he despairs that we could understand how the reduction works.



His discussion of tertiary qualities is similar. If we knew the
"Figure, Size, Texture, and Motion of the minute Constituent
parts of any two Bodies," we would then be able to derive
tertiary qualities; we would be able to deduce that opium causes
sleep, and we would understand why (E IV.iii.25, pp. 555-56; see
also E IV.iii.13, p. 545). In all of these passages, then, and in many
similar ones, Locke appears to accept at least some components of the
corpuscular hypothesis--that material bodies are compounded from
minute particles, and certain observable qualities are reducible to
the particles' primary qualities of size, shape, and motion.
This tendency to speak as though the corpuscular hypothesis is true,
either in whole or in part, has been termed Locke's
"dogmatic" side (Downing 2007).



In apparent tension with this so-called dogmatic side is what has been
termed his "agnostic" or "skeptical" side. The
following features of his discussion seem to suggest that he has
reasons either for remaining agnostic about whether the corpuscular
hypothesis is true, or more seriously, for believing that it is wholly
unable to explain the phenomena it purports to explain and therefore
cannot be true.



First, he refers to the corpuscular hypothesis as a hypothesis, and
one that falls well short of providing us with scientific knowledge.
Further, he remarks that it is not his aim to adjudicate among
competing hypotheses.



> 
> I have here instanced in the corpuscularian Hypothesis, as that which
> is thought to go farthest in an intelligible Explication of the
> Qualities of Bodies; and I fear the Weakness of humane Understanding
> is scarce able to substitute another, which will afford us a fuller
> and clearer discovery of the necessary Connexion, and
> *Co-existence*, of the Powers, which are to be observed united
> in several sorts of them. This at least is certain, that which ever
> Hypothesis be clearest and truest, (for that it is not my business to
> determine,) our Knowledge concerning corporeal Substances, will be
> very little advanced by any of them, till we are made see, what
> Qualities and Powers of Bodies have a *necessary Connexion or
> Repugnancy* one with another; which in the present State of
> Philosophy, I think, we know but to a very small degree (E IV.iii.16,
> pp. 547-548).
> 



The hypothetical status of all physical theories is underscored also
in *Some Thoughts Concerning Education*: "The systems of
*natural philosophy*...are to be read, more to know the
*hypotheses*...than with hopes to gain thereby a
comprehensive, scientifical, and satisfactory knowledge of the works
of nature" (Locke, quoted in Rogers 1982, p. 230). Still, while
all physical theories are ultimately hypotheses, it is useful to bear
in mind Peter Anstey's remarks about this one's genesis.
The hypothesis originated among theorists championing an experimental
method over a purely speculative one, and while they did not exclude
speculation entirely, they circumscribed its role, engaging in it only
after observation and experiment had already given the theory a solid
foundation. Furthermore, the corpuscular hypothesis had credibility
insofar as its theorists avoided the question about matter's
infinite divisibility and thereby adhered to their prohibition against
questions they thought could never be answered experimentally (see
Anstey, 2011, pp. 4-5).



Second, if Locke indeed identifies material bodies' real
essences with the primary qualities of their constituent corpuscles,
that view of real essences combined with his pessimism about ever
discovering them implies pessimism about the corpuscular hypothesis.
Specifically, it implies pessimism about the claims that bodies are
made of corpuscles and that those bodies' observable qualities
are reducible to the corpuscles' qualities. In the same passages
where Locke seems to support or assume the corpuscular
hypothesis's central tenets--that observable bodies are
made up of corpuscles and that those corpuscles have a restricted set
of inherent properties--he simultaneously appears very skeptical
about the possibility of reducing observable properties such as color
and taste to that restricted set of primary properties.



Third, Locke arguably believes that the corpuscular hypothesis's
limitations are so serious that they amount to fatal flaws, an
interpretation that Wilson (1979) was perhaps the first to defend.
Although Wilson develops her line of argument mainly in connection
with difficulties Locke raises about the corpuscular
hypothesis's purported ability to explain sensation and more
generally, the relation between thought and
 matter,[17]
 some other phenomena are troublesome as well. Locke appears to
consider such phenomena so obscure that we can attempt to understand
them only by attributing them to God's direct action.



> 
> The coherence and continuity of the parts of Matter; the production of
> Sensation in us of Colours and Sounds, *etc.* by impulse and
> motion; nay, the original Rules and Communication of Motion being
> such, wherein we can discover no natural connexion with any
> *Ideas* we have, we cannot but ascribe them to the arbitrary
> Will and good Pleasure of the Wise Architect (E IV.iii.29, pp.
> 559-560).
> 



Elsewhere, Locke will use the term 'superaddition' to
refer to God's role. For the moment, superadded properties may
be neutrally described as those that God confers by fiat. Section
3.3., which reviews some of the main positions in the debate about
Locke's stance toward the corpuscular hypothesis, will also
address the various interpretations of superaddition.
 [18]
 First, however, we will examine the problematic phenomena.


### 3.2 Limitations of the corpuscular hypothesis



This section examines the four phenomena that Locke seems to consider
too obscure for the corpuscular hypothesis to illuminate. The
above-quoted passage mentioned three of these phenomena-- the
production of sensation, the communication of motion, and
 cohesion.[19]
 Locke discusses the fourth, gravity, only outside the
*Essay*.


#### 3.2.1 Sensation



As we saw in passages discussed earlier, in connection with
*scientia's* impossibility, Locke finds the production of
sensation to be utterly obscure. One side of the difficulty is the
nature of the mind. While in all probability, it is immaterial, Locke
allows the possibility that God superadded the power of thought
directly to matter. The other side of the difficulty concerns the
nature of secondary qualities as powers to produce sensations. The
appeal of the corpuscular hypothesis lay largely in its reductive
promise. Reduction was expected particularly for secondary qualities,
such as colors and sounds, but was also anticipated for ideas of
macro-level primary qualities, including visual sensations of shapes
and sizes, and tertiary qualities. All would be reduced to the primary
qualities of bodies' component corpuscles as they interact with
one another and our perceptual systems.



One part of the corpuscular hypothesis's purported explanation
*is* conceivable, namely, the interactions among the primary
qualities of bodies, which are supposed to be part of the causal basis
of our sensations:



> 
> That the size, figure, and motion of one Body should cause a change in
> the size, figure, and motion of another Body, is not beyond our
> Conception; the separation of the Parts of one Body, upon the
> intrusion of another; and the change from rest to motion, upon
> impulse; these, and the like, seem to us to have some
> *connexion* one with another (E IV.iii.13, p. 545).
> 



Indeed, we are capable of discerning necessary connections in two
instances, as noted earlier. (One case involves only primary
qualities-- "Figure necessarily presupposes
Extension" (E IV.iii.14, p. 546)--while the other involves
tertiary and primary qualities--"receiving or communicating
Motion by impulse, supposes Solidity" (E IV.iii.14, p. 546).) If
we knew more about the primary qualities of bodies, we might multiply
such instances: "And if we knew these primary Qualities of
Bodies...we might be able to know a great deal more of these
Operations of them one upon another". That is, if we knew real
essences, we could derive more necessary connections, to know the
causal relation between opium and sleep, for instance, and as
certainly as we now know that impulse requires solidity.



Still, knowing real essences would not give us any genuine knowledge
of how *sensations* are produced by primary qualities. While
corpuscular theorists such as Galileo (*The Assayer*) sketched
a reductive account of our sensations of taste in terms of particles
striking our tongues, Locke suggests that any attempt to discover the
process's details will be foiled. For as far as we can imagine,
a body that strikes other bodies can produce "nothing but
Motion"
 [20]
 (E IV.iii.6, pp. 540-541), and motion may itself be hopelessly
obscure, as indicated below. The roles of shape and size are equally
obscure; we cannot imagine how they could help produce sensations.



> 
> We are so far from knowing what figure, size, or motion of parts
> produce a yellow Colour, a sweet Taste, or a sharp Sound, that we can
> by no means conceive how any *size, figure, or motion* of any
> Particles, can possibly produce in us the *Idea* of any
> *Colour, Taste*, or *Sound* whatsoever; there is no
> conceivable *connexion* betwixt the one and the other (E
> IV.iii.13, p. 545).
> 



Although Locke mentions only secondary qualities here, his point
presumably applies to all sensations, including our sensations of
macro-level primary qualities, such as the shape and size of a
snowball or lump of gold. For again, genuine knowledge is knowledge of
necessary connections, with the conceptual relations in geometry being
the model, and it does not seem possible to discover such connections
between any sensation and the sizes, shapes, and textures that are
alleged to cause them. Macro-level primary qualities to micro-level
ones and accordingly resemble
 them.[21]
 but the idea of a quality is nonetheless very different than the
quality itself.



Locke finds that our only way of understanding the production of
sensation is to attribute the process to God. If we try to understand
how motion could produce a color, sound, or taste, "we are fain
to quit our Reason, go beyond our *Ideas*, and attribute it
wholly to the good pleasure of our Maker" (E IV.iii.6, p.
540-541; see also IV.iii.28, p. 559). Locke was certainly not
alone in grappling with this problem. Commenting upon
Descartes's account, Walter Charleton (1654, p. 197) had
observed earlier that even a detailed knowledge of light behavior
would still leave us with the "superlative difficulty" of
understanding why a certain reflection or refraction should be
"transformed into a Vermillion rather than a Blew," and
further details about our sensory apparatus do not reveal "any
*Analogy* betwixt the Retina Tunica ... and any one
Colour". Locke was also not alone in resorting to a divine
cause; in *De gravitatione*'s creation account of bodies,
for example, Newton suggested a divine basis for bodies'
abilities to stimulate perceptions in minds.


#### 3.2.2 Gravity



Newton's *Principia* implied the possibility of
unmediated action at a distance, and with its publication, gravity
became the most nettlesome phenomenon for the orthodox version of the
corpuscular hypothesis due to its proviso of contact action. Locke was
initially sympathetic to the proviso, writing in the first three
editions of his *Essay*, "How bodies operate one upon
another...is manifestly by impulse and nothing else. It being
impossible to conceive that body should operate on *what it does
not touch*" (E II.viii.11, editions 1-3). Yet for the
fourth edition, he replaced that claim about how bodies do operate
with one about how we can conceive of them operating: "How
Bodies produce *ideas* in us is manifestly *by impulse*,
[this being] the only way which we can conceive Bodies [to]
operate" (E II.viii.11, edition 4). He also omitted a clause
denying unmediated action at a distance, which had appeared in
II.viii.12 of previous
 editions.[22]
 These subtle emendations reflect a dramatic shift, one expressed
directly in his correspondence with Stillingfleet.



> 
> The gravitation of matter towards matter, by ways inconceivable to me,
> is not only a demonstration that God can, if he pleases, put into
> bodies powers and ways of operation, above what can be derived from
> our idea of body, or can be explained by what we know of matter, but
> also an unquestionable and every where visible instance, that he has
> done so (*Second Reply to the Bishop of Worcester*, 1699,
> *The Works of John Locke*, Vol. IV, p. 467).
> 



The phenomenon of gravity--as explained by "Mr.
Newton's incomparable book"
(*ibid.*)--apparently led Locke to abandon the contact
action proviso and to attribute to matter the power of acting
distantly, even though he considered the process by which such
interactions could occur so obscure that it drove him to invoke
superaddition. This is the prevailing interpretation of Locke (and was
assumed by Leibniz, who targeted Locke for it in *Against Barbaric
Physics*), though not all commentators agree, as indicated in a
subsequent section.


#### 3.2.3 Motion



Locke takes the notion of impulse, in which bodies communicate motion
to one another by surface impact, to be, along with extension and
cohesion, fundamental to our concept of
 body.[23]
 Indeed, regardless of how motion may actually be communicated,
impulse is the only means by which we can conceive of its being
communicated, a view of our conceptual abilities that Locke maintains,
as we saw, despite his changing thoughts about gravity. Impulse is
also fundamental to the corpuscular hypothesis's explanation of
phenomena, being either the exclusive means of interaction among
bodies, as adherents of the contact action proviso hold, or the means
of at least many interactions. Yet how exactly does a moving body
communicate motion to a resting one simply by impacting it? When we
attempt to discover the precise nature of the process, Locke suggests,
we find that it is just as mysterious as the process by which the mind
moves the body.



> 
> Another *Idea* we have of Body, is the power of
> *communication of Motion by impulse*; and of our Souls, the
> power of *exciting of Motion by Thought*.... But if here
> again we enquire how this is done, we *are equally in the
> dark*. For in the communication of Motion by impulse, wherein as
> much Motion is lost to one Body, as is got to the other, which is the
> ordinariest case, we can have no other conception, but of the passing
> of Motion out of one Body into another; which, I think, is as obscure
> and unconceivable, as how our Minds move or stop our Bodies by
> Thought....The increase of Motion by impulse, which is observed
> or believed sometimes to happen, is yet harder to be understood. We
> have by daily experience clear evidence of Motion produced both by
> impulse, and by thought; but the manner how, hardly comes within our
> comprehension; we are equally at a loss in both (E II.xxiii.28, p.
> 311).
> 



Since the corpuscular hypothesis holds impulse to be the primary if
not the sole means by which bodies causally interact, then any
phenomenon that the corpuscular hypothesis purports to explain by
impulse will remain obscure if impulse itself remains obscure. All of
the hypothesis's reductions of observable primary, secondary,
and tertiary qualities, would inherit impulse's obscurity; thus
Locke appears to be suggesting here that the corpuscular hypothesis
cannot fulfill its promise of explaining and reducing those properties
and powers.


#### 3.2.4 Cohesion



Since the claim that observable bodies are made up of particles is
central to the corpuscular hypothesis, an immediate question for its
proponents asks how the particles cohere into compound bodies.
Plenists have some resources for answering that question, though those
resources may introduce worse difficulties. Descartes, for instance,
though he speaks in terms of particles, understands an individual body
as an area of extension moving as one with respect to surrounding
areas. It is not possible for any particle to move away into empty
space, there being no such thing as empty space once matter is
identified with extension; with every bit of matter pressed from all
sides by other matter, there is no problem about cohesion per se,
though there is certainly a problem about individuating bodies from
one another. In the same vein, Malebranche can invoke the pressure of
the air to explain the coherence of bodies, and then invoke the
pressure of an aether to explain the coherence of air particles. Locke
objects that this explanation fails because it leaves us with the
question of what causes the particles of the aether to cohere (E
II.xxiii.23, p, 308). The objection reveals Locke's atomist
sympathies, drawing its power from the presumption that there is such
a thing as empty space into which the aether particles could move. The
problem for atomist versions of the corpuscular hypothesis is that the
restricted set of properties that they allow the particles
--size, shape, and motion--provides no obvious resources for
explaining how the particles cohere with one another to form compound
bodies. In various forms, the problem about cohesion has dogged
atomists since ancient times.



The problem arises in two forms, which, to borrow James Hill's
terminology (Hill 2004), may be called the limited and the
foundational problems. The limited problem, arising for those who take
corpuscles to be genuine atoms, that is, to be indivisible, is the
problem of explaining how those indivisible corpuscles cohere with one
another. This is the problem one finds in Newton's writings.
Although Rule 3 of the *Principia* allows the possibility that
the least parts of matter could turn out to be divisible, his atomist
sympathies are evident throughout his writings. He speculates in Query
31 that in all probability, bodies are made up of hard particles that
only God could divide, and in the body of the *Opticks* (Book
II, Part III, Proposition VII) he suggests that more powerful
microscopes might permit us to see the larger particles. In answer to
the problem of how those naturally indivisible particles cohere, he
rejects the ancient solution of hooked particles as begging the
question, proposing instead some short-range forces modeled on the
gravitational force (Query 31). Newton's speculations about such
forces are driven by an absence of any resolution to the problem about
cohesion within the corpuscular theory itself.



The foundational problem pushes the question about cohesion into the
corpuscles themselves. The problem was raised by Joseph Glanvill:
"If it be pretended...that the parts of solid bodies are
held together by *hooks*, and *angulous* involutions; I
say, this comes not home: For the *coherence* of the parts of
these *hooks*...will be of as difficult a conception, as
the former" (Glanvill, *The Vanity of Dogmatizing*, p.
18, quoted in Hill 2004, p. 616). Without any grounds for asserting
that the divisibility of matter bottoms out in indivisible corpuscles,
then, the question arises of how the parts of a corpuscle could
cohere, how the parts of those parts could cohere, and so on, ending
in the question of how extended bodies are possible at all.



Whatever his ultimate view of the corpuscular hypothesis, Locke
inevitably faces the atomist's problem about cohesion, accepting
as he does void space (see II.xiii.11, 12-14, 21-23), and
holding that our ideas of body depend fundamentally upon cohesion. One
of the ideas "proper and peculiar" to body, he writes, is
"*the cohesion of solid*, and consequently separable
*parts*" (E II.xxiii.17, p. 306), and the extension of
body, as opposed to the extension of space, is "nothing, but the
cohesion or continuity of solid, separable, moveable Parts" (E
II.iv.5, p. 126). Yet we have no understanding of cohesion, and so our
idea of body does not rest on any genuine understanding of it. In
trying to understand how bodies are extended, we are as much in the
dark as when we try to understand how the soul thinks.



> 
> 'Tis *as easie* for him *to have a clear*
> Idea*, how the Soul thinks, as how Body is extended*. For since
> Body is no farther, nor otherwise extended, than by the union and
> cohesion of its solid parts, we shall very ill comprehend the
> *extension* of Body, without understanding wherein consists the
> union and cohesion of its parts; which seems to me as
> incomprehensible, as the manner of Thinking, and how it is performed
> (E II.xxiii.24, p. 309).
> 



One scholarly debate about cohesion concerns the question of whether
Locke acknowledged only the limited problem, as one would expect from
commentators who read him as accepting atomism (e.g. Mandelbaum 1964,
p. 1), or whether he looked further, to the more serious, foundational
problem, as Hill argues (2004). A related controversy, to be discussed
in the next section, concerns the question of whether Locke concludes
that the corpuscular hypothesis simply cannot resolve the problem
(e.g., Hill 2004; Downing 2007, p. 408) or instead remains agnostic on
the issue (e.g., McCann in Chappell 1998, p. 244).



The four problematic phenomena winnow away the components of the
corpuscular hypothesis, as defined at the outset of this article. The
problem about sensation threatens the corpuscular hypothesis's
promise of reducing secondary, tertiary, and macro-level primary
qualities to microlevel primary qualities. Newton's results
about gravitational phenomena cast grave doubts upon the contact
action proviso, and according to some commentators, those results led
Locke to abandon the belief that impulse is the only means of causal
interaction. A corpuscular theorist might hope to preserve some part
of the theory by insisting that impulse is still the means by which
most other causal interactions are effected; but this runs up against
the problem about impulse, in that the process by which motion is
communicated seems utterly obscure. Finally, even the core claim that
observable bodies are composed of tiny corpuscles is threatened by the
problem of cohesion. This last problem threatens to be the most
serious of four phenomena, for as James Hill has pointed out (2004, p.
628), the problems about gravity, sensation, and motion arise
subsequent to our having conceived of body, whereas the problem about
cohesion may thwart our very ability to conceive clearly of body.


### 3.3 Main positions in the debate



This section considers some main responses to the tension between
Locke's seeming acceptance of the corpuscular hypothesis, most
notable in his apparent identification of a material substance's
real essence with the size, shape, and texture of its insensible
parts, and his pessimism about the hypothesis's explanatory
power, most notable in his remarks about the four phenomena discussed
above.



One approach to the tension is to understand it as a genuine
inconsistency. Margaret Wilson has defended such an interpretation,
though in the 1979 paper that launched the debate, her intent is to
show how acutely Locke understood the explanatory limitations of
"Boylean mechanism". Specifically, Wilson argues, the
inconsistency reveals Locke's recognition "that some
presumed properties of matter *cannot be conceived* as
'natural' consequences of Boylean primary qualities"
(Wilson 1979, p. 197). Thus, our ignorance about bodies has more
profound causes than our ignorance about the primary qualities of a
body's constituent corpuscles. In accordance with her view that
Locke's agnostic tendencies (just like his dogmatic ones) are
genuine, Wilson interprets Locke's concept of superaddition
robustly, as a sort of divine action that goes beyond the corpuscular
hypothesis. According to this "non-essentialist" or
"divine annexation" reading, Locke understands superadded
properties as properties that God has annexed to matter by fiat, and
that bear no intrinsic connection to matter's real essence. This
reading implies a distinction in etiology for superadded qualities.
Whereas the other qualities of matter are either given initially, as
those qualities constituting the real essence, or else flow from the
real essence, superadded qualities are added on independently of the
real essence, such that the substance would have been complete without
them. This view implies that while superadded properties are
consistently present, they are inexplicable by physical theory and,
accordingly, are evidence of divine action.
 [24]



Some other interpretations absolve Locke of inconsistency, either by
emphasizing his so-called dogmatic side while downplaying or
reinterpreting his agnostic tendencies, or emphasizing his agnosticism
while downplaying his dogmatism. One line of interpretation, then,
reads Locke as in some manner accepting the corpuscular hypothesis
(Mandelbaum 1964, chapter 1; Osler 1970, p. 12; Ayers 1975; McCann
1994, SS1 and p. 85; McCann 2002, pp. 354-355). According to
weaker versions of this reading, Locke's project is the
naturalistic one of pursuing the philosophical implications of the
best available scientific theory, and developing an epistemological
basis for it. McCann, for example, reads Locke as defending the
atomist version of the corpuscular philosophy over its Cartesian
competitor by providing an epistemology for it. While Descartes had
provided an epistemology for his plenist version, there was nothing
comparable for the atomist version associated with Gassendi and Boyle
until Locke supplied it (McCann 2002, pp. 354-355). According to
Ayers' stronger interpretation, Locke accepts "pure
mechanism," that is, the orthodox version of the corpuscular
hypothesis, which includes the contact action proviso. According to
this view, all of matter's qualities flow from its real essence
(Ayers 1981).



Since this line of interpretation seeks to downplay Locke's
agnostic tendencies, one challenge is to account for Locke's
pessimism about the possibility of our knowing real essences.
Mandelbaum meets the challenge by confining Locke's pessimism to
the real essences of particular material substances; we are able to
know "the general properties possessed by all material
substances," and are ignorant only of the "particular
sizes, shapes, number, or motions of the particles which go to make up
any specific object" (Mandelbaum 1964, p. 54). A related
challenge is to account for Locke's appeals to superaddition,
since *prima facie*, Locke's reason for invoking God is
that he thinks the corpuscular hypothesis has no resources for
explaining the four problematic phenomena. Ayers responds by rejecting
Wilson's divine annexation interpretation of superaddition in
favor of a deflationary one. According to Ayers' "divine
architect" interpretation, Locke makes no distinction in
etiology by calling a property superadded; he means only that God
selected the property with particular care when first creating matter.
To diffuse the effect of Locke's remarks to Stillingfleet, in
which Locke appears to embrace action at a distance, Ayers points to
Locke's late manuscript, "The Elements of Natural
Philosophy," interpreting certain passages as referring
gravitational effects to an undetectable medium (Ayers 1981, pp.
212-214). This move has been challenged by Stuart. Claiming that
the manuscript was probably written for the education of a child,
Stuart denies that it could trump Locke's remarks to
Stillingfleet (see Stuart 1998, pp. 378-379).



Another way to absolve Locke of inconsistency is to emphasize his
agnosticism or skepticism, while downplaying or reinterpreting
passages that appear to commit him to the corpuscular hypothesis.
Interpretations in this vein tend to emphasize Locke's pessimism
about our ability to know real essences, to discern necessary
connections, and consequently, have *scientia* in natural
philosophy. Commentators pursuing this line include Downing (1998,
2007), Jolley (2002), and Connolly (2015); the latter argues for an
even greater epistemic humility, one extending to real essences, along
with superaddition (mentioned briefly below).



The central challenge facing such interpretations is to account for
the passages in which Locke speaks as though he accepts the
corpuscular hypothesis, most notably those in which he appears to
identify the real essences of material substances with the
corpuscles' primary qualities. Jolley (2002) accounts for
Locke's so-called dogmatic tendency in strategic terms.
Locke's *Essay* targets both Aristotelians and
Cartesians, and though agnosticism, which targets the Cartesians, is
ultimately the dominant tendency in his thought, Locke emphasizes the
explanatory power of the corpuscular hypothesis whenever he has the
Aristotelians in his sights. Downing (1998, 2007), meanwhile,
interprets Locke's *Essay* as developing metaphysical
distinctions that constrain physical theory, and then downplays his
dogmatic side by taking the corpuscular hypothesis to be truly only a
hypothesis for him, and denying that real essence can be identified
with primary qualities. Properly understood, Downing argues, the
distinction between real and nominal essence is a metaphysical
distinction. It is thus more fundamental than the distinction between
primary and secondary qualities, which belongs to a particular
physical theory, the corpuscular hypothesis. To be worth its salt, a
physical theory must meet the metaphysical constraint provided by the
real-nominal essence distinction. That is, the physical theory must
provide some way of making sense of the notion that material bodies
have an internal constitution that is inaccessible to us while
producing qualities that are accessible. Locke explains his
metaphysical distinction using one physical theory, the corpuscular
hypothesis, as an illustration, and he often appears to accept or even
defend that hypothesis. Still, this is only an appearance, an
appearance due to a certain advantage that the corpuscular hypothesis
has over other physical hypotheses: it is the theory best suited to
our sensory capacities and understanding. Despite its unique status,
Locke sees it as a mere hypothesis, one crippled by the explanatory
limitations evidenced by the problematic phenomena discussed earlier.
For Downing, then, Locke's dogmatic tendencies disappear,
leaving only his agnostic side.



Taking a different approach to the situation, Jacovides (2017) treats
Locke's descriptions of what we perceive, intuit, and are
capable of conceiving as providing a case for testing some Kuhnian
theses; as the normal science of his day, the corpuscularian
hypothesis sets limits upon what Locke can conceive.


## 4. Locke and Newton



Locke and Newton probably first met in 1689 (though the exact date is
not known; Westfall 1980, p. 488; Rogers 1982, p. 219), and their main
works were written independently of one another; Locke's
*Essay*,though published subsequently, was essentially complete
by the time he read the *Principia*. A noticeable intellectual
affinity may nevertheless be seen in those works, and the opportunity
for mutual influence followed as they established a friendship,
exchanging views on a wide variety of subjects, not least certain
unorthodox theological convictions (see Westfall 1980, 490-91).
The influence did not run in one direction alone; in a draft passage
probably penned shortly after the *Principia*'s second
edition, for example, Newton adopts a Lockean tone as he denies that
any ideas are
 innate.[25]
 As for Newton's influence upon Locke, the best-known example
concerns action at a distance, as noted below, though there are deeper
questions about methodology.


### 4.1 Epistemology and methodology



A good deal of resemblance to Locke's epistemological approach
can be seen in Newton, who holds that, revelation apart, we must
gather what knowledge we can from our perceptions, and anything like a
real essence eludes us. In the early manuscript, *De
gravitatione*, for instance, Newton denies knowing the
"essential and metaphysical constitution" of matter
(Newton, 2004, p. 27). He reiterates this position in later texts,
including the 1713 General Scholium:



> 
> 
> We certainly do not know what is the substance of any thing. We see
> only the shapes and colors of bodies, we hear only their sounds, we
> touch only their external surfaces....But there is no direct
> sense and there are no indirect reflected actions by which we know
> innermost substances (*Principia*, 942).
> 
> 
> 



Locke and Newton also share the problem of an evidentiary deficit,
insofar as they subscribe to corpuscularianism. Thinkers who did
subscribe tended to rely on transduction (also called
transdiction)--an inductive inference that is empirical insofar
as it relies upon observed cases, but which generalizes not only to
unobserved cases but to those that are
 unobservable.[26])
 Newton's Rule 3 licensed such inferences, from the
*Principia*'s second edition onward, for qualities of
invariable intensity, i.e., extension, impenetrability, hardness,
mobility, and the *vis inertiae*. The rule explicitly permits
inferences to the realm of the unobservable: "Because the
hardness of the whole arises from the hardness of the parts, we justly
infer from this not only the hardness of the undivided particles of
bodies that are accessible to our senses, but also of all other
bodies" (*Principia*, Book 3, p.
 795[27]).
 As for Locke and the problem of transduction, how serious it is for
him depends largely upon the status of the corpuscular hypothesis. If
he takes an agnostic or skeptical stance toward it, then he owes no
solution to the problem.



Although the endeavors of Locke and Newton are often considered
complementary, there are questions about how Locke responded to the
*Principia*, both in terms of its methodology and its
epistemological implications. With regard to the former, how far did
Locke go in absorbing or incorporating the methodology that Newton had
forged? As a number of commentators have emphasized, the new science
was itself in flux at this time, with natural history being gradually
sidelined by Newton's approach, an experimentalism that was
theoretical and mathematical (see, e.g., Anstey 2011; Roux, 2013).
Given that Locke had had Boyle as his mentor but was greatly impressed
by Newton's *Principia*, it is natural to ask whether a
similar shift was occurring in Locke's own thought. Although
Locke was influenced by Newton, his allegiances to certain older ideas
ran deep. De Pierris (2006) explains Locke's failure to adopt
the method of inductive proof in terms of his devotion to the ideal of
demonstrative knowledge, as combined with a belief in hidden primary
qualities.



With regard to its epistemological implications, what precisely did
Locke take the *Principia* to have achieved? To pose the
question in the starkest possible terms, might Newton's
*Principia* have tempted Locke to retreat from his belief that
natural philosophy cannot be made a science (much as it led him to
retreat from the contact action proviso)? Perhaps Locke categorized
Newton's epistemological achievement in natural philosophy as a
contribution to sensitive knowledge alone. That is, perhaps he saw
Newton's "mighty Designs in advancing the Sciences"
(*Essay*, Epistle to the Reader, pp. 9-10) as confined to
providing a firm basis for natural philosophy through his experimental
method. But did he perhaps instead take Newton's mathematical
methods as offering the demonstrations needed to push natural
philosophy into the domain of demonstrative knowledge and hence
scientia?



Commentators interpreting Locke as seeing Newton's contribution
in terms of sensitive knowledge alone include Yolton (1969); Woolhouse
(1994); and Downing (1997, see especially pp. 292-93). Winkler
(2008), however, reads Locke's correspondence with Stillingfleet
and other writings from the 1690s as a defense of Newton's
mathematical physics, and one that draws from his mathematical
demonstrations a greater optimism about the possibility of certain
knowledge in natural philosophy. This move has been contested by
Domski (2012), who argues that Locke advocated Newton's
mathematical methods only in connection with astronomy, whose objects
are unavailable for experimentation; and that Locke maintained his
emphasis upon natural historical methods for questions about
terrestrial bodies. The debate over the extent to which Locke
prioritized natural history over experimentalism directly concerns his
stance on hypotheses (a controversy mapped by Priselac, 2017), while a
related debate concerns the relationship between medicine and natural
philosophy. Opposing Yost (1951) among others, Lu-Adler (2021) has
recently argued that Locke did not assume that the scientific
methodology most appropriate to medicine could be generalized to
physics.


### 4.2 Ontology



Locke's discussions of space, body, mind, and God bear some
striking similarities to Newton's, raising questions about the
extent of those similarities as well as points of influence. Newton
presents his mature concept of space in the *Principia*, having
formulated important aspects of it earlier in the manuscript *De
gravitatione*. In both texts, space is distinct from body and is
real, infinite, three-dimensional, and homogeneous. Whereas body has
separable parts, Newton's eternal, infinite space is immobile
and indivisible, having merely distinguishable parts. Although space
is neither substance nor attribute, it would be a mistake to suppose
that it is nothing; as he emphasizes in *De gravitatione*
(2004, p. 21, 22), by explaining space's properties he has shown
that it is something.



Unlike Newton, Locke initially accepted a relationist view of space;
in journal entries of 1676-78 and the Early Draft of the
*Essay,* space is nothing more than a relation between bodies
that do not touch (see Gorham, 2020, p. 221). In the *Essay*,
however, his discussion of space and its relation to body is strongly
reminiscent of the *Principia*'s absolutist conception.
Locke confesses himself to be among those "who persuade
themselves ... that they can think on Space, without anything in
it, that resists, or is protruded by Body" (E II.iii.5, p. 126).
Declaring that explicating extension in terms of *partes extra
partes* would be tautological (E II.iii.15, p. 173) and declining
to answer the question of whether space is substance or accident (E
II.iii.17, p. 174), Locke describes our idea of "pure
Space" (E II.iii.5, p. 126) as an extension consisting of
"the continuity of unsolid, inseparable, and immoveable
Parts" (*ibid*.; see also E II.xiii.13, 14, which further
discuss space's perpetual rest and the impossibility of
separating its parts, either actually or mentally). Lacking solidity,
space poses no resistance to a body's motion (E II.xiii.12, p.
172). Again recalling Newton, who takes space to be a consequence of
God's existence (2004, p. 21), Locke invokes God when defending
his conception of space: "Those who assert the impossibility of
Space existing without Matter, must not only make Body infinite, but
must also deny a power in God to annihilate any part of Matter"
(E II.xiii.21, p. 176).



Analyzing remarks such as the above, most commentators have concluded
that by the time of the published *Essay*, Locke had fully
abandoned the relationist view (see Gibson, 1960; Di Biase, 2016;
Gorham and Slowik, 2014; Gorham, 2020). However, Locke tends to
describe our ideas rather than things themselves, and the gap between
the two has led a few commentators to conclude that Locke retained his
earlier relationism or, at least, some inclination toward it (e.g.,
Thomas, 2016; see also Rogers, 1978).



Locke's concept of body also overlaps considerably with
Newton's. In developing a concept of body, Locke notoriously
finds the related, general problem about a substratum or substance
 vexing,[28]
 whereas Newton dispenses with the problem more easily. In *De
gravitatione*, Newton eliminates the unintelligible notion of
prime matter by associating perceived properties with determined
quantities of
 extension.[29]
 Both thinkers attack Descartes' identification of matter with
extension, instead sympathizing strongly with the notion of atoms in a
void. Further, Newton's list in Rule 3 of body's universal
qualities includes not only extension, hardness, impenetrability, and
mobility, but also the *vis inertiae*--the inherent force
or power of resisting (*Principia*, Definition 3), which some
commentators take Newton to identify with
 mass.[30]
 Locke's concept, however, emphasizes extension, mobility, and
solidity, without mentioning mass. Still, since Locke explicates
solidity in terms of impenetrability, which arises from resistance (E
II.iv.1), there may be grounds for holding that his concept does after
all include mass, a position defended by Woolhouse
 (2005).[31]
 At the least, Locke's discussion of action by impulse
presupposes mass, as Stein has noted. Nevertheless, presupposing the
concept may not amount to including it in the concept of body, since
as Stein points out, mass for Locke "cannot be construed to
correspond to a simple idea, but can only be understood as a power
'mediately perceivable'" (Stein 1990, p. 36).



The question about gravity's relation to body is also a
persistent source of controversy for both
 thinkers.[32]
 One point of clarity and consensus is that neither Locke nor Newton
considers the power of gravitational attraction to be essential to
matter. Newton consistently denies that it is essential (for instance
in his explanatory remarks following Rule 3 of the *Principia*)
and Locke refers to it only as power superadded or endowed by God.
Locke's concept of superaddition is controversial, as noted
earlier, but he does appear to embrace action at a distance in his
letter to Stillingfleet. Newton's writings contain no such
dramatic pronouncements. So, while a few commentators interpret Newton
as accepting action at a distance, grounded either in a superadded
property (Henry 1994) or in a relational quality of matter (Schliesser
2011), his more favorable remarks are at best indirect. Most
commentators interpret him as at least having very grave misgivings
about distant action, so perhaps Locke was wise to credit his change
of heart about gravity to "Mr. Newton's incomparable
book," rather than to Newton himself.



With respect to mind, Locke and Newton are both circumspect about its
nature and yet may be considered substance dualists. Although Newton
indicates in *De gravitatione* that he does not pretend to know
the substantial foundation of minds, he consistently presents minds as
immaterial, lacking certain characteristics of body, namely, hardness,
impenetrability and resistance. Similarly, while Locke explicitly
allows the possibility of thinking matter in his *Essay* and
discusses it at length with Stillingfleet, he emphasizes that in all
probability, the soul is immaterial (E IV.iii.6, pp. 540-541).
Does Locke share Newton's view that even immaterial spirits are
spatially extended, such that a mind can co-occupy place with a body?
A Cartesian interpretation does not seem impossible given
Locke's remarks about the possibility of thinking matter, which
contrast matter against an immaterial soul that is
 unextended.[33]
 Still, at several passages in the *Essay*, Locke seems to
agree with Newton. At E II xxvii.2 (p. 329), he locates minds
spatially: "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
exists".



Finally, Locke seems to share Newton's belief in God's
substantial ubiquity and its implications. As the above-quoted
passage's discussion of finite spirits continues, he remarks
more generally on the possibility of co-presence: "These three
sorts of substances, as we term them, do not exclude one another out
of the same place" (*ibid*.). His later remarks about the
"infinite Space ... possessed by God's infinite
Omnipresence" (E II.xvii.20, p. 222) are similar, but by framing
them in terms of third parties' ideas, Locke leaves more room
there for interpretive controversy.