John Locke has been widely hailed for providing an epistemologicalfoundation for the experimental science of his day, articulating thenew, probabilistic form of knowledge appropriate to it. Yet, while heis in important respects a devotee of that new science, there are alsosignificant tensions in his thought. He stands behind its experimentalmethods as he targets the earlier, speculative or rationalistphilosophies for relying on methodologies and epistemologicalexpectations unsuited to natural philosophy. He also frequentlyappears to embrace the new science’s corpuscular hypothesis,whose powers and minute particles figure prominently in his attempt tounderstand why we cannot hope for demonstrative certainty aboutnatural phenomena. Still, the new science’s methodology wasevolving. Just how far did Locke travel with that evolution, and whataspects of his thought prevented him from going further? As for thecorpuscular hypothesis, what exactly was his stance toward it? Hefrequently speaks of particles and powers as if they belonged toestablished knowledge, and yet in explaining the hypothesis’sflaws, he seems to consider them fatal. This article will mainlyemphasize the second of those related questions, though both havespurred scholarly investigation and debate.

• 1. Introduction
• 2. Locke on knowledge in natural philosophy:Scientia and human knowledge
  • 2.1 Historical roots ofscientia
  • 2.2Scientia in natural philosophy and obstacles to human attainment
  • 2.3 Human knowledge in natural philosophy (sensitive knowledge)
• 3. Tension in Locke’s thought and a consequent debate
  • 3.1 Tension in Locke’s thought
  • 3.2 Limitations of the corpuscular hypothesis
  • 3.3 Main positions in the debate
• 4. Locke and Newton
  • 4.1 Epistemology and methodology
  • 4.2 Ontology
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Introduction

Two features of Locke’s intellectual landscape are most salientfor understanding his philosophy of science, one of which concerns thenew science’s methodology, and the other its content. First,then, is the new methodological approach to understanding the naturalworld. This approach is accompanied by profound shifts in disciplinaryboundaries and in conceptions of induction and scientific knowledge.Locke’s reaction is mostly progressive. Impressed byexperimental methods and cognizant of their poor fit with theAristotelian ideal, he defines a distinct kind of knowledge, oneinferior to genuine scientific knowledge but appropriate to humansensory capacities. In so doing, he develops an epistemological basisfor the new, experimental philosophy. Yet his reaction also has itsconservative aspect, one that some see as having limited him in theface of the new science’s evolving methodology. He retains anideal notion of scientific knowledge as demonstrative and certain, andwhile the speculative systems of the Aristotelians and the Cartesians.are the two main targets of hisEssay, he shares that idealwith them.

The second salient feature is the dominant scientific theory ofLocke’s day: the new science’s corpuscular hypothesis. Asdefined for the purposes of this article, the corpuscular hypothesis(i) takes observable bodies to be composed of material particles orcorpuscles, (ii) takes impulse (action by surface impact) to be eitherthe primary or sole means of communicating motion, and (iii) attemptsto reduce colour and other qualities at the level of observable bodiesto the primary or inherent properties of the particles composing thosebodies. In what may be called its orthodox version (“puremechanism,” as Ayers (1981, p. 212) calls it) the corpuscularhypothesis restricts those inherent properties to size, shape, number,and motion, and holds that all other qualities and operations areexplicable in terms of that restricted set of properties. The orthodoxversion thus implies aproviso of contact action—thatbodies causally interact only locally, by impact, such that unmediatedaction at a distance is deemed impossible. (Although a number ofcommentators use the terms ‘corpuscular hypothesis’ and‘mechanism’ interchangeably, distinguishing them hascertain benefits. For instance, it permits us to classify Isaac Newtonamong the corpuscularian theoristis without engaging the debate aboutwhether he adhered to the contact action proviso. The definitionsgiven here also agree largely with those in the entry onJohn Locke.) Plenist and atomist versions of the corpuscular hypothesis may bedistinguished. Plenist theorists deny the void and assert a plenum ofmatter, as Descartes does by identifying matter with extension, andthough these theorists speak of particles, their particles are notatoms, being infinitely or at least indefinitely divisible. Atomisttheorists, by contrast, accept the void and take the particles orcorpuscles comprising compound bodies to be indivisible, or at leastprobably so. Since Locke’s sympathies clearly lie with theatomist version, the term ‘corpuscular hypothesis’ shallrefer to that variant throughout this article unless indicatedotherwise. Locke develops central theses of theEssay inclose conjunction with the corpuscular hypothesis. The most notable ofthese is the distinction between real and nominal essences, the formerdenoting a substance’s internal constitution and the latterdenoting the observable properties we use to name or categorize it.Although Locke develops that distinction in connection with theprimary-secondary quality distinction associated with corpusculartheorists, including his mentor, Robert Boyle, it does not follow thatthe two distinctions are interchangeable. Locke often treats thehypothesis with skepticism, and its status and purpose are a source ofcontroversy.

This article examines questions connected with the two salientfeatures noted, and in connection with the first, it also examinesLocke’s relationship to Newton, a figure instrumental to thechanging conceptions of scientific knowledge. Section 2 addressesquestions connected to those conceptions. What does Locke take science(scientia) or scientific knowledge to be generally, why doeshe think thatscientia in natural philosophy is beyond thereach of human beings, and what characterizes the conception of humanknowledge that he develops for natural philosophy? Section 3 addressesthe question provoked by Locke’s apparently conflictingtreatments of the corpuscular hypothesis. Does he accept or defend thecorpuscular hypothesis? If not, what is its role in his thought, andwhat explains its close connection to key theses of theEssay? Since a scholarly debate has arisen about the statusof the corpuscular hypothesis for Locke, Section 3 reviews some mainpositions in that debate. Section 4 considers the relationship betweenLocke’s thought and Newton’s. All citations ofAnEssay Concerning Human Understanding are indicated by‘E’, followed by the book and section numbers. Pagenumbers 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 aconception of human knowledge suitable for the experimental science ofhis day, one that in natural philosophy will replace the old,Aristotelian conception. According to the Aristotelian conception,scientific knowledge—scientia—is certainknowledge of necessary truths, which can be expressed in syllogisticform, the conclusion following from self-evident premises. In thedomain of natural philosophy, it is certain knowledge of realessences. Although Locke does not seriously entertain a radicalskepticism—finding “a very manifest difference betweendreaming of being in the Fire, and being actually in it” (EIV.ii.14, pp.537–538)—he recognizes that the demands ofscientia are too stringent for the new, experimental science.Nevertheless, the concept ofscientia plays an important roleas he develops his conception of the probabilistic sort of knowledgethat is possible for humans in the domain of natural philosophy: itserves as a foil as he explains why humans must settle forprobabilistic knowledge in natural philosophy, wherescientiawill always remain out of reach.Scientia can serve as a foilbecause it is attainable for human beings in certain domains; howeverLocke thinks that for nobler spirits, notably angels, it is alsoattainable within natural philosophy.

This section begins by reviewing the history of the concept ofscientia and the factors that prevent human beings fromattaining it in natural philosophy, which impel Locke to develop analternative conception of human knowledge there. This section alsoexplains what Locke thinksscientia in natural philosophywould amount to and the lesser human knowledge that must serve us inits place.

2.1 Historical roots ofscientia

The conception of genuine scientific knowledge that Locke inherits andto some extent retains,scientia, has its roots in Aristotle,specifically in the beginning of Book I, §2 of thePosterior Analytics.^[1] For Aristotle, scientific knowledge can have only necessary truths asits objects and requires a knowledge of causes. Accordingly,scientific knowledge of a fact requires knowing its necessity byknowing its necessary relations to its causes. Although there aremanifold causes, the main one at issue here is the formalcause—the nature or essence—as Aristotle indicates elsewhere.^[2]

We suppose ourselves to possess unqualified scientific knowledge of athing, as opposed to knowing it in the accidental way in which thesophist knows, when we think that we know the cause on which the factdepends, as the cause of that fact and of no other, and, further, thatthe fact could not be other than it is….The proper object ofunqualified scientific knowledge is something which cannot be otherthan it is (Aristotle,Posterior Analytics, I.2).

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

We do know by demonstration. By demonstration I mean a syllogismproductive of scientific knowledge, a syllogism….The premissesmust be primary and indemonstrable; otherwise they will requiredemonstration in order to be known, since to have knowledge, if it benot accidental knowledge, of things which are demonstrable, meansprecisely to have a demonstration of them. The premisses must be thecauses of the conclusion, better known than it, and prior to it; itscauses, since we possess scientific knowledge of a thing only when weknow its cause; prior, in order to be causes; antecedently known, thisantecedent knowledge being not our mere understanding of the meaning,but knowledge of the fact as well (Aristotle,PosteriorAnalytics, I.2).

The stipulation that the premises of a scientific demonstration mustbe indemonstrable, that is, self-evident, leads to aprimafacie difficulty. This conception of scientific knowledge isintended to encompass not only conceptual propositions, but alsopropositions about the real natures or essences of substances, whichare propositions about the world. As with any other demonstration, ademonstration in natural philosophy must have premises that areself-evident, since otherwise a regress would ensue. Propositionsabout the world are experience-based, however, and therefore, it isnot clear how a demonstration’s premises could beself-evident.

It would be overstating matters to say that this problem appears onlyfrom a contemporary perspective; there was some recognition of it inthe ancient and medieval periods.^[3] Still, before the advent of the experimental science, the problem wasnot strongly felt because experience was understood in a differentway. For one thing, the notion of an experiment—an artificiallyconstructed, single event or series of events designed to test for apredicted outcome—did not exist. Moreover, among the scholasticAristotelians, a single, naturally occurring event could not by itselfbe regarded as revealing of natural processes; such an event could bea “monster,” that is, an event that conflicts with naturerather than having been produced by it.^[4] Events as generally experienced, however, were regarded as beingrevealing of nature and consequently, they could provide the universaltruths needed as premises in the syllogism.^[5] How was the gap between events as usually experienced—whichstill amounts to a limited sample of evidence—and the universalclaim derived from them bridged? It is only from the modern andcontemporary perspectives that such a gap exists to be bridged. ForAristotle and medieval thinkers, human faculties are so constituted asto be able to apprehend nature, that is, to discern the essences of substances.^[6] In short, then, because the internal essences that form the contentof natural philosophy are real, and because our faculties areconstituted to apprehend those real essences, natural philosophy canbe a science; it is a domain in which certain, demonstrative knowledgecan be had, despite its dependence upon experience.

The exemplars of scientific knowledge, to be sure, are conceptualdisciplines, not only geometry, but also rational theology, the latterbeing the quintessential science for the medievals.^[7] Yet for the most part, natural philosophy stands side by side withconceptual disciplines, even into the modern period. Bacon, thoughassociated with induction, accepts the demonstrative conception ofscientific knowledge, and so does Galileo, who uses experiments toreveal 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 ofscientia comes under increasing pressure.^[9] Some thinkers resist the pressure, most notably Descartes, whoderives his laws of nature by an a priori reflection upon God’snature, and, placing his confidence in these rationalist methods,denies that his third law of nature is undermined when observations ofcolliding bodies conflict with it.^[10] Yet for the experimentalists themselves, including Locke’smentor, Boyle, observations and experiments are the primary route toknowledge. (It should be noted, however, that most advocates ofexperimentalism did not consider speculation wholly illegitimate,instead insisting that it be delayed until considerable experimentaland observational evidence had been amassed; see Anstey (2011, pp.4,5).) It is this approach that puts natural philosophy on the path toHume’s problem of induction and most influences Locke.^[11]

2.2Scientia in natural philosophy and obstacles to human attainment

As indicated earlier,scientia serves as the backdrop againstwhich Locke develops the conception of the type of knowledge that ishumanly possible in natural philosophy. Having retained the ideal ofscientia exemplified by geometry while also havingassimilated the significance of Boyle’s experimental method,Locke is driven to his characteristic pessimism about the kind andextent of knowledge possible for us in natural philosophy. “Themeanest, and most obvious Things that come in our way, have darksides, that the quickest Sight cannot penetrate into” (EIV.iii.22, p. 553). Due to the weakness of our faculties, Lockesuspects, “natural Philosophy is not capable of being made aScience” (E IV.xii.10, p. 645). This section considersLocke’s general notion ofscientia, what would berequired forscientia in natural philosophy, and theobstacles that prevent human beings from attainingscientiain that domain.

2.2.1Scientia in general

Locke takes knowledge generally to consist in “the perception ofthe connexion and agreement, or disagreement and repugnancy of any ofour Ideas” (E IV.i.1–2, p. 525), and among the three kindsof knowledge that he distinguishes—intuitive, demonstrative, andsensitive—the former two are kinds of certain knowledge.Intuitive and demonstrative knowledge differ in the number ofintuitions involved, and consequently, differ in their degree ofcertainty (E IV.ii.14, pp.537–538). Intuitive knowledge is themost certain because the truth is grasped immediately. There are nointermediate steps, and doubt is impossible because the mind can nomore avoid recognizing the truth than the open, functioning eye couldavoid seeing light when turned toward the sun (see E IV.ii.1, p. 531).Demonstrative knowledge, though also qualifying as certain, is less sobecause it involves intermediate steps. We cannot grasp immediatelythat the three angles of any triangle are equal to two right trianglesbut must instead construct the steps of a proof. Upon doing so andgrasping the connections among the proof’s steps, we havedemonstrative knowledge (E IV.ii.2–3, p.531–532).Intuitive and demonstrative knowledge are forms ofscientia,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 ofscientia, we mustconsider its objects: real essences and the necessary connections thatflow from them. According to the Aristotelian view, a single essenceboth grounds the properties of a thing, making it what it is, andprovides the basis for classifying it. Repudiating that view, Lockedraws a distinction between real and nominal essences.^[12] Whereas the nominal essence consists of the set of observablequalities we use to classify a thing (which implies that the nominalessence could vary across time or communities), the real essence (orreal or internal constitution, as he sometimes writes) is that whichmakes a thing what it is.

Essence may be taken for the being of any thing, whereby it is what itis. And thus the real internal, but generally in Substances, unknownConstitution of Things, whereon their discoverable Qualities depend,may be called theirEssence. This is the proper originalsignification of the Word, as is evident from the formation of it;Essentia, in its primary notation signifying properlyBeing. And in this sense it is still used, when we speak oftheEssence of particular things, without giving them anyName (E III.iii.15, p. 417).

Discussion of real essence often focuses upon the real essences ofmaterial substances, and in that case we say that the real essence isthe causal ground of the substance’s perceivable properties (forfurther details and a useful chart, see the entry by Jones,“Locke on Real Essence”). As will be discussed in asubsequent section, many commentators interpret Locke as identifying amaterial substance’s real essence with some subset of itsconstituent corpuscles’ primary qualities, but thatidentification presums Locke’s acceptance of the corpuscularhypothesis (e.g., Osler 1970, p. 12; Mandelbaum 1964, p.1). Accordingto another interpretation, the real-nominal essence distinction ismetaphysical, and thus more fundamental than the primary-secondaryquality distinction, which is a physical distinction, belonging as itdoes to a particular physical theory, the corpuscular hypothesis. Yetleaving that debate aside for the moment, we may note from the firstsentence of the above-quoted passage that Locke does not restrict thenotion of real essence to substances. This means that we can speak of,say, the real essence of a triangle, understanding it as that whichgrounds the triangle’s qualities, making it what it is.

Since a substance’s qualities flow from its real essence, havingscientific knowledge of a substance requires knowing both its realessence and the necessary connections between that and its otherqualities. Geometry serves as an exemplar, as it did for so many ofLocke’s predecessors. In knowing what a triangle is, we cannotconceive things being otherwise than that the sum of its three anglesequals the sum of two right angles.

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

Thus theIdea of a right-line Triangle necessarily carrieswith it an equality of its Angles to two right ones. Nor can weconceive this Relation, this connexion of these twoIdeas, tobe possibly mutable, or to depend on any arbitrary Power, which ofchoice 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 necessaryconnections, and Locke is adamant that we can have the same level ofcertainty there as in geometry.

Where there is no Property, there is no Injustice, is aProposition as certain as any Demonstration inEuclid: FortheIdea ofProperty, being a right to any thing;and theIdea to which the NameInjustice is given,being the Invasion or Violation of that right; it is evident,that…I can as certainly know this Proposition to be true, asthat a Triangle has three Angles equal to two right ones (E IV.iii.18,pp. 549–50).

2.2.2Scientia in natural philosophy

What would be required forscientia in natural philosophy?Sincescientia generally concerns real essences, and sincenatural philosophy for Locke concerns material substances and theirpowers,scientia in natural philosophy would be knowledge ofmaterial substances’ real essences and their necessaryconnections to the qualities flowing from them.

If we could havescientia in natural philosophy, we couldknow a substance’s qualities without making observations orexperiments. To take one of Locke’s frequent examples, if wecould know gold’s real essence, we would then know itsqualities, even if not a single sample of gold existed.

Had we suchIdeas of Substances, as to know what realConstitutions produce those sensible Qualities we find in them, andhow those Qualities flowed from thence, we could, by the specifickIdeas of their real Essences in our own Minds, more certainlyfind out their Properties, and discover what Qualities they had, orhad not, than we can now by our Senses: and to know the Properties ofGold, it would be no more necessary, thatGoldshould exist, and that we should make Experiments upon it, than it isnecessary for the knowing the Properties of a Triangle, that aTriangle should exist in any Matter, theIdea in our Mindswould 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 ableto deduce a substance’s tertiary qualities, that is, its powersto produce certain effects in other substances.^[13] If we knew the real essences of opium and hemlock, then just as if wewere performing a geometric deduction, or just as a locksmithunderstands why a given key will open one lock rather than another, wecould deduce that opium produces sleep, that hemlock causes death andwhy each substance produces its effects.

I doubt not but if we could discover the Figure, Size, Texture, andMotion of the minute Constituent parts of any two Bodies, we shouldknow without Trial several of the Operations one upon another, as wedo now the Properties of a Square, or a Triangle. Did we know theMechanical affections of the Particles ofRhubarb, Hemlock,Opium, and aMan, as a Watchmaker does those of a Watch,whereby it performs its Operations, and of a File which by rubbing onthem will alter the Figure of any of the Wheels, we should be able totell before Hand, thatRhubarb will purge,Hemlockkill, andOpium make a Man sleep….The dissolving ofSilver inaqua fortis, and Gold inaqua Regia, andnotvice versa, would be then, perhaps, no more difficult toknow, that it is to a Smith to understand, why the turning of one Keywill 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 atlength inThe 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 areinitially less clear; Locke seems to be saying that while having moreacute senses would not eliminate the secondary quality of sound, butmight do away with the secondary quality of color. In a passage wherehe imagines our having very acute senses, including“microscopical eyes,” he unquestionably assumes that wewould still experience the secondary quality of sound: “If ourSense of Hearing were but 1000 times quicker than it is, how would aperpetual noise distract us” (E II.xxiii.12, pp. 302–303).Yet, in a preceding passage, he suggested that if our faculties weredesigned for detecting real essences, we would not experience color atall:

Had we Senses acute enough to discern the minute particles of Bodies,and the real Constitution on which their sensible Qualities depend, Idoubt not but they would produce quite differentIdeas in us;and that which is now the yellow Colour of Gold, would disappear, andinstead of it we should see an admirable Texture of parts of a certainSize and Figure. This Microscopes plainly discover to us: for what toour naked Eyes produces a certain Colour, is by thus augmenting theacuteness of our Senses, discovered to be quite a different thing; andthe thus altering, as it were, the proportion of the Bulk of theminute parts of a coloured Object to our usual Sight, producesdifferentIdeas, from what it did before….Blood to thenaked Eye appears all red; but by a good Microscope, wherein itslesser parts appear, shews only some few Globules of Red, swimming ina pellucid Liquor; and how these red Globules would appear, if Glassescould 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, ornot always, imagining that microscopical eyes would do away with coloraltogether; rather, it might in some cases enable us to view tinierparticles having different colors than those we perceive in theaggregate object. In his example, only as an aggregate body does bloodas an aggregate body appears uniformly red; under a microscope, onlythe globules appear red, while some other parts of it seemtranslucent. Once the microscope is used, color is not eliminated fromthe experience of seeing blood, but is instead seen as beingdifferently distributed. A pixelated painting provides a roughanalogy; a shape seen from afar may appear uniformly green, but upclose is seen as comprising tiny blue and yellow dots.

2.2.3 Obstacles to human attainment

Scientia in natural philosophy would require knowledge ofboth real essences and their necessary connections among qualities,yet neither is possible for human beings, Locke concludes. Oneobstacle toscientia, then, is that real essences escape us.God has given us sensory capacities that are suitable for suchpractical endeavors as finding our way to the “market andexchange,” but as the “microscopical eyes” passageindicates, they are not useful for detecting the minute parts ofbodies.

Another obstacle is that we are almost entirely unable to discern thenecessary causal connections among the qualities of substances. (AndLocke does take those connections to be necessary—but does heconstrue them in terms of nomological or logical necessity? Given hisview that demonstrative knowledge in natural philosophy is possiblefor immortals and serves as an ideal for mortals, he seems to bethinking in terms of logical necessity, much like the Aristotelians hewas reacting against (see Ott, 2009, p. 13). Locke does find twoinstances in which we can discern necessary connections betweenqualities of bodies: “Some few of the primary Qualities have anecessary dependence, and visible connexion one with another, asFigure necessarily presupposes Extension, receiving or communicatingMotion by impulse, supposes Solidity” (E IV.iii.14, p. 546).Apart from these two exceptions, however, necessary connections escapeus. In part, this is due to the first obstacle, our inability todiscover real essences, due to the minuteness of particles. It is alsodue, however, to the remoteness of so many bodies, those which lie“beyond this our Earth and Atmosphere…even beyond theSun, 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’sspeculations, all things might be causally interconnected in complexways, in which case knowing one real essence would require knowing allthose with which it is causally connected.

Still, while human beings cannot attainscientia in naturalphilosophy, there are other epistemic agents who can. God certainlyknows real essences (E III.vi.3, p. 440), and “’tispossible Angels have” ideas of real essences as well (EIII.vi.3, p. 440).

It be not to be doubted, that Spirits of a higher rank than thoseimmersed in Flesh, may have as clearIdeas of the radicalConstitution of Substances, as we have of a Triangle, and so perceivehow all their Properties and Operations flow from thence, but themanner how they come by that Knowledge, exceeds our Conceptions (EIII.xi.22, p. 520).

That Locke finds it natural to speak in the same breath of matter andspirits marks him as belonging to the age of natural philosophy ratherthan science. It is because knowledge of necessary connections can bereferred to these higher epistemic agents thatscientia is sostrongly entrenched as an ideal, even though he recognizes the needfor a quite different conception of knowledge.

2.3 Human knowledge in natural philosophy (sensitive knowledge)

The conclusion that no intuitive or demonstrative knowledge ofsubstances is possible for us because their real essences andnecessary connections remain out of reach leaves Locke at acrossroads. 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 (EIV.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 thebar by admitting a third kind of knowledge, which lacks certainty:sensitive knowledge.^[14]

Sensitive knowledge is knowledge of the “effects [that] comeevery day within the notice of our Senses,” without anunderstanding of their causes; “we must be content to beignorant of” those causes (E IV.iii.29, pp. 559–560).Instead of knowing real essences, the causal basis of the propertieswe perceive, we know only those perceived properties, from which weconstruct nominal essences. Instead of employing deduction, we areforced to rely upon “trials”—observations andinduction. Instead of knowing the necessary connections holdingbetween a substance’s real essence and its other qualities,including its tertiary qualities (which might include, recall, causalconnections with substances beyond the remotest star), we know onlythe co-existences of properties. And from the mere, regularco-existence of properties found in observed cases, Locke observes, wecould not know with certainty that the same set will be foundco-existing in the next case.

For all the Qualities that areco-existent in any Subject,without this dependence and evident connexion of theirIdeasone with another, we cannot know certainly any two toco-exist any farther, than Experience, by our Senses, informsus. Thus though we see the yellow Colour, and upon trial find theWeight, Malleableness, Fusibility, and Fixedness, that are united in apiece of Gold; yet because no one of theseIdeas has anyevidentdependence, or necessary connexion with the other, wecannot certainly know, that where any four of these are, the fifthwill be there also, how highly probable soever it may be (E IV.iii.14,p. 546).

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

Herein therefore is founded thereality of our KnowledgeconcerningSubstances, that all our complexIdeas ofthem must be such, and such only, as are made up of such simple ones,as have been discovered to co-exist in Nature. And ourIdeasbeing thus true, though not, perhaps, very exact Copies, are yet theSubjects ofreal (as far as we have any)Knowledgeof them (E IV.iv.12, p. 568).

As for general claims about substances based upon observed particularmatters of fact, these too can qualify as real knowledge. Admittedly,when four of the five properties previously found co-existing togetheroccur again, it is only probable that the fifth will be present aswell. Yet we can still form an abstract idea of gold, as a substancehaving all five properties, and call this general claim knowledgebecause “whatever have once had an union in Nature, may beunited again” (E IV.iv.12, p. 568).

Has contemporary science enabled us to go beyond sensitive knowledge?In particular, have discoveries about compounds, elements, andsubatomic particles provided us with knowledge of real essences? Muchof the force of this question derives, to paraphrase Nicholas Jolley,from the fact that many of those discoveries about matter’sstructure were not conceived empirically, but only confirmedempirically; they were initially conceived as possibilities throughthe hypothetico-deductive model, and the predictions deduced from themodels were then compared to empirical data (Jolley 2002, p. 69). Yetas Jolley also points out, these commentators may have missed the fullimport of Locke’s geometric model; in a passage quoted earlier,Locke tells us explicitly that if we knew the real essence of gold, wecould deduce its qualities even if the metal did not exist.^[16] So while the predictions of any model developed via thehypothetico-deductive model must survive the test of observations,observations in Locke’sscientia arewhollyunnecessary. To put the point another way, Locke takes naturalphilosophy to be an empirical domain only for human beings, whosefaculties are impoverished. For nobler spirits, it would resemblegeometry.

3. Tension in Locke’s thought and a consequent debate

The last few decades have seen a lively debate about the role of thecorpuscular hypothesis in Locke’sEssay. This sectionexamines the sources of that debate and reviews some of the mainpositions figuring in it.

3.1 Tension in Locke’s thought

As we have seen, Locke develops some central theses of hisEssay in connection with the corpuscular hypothesis. In histheory of ideas, corpuscles provide at least a structural basis forsimple ideas, and depending upon one’s interpretation, there maybe a causal relationship as well. Further, and of particular interesthere, Locke often appears to identify a material substance’sreal essence with the set or some subset of its componentparticles’ primary qualities. In the following well-knownpassage, for instance, he points to the primary qualities of abody’s parts—their bulk or solidity, motion, andshape—as the causal ground of the qualities we perceive.

The particularBulk, Number, Figure, and Motion of the parts ofFire, or Snow, are really in them whether any ones Sensesperceive them or no: and therefore they may be calledrealQualities, because they really exist in those Bodies. ButLight, Heat, Whiteness, orColdness, are no more reallyin them, than Sickness or Pain is in Manna. Take away theSensation of them; let not the Eyes see Light, or Colours, nor theEars hear Sounds; let the Palate not Taste, nor the Nose Smell, andall Colours, Tastes, Odors, and Sounds, as they are such particularIdeas, 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 primaryqualities just prior to the “microscopical eyes” passage.There, he suggests that instead of seeing colors (or instead of seeingthem as we currently do), we could discover bodies’ internalconstitutions, if only we knew the “texture and motion of theminute Parts of corporeal things” (E II.xxiii.12, pp.302–303). A commitment to the corpuscular hypothesis is againsuggested when he despairs of understanding the production ofsecondary qualities: even if “we could discover the size,figure, or motion of those invisible parts, which immediately producethem [secondary qualities],” we still cannot discover any“undoubted Rules” concerning their production orconnection, nor “conceive how anysize, figure, ormotion of any Particles, can possibly produce in us theIdea of anyColour, Taste, orSound”(E IV.iii.13, p. 545). Here, he appears to despair of understandinghow secondary qualities are produced by primary ones; he appears totake 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 Constituentparts of any two Bodies,” we would then be able to derivetertiary qualities; we would be able to deduce that opium causessleep, and we would understand why (E IV.iii.25, pp. 555–56; seealso E IV.iii.13, p. 545). In all of these passages, then, and in manysimilar ones, Locke appears to accept at least some components of thecorpuscular hypothesis—that material bodies are compounded fromminute particles, and certain observable qualities are reducible tothe 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 beentermed his “agnostic” or “skeptical” side. Thefollowing features of his discussion seem to suggest that he hasreasons either for remaining agnostic about whether the corpuscularhypothesis is true, or more seriously, for believing that it is whollyunable to explain the phenomena it purports to explain and thereforecannot be true.

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

I have here instanced in the corpuscularian Hypothesis, as that whichis thought to go farthest in an intelligible Explication of theQualities of Bodies; and I fear the Weakness of humane Understandingis scarce able to substitute another, which will afford us a fullerand clearer discovery of the necessary Connexion, andCo-existence, of the Powers, which are to be observed unitedin several sorts of them. This at least is certain, that which everHypothesis be clearest and truest, (for that it is not my business todetermine,) our Knowledge concerning corporeal Substances, will bevery little advanced by any of them, till we are made see, whatQualities and Powers of Bodies have anecessary Connexion orRepugnancy one with another; which in the present State ofPhilosophy, 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 alsoinSome Thoughts Concerning Education: “The systems ofnatural philosophy…are to be read, more to know thehypotheses…than with hopes to gain thereby acomprehensive, scientifical, and satisfactory knowledge of the worksof nature” (Locke, quoted in Rogers 1982, p. 230). Still, whileall physical theories are ultimately hypotheses, it is useful to bearin mind Peter Anstey’s remarks about this one’s genesis.The hypothesis originated among theorists championing an experimentalmethod over a purely speculative one, and while they did not excludespeculation entirely, they circumscribed its role, engaging in it onlyafter observation and experiment had already given the theory a solidfoundation. Furthermore, the corpuscular hypothesis had credibilityinsofar as its theorists avoided the question about matter’sinfinite divisibility and thereby adhered to their prohibition againstquestions they thought could never be answered experimentally (seeAnstey, 2011, pp. 4–5).

Second, if Locke indeed identifies material bodies’ realessences with the primary qualities of their constituent corpuscles,that view of real essences combined with his pessimism about everdiscovering them implies pessimism about the corpuscular hypothesis.Specifically, it implies pessimism about the claims that bodies aremade of corpuscles and that those bodies’ observable qualitiesare reducible to the corpuscles’ qualities. In the same passageswhere Locke seems to support or assume the corpuscularhypothesis’s central tenets—that observable bodies aremade up of corpuscles and that those corpuscles have a restricted setof inherent properties—he simultaneously appears very skepticalabout the possibility of reducing observable properties such as colorand taste to that restricted set of primary properties.

Third, Locke arguably believes that the corpuscular hypothesis’slimitations are so serious that they amount to fatal flaws, aninterpretation that Wilson (1979) was perhaps the first to defend.Although Wilson develops her line of argument mainly in connectionwith difficulties Locke raises about the corpuscularhypothesis’s purported ability to explain sensation and moregenerally, the relation between thought and matter,^[17] some other phenomena are troublesome as well. Locke appears toconsider such phenomena so obscure that we can attempt to understandthem only by attributing them to God’s direct action.

The coherence and continuity of the parts of Matter; the production ofSensation in us of Colours and Sounds,etc. by impulse andmotion; nay, the original Rules and Communication of Motion beingsuch, wherein we can discover no natural connexion with anyIdeas we have, we cannot but ascribe them to the arbitraryWill and good Pleasure of the Wise Architect (E IV.iii.29, pp.559–560).

Elsewhere, Locke will use the term ‘superaddition’ torefer to God’s role. For the moment, superadded properties maybe neutrally described as those that God confers by fiat. Section3.3., which reviews some of the main positions in the debate aboutLocke’s stance toward the corpuscular hypothesis, will alsoaddress 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 considertoo obscure for the corpuscular hypothesis to illuminate. Theabove-quoted passage mentioned three of these phenomena— theproduction of sensation, the communication of motion, and cohesion.^[19] Locke discusses the fourth, gravity, only outside theEssay.

3.2.1 Sensation

As we saw in passages discussed earlier, in connection withscientia’s impossibility, Locke finds the production ofsensation to be utterly obscure. One side of the difficulty is thenature of the mind. While in all probability, it is immaterial, Lockeallows the possibility that God superadded the power of thoughtdirectly to matter. The other side of the difficulty concerns thenature of secondary qualities as powers to produce sensations. Theappeal of the corpuscular hypothesis lay largely in its reductivepromise. Reduction was expected particularly for secondary qualities,such as colors and sounds, but was also anticipated for ideas ofmacro-level primary qualities, including visual sensations of shapesand sizes, and tertiary qualities. All would be reduced to the primaryqualities of bodies’ component corpuscles as they interact withone another and our perceptual systems.

One part of the corpuscular hypothesis’s purported explanationis conceivable, namely, the interactions among the primaryqualities of bodies, which are supposed to be part of the causal basisof our sensations:

That the size, figure, and motion of one Body should cause a change inthe size, figure, and motion of another Body, is not beyond ourConception; the separation of the Parts of one Body, upon theintrusion of another; and the change from rest to motion, uponimpulse; these, and the like, seem to us to have someconnexion one with another (E IV.iii.13, p. 545).

Indeed, we are capable of discerning necessary connections in twoinstances, as noted earlier. (One case involves only primaryqualities— “Figure necessarily presupposesExtension” (E IV.iii.14, p. 546)—while the other involvestertiary and primary qualities—“receiving or communicatingMotion by impulse, supposes Solidity” (E IV.iii.14, p. 546).) Ifwe knew more about the primary qualities of bodies, we might multiplysuch instances: “And if we knew these primary Qualities ofBodies…we might be able to know a great deal more of theseOperations of them one upon another”. That is, if we knew realessences, we could derive more necessary connections, to know thecausal relation between opium and sleep, for instance, and ascertainly as we now know that impulse requires solidity.

Still, knowing real essences would not give us any genuine knowledgeof howsensations are produced by primary qualities. Whilecorpuscular theorists such as Galileo (The Assayer) sketcheda reductive account of our sensations of taste in terms of particlesstriking our tongues, Locke suggests that any attempt to discover theprocess’s details will be foiled. For as far as we can imagine,a body that strikes other bodies can produce “nothing butMotion”^[20] (E IV.iii.6, pp. 540–541), and motion may itself be hopelesslyobscure, as indicated below. The roles of shape and size are equallyobscure; we cannot imagine how they could help produce sensations.

We are so far from knowing what figure, size, or motion of partsproduce a yellow Colour, a sweet Taste, or a sharp Sound, that we canby no means conceive how anysize, figure, or motion of anyParticles, can possibly produce in us theIdea of anyColour, Taste, orSound whatsoever; there is noconceivableconnexion betwixt the one and the other (EIV.iii.13, p. 545).

Although Locke mentions only secondary qualities here, his pointpresumably applies to all sensations, including our sensations ofmacro-level primary qualities, such as the shape and size of asnowball or lump of gold. For again, genuine knowledge is knowledge ofnecessary connections, with the conceptual relations in geometry beingthe model, and it does not seem possible to discover such connectionsbetween any sensation and the sizes, shapes, and textures that arealleged to cause them. Macro-level primary qualities to micro-levelones and accordingly resemble them.^[21] but the idea of a quality is nonetheless very different than thequality itself.

Locke finds that our only way of understanding the production ofsensation is to attribute the process to God. If we try to understandhow motion could produce a color, sound, or taste, “we are fainto quit our Reason, go beyond ourIdeas, and attribute itwholly to the good pleasure of our Maker” (E IV.iii.6, p.540–541; see also IV.iii.28, p. 559). Locke was certainly notalone in grappling with this problem. Commenting uponDescartes’s account, Walter Charleton (1654, p. 197) hadobserved earlier that even a detailed knowledge of light behaviorwould still leave us with the “superlative difficulty” ofunderstanding why a certain reflection or refraction should be“transformed into a Vermillion rather than a Blew,” andfurther details about our sensory apparatus do not reveal “anyAnalogy betwixt the Retina Tunica … and any oneColour”. Locke was also not alone in resorting to a divinecause; inDe 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’sPrincipia implied the possibility ofunmediated action at a distance, and with its publication, gravitybecame the most nettlesome phenomenon for the orthodox version of thecorpuscular hypothesis due to its proviso of contact action. Locke wasinitially sympathetic to the proviso, writing in the first threeeditions of hisEssay, “How bodies operate one uponanother…is manifestly by impulse and nothing else. It beingimpossible to conceive that body should operate onwhat it doesnot touch” (E II.viii.11, editions 1–3). Yet for thefourth edition, he replaced that claim about how bodies do operatewith one about how we can conceive of them operating: “HowBodies produceideas in us is manifestlyby impulse,[this being] the only way which we can conceive Bodies [to]operate” (E II.viii.11, edition 4). He also omitted a clausedenying unmediated action at a distance, which had appeared inII.viii.12 of previous editions.^[22] These subtle emendations reflect a dramatic shift, one expresseddirectly 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 intobodies powers and ways of operation, above what can be derived fromour idea of body, or can be explained by what we know of matter, butalso an unquestionable and every where visible instance, that he hasdone 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 contactaction proviso and to attribute to matter the power of actingdistantly, even though he considered the process by which suchinteractions could occur so obscure that it drove him to invokesuperaddition. This is the prevailing interpretation of Locke (and wasassumed by Leibniz, who targeted Locke for it inAgainst BarbaricPhysics), though not all commentators agree, as indicated in asubsequent section.

3.2.3 Motion

Locke takes the notion of impulse, in which bodies communicate motionto one another by surface impact, to be, along with extension andcohesion, 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 beingcommunicated, a view of our conceptual abilities that Locke maintains,as we saw, despite his changing thoughts about gravity. Impulse isalso fundamental to the corpuscular hypothesis’s explanation ofphenomena, being either the exclusive means of interaction amongbodies, as adherents of the contact action proviso hold, or the meansof at least many interactions. Yet how exactly does a moving bodycommunicate motion to a resting one simply by impacting it? When weattempt to discover the precise nature of the process, Locke suggests,we find that it is just as mysterious as the process by which the mindmoves the body.

AnotherIdea we have of Body, is the power ofcommunication of Motion by impulse; and of our Souls, thepower ofexciting of Motion by Thought…. But if hereagain we enquire how this is done, weare equally in thedark. For in the communication of Motion by impulse, wherein asmuch Motion is lost to one Body, as is got to the other, which is theordinariest case, we can have no other conception, but of the passingof Motion out of one Body into another; which, I think, is as obscureand unconceivable, as how our Minds move or stop our Bodies byThought….The increase of Motion by impulse, which is observedor believed sometimes to happen, is yet harder to be understood. Wehave by daily experience clear evidence of Motion produced both byimpulse, and by thought; but the manner how, hardly comes within ourcomprehension; we are equally at a loss in both (E II.xxiii.28, p.311).

Since the corpuscular hypothesis holds impulse to be the primary ifnot the sole means by which bodies causally interact, then anyphenomenon that the corpuscular hypothesis purports to explain byimpulse will remain obscure if impulse itself remains obscure. All ofthe hypothesis’s reductions of observable primary, secondary,and tertiary qualities, would inherit impulse’s obscurity; thusLocke appears to be suggesting here that the corpuscular hypothesiscannot fulfill its promise of explaining and reducing those propertiesand powers.

3.2.4 Cohesion

Since the claim that observable bodies are made up of particles iscentral to the corpuscular hypothesis, an immediate question for itsproponents asks how the particles cohere into compound bodies.Plenists have some resources for answering that question, though thoseresources may introduce worse difficulties. Descartes, for instance,though he speaks in terms of particles, understands an individual bodyas an area of extension moving as one with respect to surroundingareas. It is not possible for any particle to move away into emptyspace, there being no such thing as empty space once matter isidentified with extension; with every bit of matter pressed from allsides by other matter, there is no problem about cohesion per se,though there is certainly a problem about individuating bodies fromone another. In the same vein, Malebranche can invoke the pressure ofthe air to explain the coherence of bodies, and then invoke thepressure of an aether to explain the coherence of air particles. Lockeobjects that this explanation fails because it leaves us with thequestion of what causes the particles of the aether to cohere (EII.xxiii.23, p, 308). The objection reveals Locke’s atomistsympathies, drawing its power from the presumption that there is sucha thing as empty space into which the aether particles could move. Theproblem for atomist versions of the corpuscular hypothesis is that therestricted set of properties that they allow the particles—size, shape, and motion—provides no obvious resources forexplaining how the particles cohere with one another to form compoundbodies. In various forms, the problem about cohesion has doggedatomists since ancient times.

The problem arises in two forms, which, to borrow James Hill’sterminology (Hill 2004), may be called the limited and thefoundational problems. The limited problem, arising for those who takecorpuscles to be genuine atoms, that is, to be indivisible, is theproblem of explaining how those indivisible corpuscles cohere with oneanother. This is the problem one finds in Newton’s writings.Although Rule 3 of thePrincipia allows the possibility thatthe least parts of matter could turn out to be divisible, his atomistsympathies are evident throughout his writings. He speculates in Query31 that in all probability, bodies are made up of hard particles thatonly God could divide, and in the body of theOpticks (BookII, Part III, Proposition VII) he suggests that more powerfulmicroscopes might permit us to see the larger particles. In answer tothe problem of how those naturally indivisible particles cohere, herejects the ancient solution of hooked particles as begging thequestion, proposing instead some short-range forces modeled on thegravitational force (Query 31). Newton’s speculations about suchforces are driven by an absence of any resolution to the problem aboutcohesion within the corpuscular theory itself.

The foundational problem pushes the question about cohesion into thecorpuscles themselves. The problem was raised by Joseph Glanvill:“If it be pretended…that the parts of solid bodies areheld together byhooks, andangulous involutions; Isay, this comes not home: For thecoherence of the parts ofthesehooks…will be of as difficult a conception, asthe former” (Glanvill,The Vanity of Dogmatizing, p.18, quoted in Hill 2004, p. 616). Without any grounds for assertingthat the divisibility of matter bottoms out in indivisible corpuscles,then, the question arises of how the parts of a corpuscle couldcohere, how the parts of those parts could cohere, and so on, endingin the question of how extended bodies are possible at all.

Whatever his ultimate view of the corpuscular hypothesis, Lockeinevitably faces the atomist’s problem about cohesion, acceptingas he does void space (see II.xiii.11, 12–14, 21–23), andholding that our ideas of body depend fundamentally upon cohesion. Oneof the ideas “proper and peculiar” to body, he writes, is“the cohesion of solid, and consequently separableparts” (E II.xxiii.17, p. 306), and the extension ofbody, as opposed to the extension of space, is “nothing, but thecohesion or continuity of solid, separable, moveable Parts” (EII.iv.5, p. 126). Yet we have no understanding of cohesion, and so ouridea of body does not rest on any genuine understanding of it. Intrying to understand how bodies are extended, we are as much in thedark as when we try to understand how the soul thinks.

’Tisas easie for himto have a clearIdea, how the Soul thinks, as how Body is extended. For sinceBody is no farther, nor otherwise extended, than by the union andcohesion of its solid parts, we shall very ill comprehend theextension of Body, without understanding wherein consists theunion and cohesion of its parts; which seems to me asincomprehensible, 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 whetherLocke acknowledged only the limited problem, as one would expect fromcommentators who read him as accepting atomism (e.g. Mandelbaum 1964,p. 1), or whether he looked further, to the more serious, foundationalproblem, as Hill argues (2004). A related controversy, to be discussedin the next section, concerns the question of whether Locke concludesthat the corpuscular hypothesis simply cannot resolve the problem(e.g., Hill 2004; Downing 2007, p. 408) or instead remains agnostic onthe issue (e.g., McCann in Chappell 1998, p. 244).

The four problematic phenomena winnow away the components of thecorpuscular hypothesis, as defined at the outset of this article. Theproblem about sensation threatens the corpuscular hypothesis’spromise of reducing secondary, tertiary, and macro-level primaryqualities to microlevel primary qualities. Newton’s resultsabout gravitational phenomena cast grave doubts upon the contactaction proviso, and according to some commentators, those results ledLocke to abandon the belief that impulse is the only means of causalinteraction. A corpuscular theorist might hope to preserve some partof the theory by insisting that impulse is still the means by whichmost other causal interactions are effected; but this runs up againstthe problem about impulse, in that the process by which motion iscommunicated seems utterly obscure. Finally, even the core claim thatobservable bodies are composed of tiny corpuscles is threatened by theproblem of cohesion. This last problem threatens to be the mostserious of four phenomena, for as James Hill has pointed out (2004, p.628), the problems about gravity, sensation, and motion arisesubsequent to our having conceived of body, whereas the problem aboutcohesion 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 betweenLocke’s seeming acceptance of the corpuscular hypothesis, mostnotable in his apparent identification of a material substance’sreal essence with the size, shape, and texture of its insensibleparts, and his pessimism about the hypothesis’s explanatorypower, most notable in his remarks about the four phenomena discussedabove.

One approach to the tension is to understand it as a genuineinconsistency. Margaret Wilson has defended such an interpretation,though in the 1979 paper that launched the debate, her intent is toshow how acutely Locke understood the explanatory limitations of“Boylean mechanism”. Specifically, Wilson argues, theinconsistency reveals Locke’s recognition “that somepresumed properties of mattercannot be conceived as‘natural’ consequences of Boylean primary qualities”(Wilson 1979, p. 197). Thus, our ignorance about bodies has moreprofound causes than our ignorance about the primary qualities of abody’s constituent corpuscles. In accordance with her view thatLocke’s agnostic tendencies (just like his dogmatic ones) aregenuine, Wilson interprets Locke’s concept of superadditionrobustly, as a sort of divine action that goes beyond the corpuscularhypothesis. According to this “non-essentialist” or“divine annexation” reading, Locke understands superaddedproperties as properties that God has annexed to matter by fiat, andthat bear no intrinsic connection to matter’s real essence. Thisreading implies a distinction in etiology for superadded qualities.Whereas the other qualities of matter are either given initially, asthose qualities constituting the real essence, or else flow from thereal essence, superadded qualities are added on independently of thereal essence, such that the substance would have been complete withoutthem. This view implies that while superadded properties areconsistently present, they are inexplicable by physical theory and,accordingly, are evidence of divine action.^[24]

Some other interpretations absolve Locke of inconsistency, either byemphasizing his so-called dogmatic side while downplaying orreinterpreting his agnostic tendencies, or emphasizing his agnosticismwhile 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; McCann1994, §1 and p. 85; McCann 2002, pp. 354–355). According toweaker versions of this reading, Locke’s project is thenaturalistic one of pursuing the philosophical implications of thebest available scientific theory, and developing an epistemologicalbasis for it. McCann, for example, reads Locke as defending theatomist version of the corpuscular philosophy over its Cartesiancompetitor by providing an epistemology for it. While Descartes hadprovided an epistemology for his plenist version, there was nothingcomparable for the atomist version associated with Gassendi and Boyleuntil Locke supplied it (McCann 2002, pp. 354–355). According toAyers’ stronger interpretation, Locke accepts “puremechanism,” that is, the orthodox version of the corpuscularhypothesis, which includes the contact action proviso. According tothis view, all of matter’s qualities flow from its real essence(Ayers 1981).

Since this line of interpretation seeks to downplay Locke’sagnostic tendencies, one challenge is to account for Locke’spessimism about the possibility of our knowing real essences.Mandelbaum meets the challenge by confining Locke’s pessimism tothe real essences of particular material substances; we are able toknow “the general properties possessed by all materialsubstances,” and are ignorant only of the “particularsizes, shapes, number, or motions of the particles which go to make upany specific object” (Mandelbaum 1964, p. 54). A relatedchallenge is to account for Locke’s appeals to superaddition,sinceprima facie, Locke’s reason for invoking God isthat he thinks the corpuscular hypothesis has no resources forexplaining the four problematic phenomena. Ayers responds by rejectingWilson’s divine annexation interpretation of superaddition infavor of a deflationary one. According to Ayers’ “divinearchitect” interpretation, Locke makes no distinction inetiology by calling a property superadded; he means only that Godselected the property with particular care when first creating matter.To diffuse the effect of Locke’s remarks to Stillingfleet, inwhich Locke appears to embrace action at a distance, Ayers points toLocke’s late manuscript, “The Elements of NaturalPhilosophy,” interpreting certain passages as referringgravitational effects to an undetectable medium (Ayers 1981, pp.212–214). This move has been challenged by Stuart. Claiming thatthe manuscript was probably written for the education of a child,Stuart denies that it could trump Locke’s remarks toStillingfleet (see Stuart 1998, pp. 378–379).

Another way to absolve Locke of inconsistency is to emphasize hisagnosticism or skepticism, while downplaying or reinterpretingpassages that appear to commit him to the corpuscular hypothesis.Interpretations in this vein tend to emphasize Locke’s pessimismabout our ability to know real essences, to discern necessaryconnections, and consequently, havescientia in naturalphilosophy. Commentators pursuing this line include Downing (1998,2007), Jolley (2002), and Connolly (2015); the latter argues for aneven greater epistemic humility, one extending to real essences, alongwith superaddition (mentioned briefly below).

The central challenge facing such interpretations is to account forthe passages in which Locke speaks as though he accepts thecorpuscular hypothesis, most notably those in which he appears toidentify the real essences of material substances with thecorpuscles’ primary qualities. Jolley (2002) accounts forLocke’s so-called dogmatic tendency in strategic terms.Locke’sEssay targets both Aristotelians andCartesians, and though agnosticism, which targets the Cartesians, isultimately the dominant tendency in his thought, Locke emphasizes theexplanatory power of the corpuscular hypothesis whenever he has theAristotelians in his sights. Downing (1998, 2007), meanwhile,interprets Locke’sEssay as developing metaphysicaldistinctions that constrain physical theory, and then downplays hisdogmatic side by taking the corpuscular hypothesis to be truly only ahypothesis for him, and denying that real essence can be identifiedwith primary qualities. Properly understood, Downing argues, thedistinction between real and nominal essence is a metaphysicaldistinction. It is thus more fundamental than the distinction betweenprimary and secondary qualities, which belongs to a particularphysical theory, the corpuscular hypothesis. To be worth its salt, aphysical theory must meet the metaphysical constraint provided by thereal-nominal essence distinction. That is, the physical theory mustprovide some way of making sense of the notion that material bodieshave an internal constitution that is inaccessible to us whileproducing qualities that are accessible. Locke explains hismetaphysical distinction using one physical theory, the corpuscularhypothesis, as an illustration, and he often appears to accept or evendefend that hypothesis. Still, this is only an appearance, anappearance due to a certain advantage that the corpuscular hypothesishas over other physical hypotheses: it is the theory best suited toour sensory capacities and understanding. Despite its unique status,Locke sees it as a mere hypothesis, one crippled by the explanatorylimitations 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) treatsLocke’s descriptions of what we perceive, intuit, and arecapable of conceiving as providing a case for testing some Kuhniantheses; as the normal science of his day, the corpuscularianhypothesis sets limits upon what Locke can conceive.

4. Locke and Newton

Locke and Newton probably first met in 1689 (though the exact date isnot known; Westfall 1980, p. 488; Rogers 1982, p. 219), and their mainworks were written independently of one another; Locke’sEssay,though published subsequently, was essentially completeby the time he read thePrincipia. A noticeable intellectualaffinity may nevertheless be seen in those works, and the opportunityfor mutual influence followed as they established a friendship,exchanging views on a wide variety of subjects, not least certainunorthodox theological convictions (see Westfall 1980, 490–91).The influence did not run in one direction alone; in a draft passageprobably penned shortly after thePrincipia’s secondedition, for example, Newton adopts a Lockean tone as he denies thatany ideas are innate.^[25] As for Newton’s influence upon Locke, the best-known exampleconcerns action at a distance, as noted below, though there are deeperquestions about methodology.

4.1 Epistemology and methodology

A good deal of resemblance to Locke’s epistemological approachcan be seen in Newton, who holds that, revelation apart, we mustgather what knowledge we can from our perceptions, and anything like areal essence eludes us. In the early manuscript,Degravitatione, 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 seeonly the shapes and colors of bodies, we hear only their sounds, wetouch only their external surfaces….But there is no directsense and there are no indirect reflected actions by which we knowinnermost substances (Principia, 942).

Locke and Newton also share the problem of an evidentiary deficit,insofar as they subscribe to corpuscularianism. Thinkers who didsubscribe tended to rely on transduction (also calledtransdiction)—an inductive inference that is empirical insofaras it relies upon observed cases, but which generalizes not only tounobserved cases but to those that are unobservable.^[26]) Newton’s Rule 3 licensed such inferences, from thePrincipia’s second edition onward, for qualities ofinvariable intensity, i.e., extension, impenetrability, hardness,mobility, and thevis inertiae. The rule explicitly permitsinferences to the realm of the unobservable: “Because thehardness of the whole arises from the hardness of the parts, we justlyinfer from this not only the hardness of the undivided particles ofbodies that are accessible to our senses, but also of all otherbodies” (Principia, Book 3, p. 795^[27]). As for Locke and the problem of transduction, how serious it is forhim depends largely upon the status of the corpuscular hypothesis. Ifhe takes an agnostic or skeptical stance toward it, then he owes nosolution to the problem.

Although the endeavors of Locke and Newton are often consideredcomplementary, there are questions about how Locke responded to thePrincipia, both in terms of its methodology and itsepistemological implications. With regard to the former, how far didLocke go in absorbing or incorporating the methodology that Newton hadforged? As a number of commentators have emphasized, the new sciencewas itself in flux at this time, with natural history being graduallysidelined by Newton’s approach, an experimentalism that wastheoretical and mathematical (see, e.g., Anstey 2011; Roux, 2013).Given that Locke had had Boyle as his mentor but was greatly impressedby Newton’sPrincipia, it is natural to ask whether asimilar shift was occurring in Locke’s own thought. AlthoughLocke was influenced by Newton, his allegiances to certain older ideasran deep. De Pierris (2006) explains Locke’s failure to adoptthe method of inductive proof in terms of his devotion to the ideal ofdemonstrative knowledge, as combined with a belief in hidden primaryqualities.

With regard to its epistemological implications, what precisely didLocke take thePrincipia to have achieved? To pose thequestion in the starkest possible terms, might Newton’sPrincipia have tempted Locke to retreat from his belief thatnatural philosophy cannot be made a science (much as it led him toretreat from the contact action proviso)? Perhaps Locke categorizedNewton’s epistemological achievement in natural philosophy as acontribution to sensitive knowledge alone. That is, perhaps he sawNewton’s “mighty Designs in advancing the Sciences”(Essay, Epistle to the Reader, pp. 9–10) as confined toproviding a firm basis for natural philosophy through his experimentalmethod. But did he perhaps instead take Newton’s mathematicalmethods as offering the demonstrations needed to push naturalphilosophy into the domain of demonstrative knowledge and hencescientia?

Commentators interpreting Locke as seeing Newton’s contributionin 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 Stillingfleetand other writings from the 1690s as a defense of Newton’smathematical physics, and one that draws from his mathematicaldemonstrations a greater optimism about the possibility of certainknowledge in natural philosophy. This move has been contested byDomski (2012), who argues that Locke advocated Newton’smathematical methods only in connection with astronomy, whose objectsare unavailable for experimentation; and that Locke maintained hisemphasis upon natural historical methods for questions aboutterrestrial bodies. The debate over the extent to which Lockeprioritized natural history over experimentalism directly concerns hisstance on hypotheses (a controversy mapped by Priselac, 2017), while arelated debate concerns the relationship between medicine and naturalphilosophy. Opposing Yost (1951) among others, Lu-Adler (2021) hasrecently argued that Locke did not assume that the scientificmethodology most appropriate to medicine could be generalized tophysics.

4.2 Ontology

Locke’s discussions of space, body, mind, and God bear somestriking similarities to Newton’s, raising questions about theextent of those similarities as well as points of influence. Newtonpresents his mature concept of space in thePrincipia, havingformulated important aspects of it earlier in the manuscriptDegravitatione. In both texts, space is distinct from body and isreal, infinite, three-dimensional, and homogeneous. Whereas body hasseparable parts, Newton’s eternal, infinite space is immobileand indivisible, having merely distinguishable parts. Although spaceis neither substance nor attribute, it would be a mistake to supposethat it is nothing; as he emphasizes inDe gravitatione(2004, p. 21, 22), by explaining space’s properties he has shownthat it is something.

Unlike Newton, Locke initially accepted a relationist view of space;in journal entries of 1676–78 and the Early Draft of theEssay, space is nothing more than a relation between bodiesthat do not touch (see Gorham, 2020, p. 221). In theEssay,however, his discussion of space and its relation to body is stronglyreminiscent of thePrincipia’s absolutist conception.Locke confesses himself to be among those “who persuadethemselves … that they can think on Space, without anything init, that resists, or is protruded by Body” (E II.iii.5, p. 126).Declaring that explicating extension in terms ofpartes extrapartes would be tautological (E II.iii.15, p. 173) and decliningto answer the question of whether space is substance or accident (EII.iii.17, p. 174), Locke describes our idea of “pureSpace” (E II.iii.5, p. 126) as an extension consisting of“the continuity of unsolid, inseparable, and immoveableParts” (ibid.; see also E II.xiii.13, 14, which furtherdiscuss space’s perpetual rest and the impossibility ofseparating 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 ofGod’s existence (2004, p. 21), Locke invokes God when defendinghis conception of space: “Those who assert the impossibility ofSpace existing without Matter, must not only make Body infinite, butmust 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 concludedthat by the time of the publishedEssay, Locke had fullyabandoned the relationist view (see Gibson, 1960; Di Biase, 2016;Gorham and Slowik, 2014; Gorham, 2020). However, Locke tends todescribe our ideas rather than things themselves, and the gap betweenthe two has led a few commentators to conclude that Locke retained hisearlier relationism or, at least, some inclination toward it (e.g.,Thomas, 2016; see also Rogers, 1978).

Locke’s concept of body also overlaps considerably withNewton’s. In developing a concept of body, Locke notoriouslyfinds the related, general problem about a substratum or substance vexing,^[28] whereas Newton dispenses with the problem more easily. InDegravitatione, Newton eliminates the unintelligible notion ofprime matter by associating perceived properties with determinedquantities of extension.^[29] Both thinkers attack Descartes’ identification of matter withextension, instead sympathizing strongly with the notion of atoms in avoid. Further, Newton’s list in Rule 3 of body’s universalqualities includes not only extension, hardness, impenetrability, andmobility, but also thevis inertiae—the inherent forceor power of resisting (Principia, Definition 3), which somecommentators take Newton to identify with mass.^[30] Locke’s concept, however, emphasizes extension, mobility, andsolidity, without mentioning mass. Still, since Locke explicatessolidity in terms of impenetrability, which arises from resistance (EII.iv.1), there may be grounds for holding that his concept does afterall include mass, a position defended by Woolhouse (2005).^[31] At the least, Locke’s discussion of action by impulsepresupposes mass, as Stein has noted. Nevertheless, presupposing theconcept may not amount to including it in the concept of body, sinceas Stein points out, mass for Locke “cannot be construed tocorrespond 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 apersistent source of controversy for both thinkers.^[32] One point of clarity and consensus is that neither Locke nor Newtonconsiders the power of gravitational attraction to be essential tomatter. Newton consistently denies that it is essential (for instancein his explanatory remarks following Rule 3 of thePrincipia)and Locke refers to it only as power superadded or endowed by God.Locke’s concept of superaddition is controversial, as notedearlier, but he does appear to embrace action at a distance in hisletter to Stillingfleet. Newton’s writings contain no suchdramatic pronouncements. So, while a few commentators interpret Newtonas accepting action at a distance, grounded either in a superaddedproperty (Henry 1994) or in a relational quality of matter (Schliesser2011), his more favorable remarks are at best indirect. Mostcommentators interpret him as at least having very grave misgivingsabout distant action, so perhaps Locke was wise to credit his changeof heart about gravity to “Mr. Newton’s incomparablebook,” rather than to Newton himself.

With respect to mind, Locke and Newton are both circumspect about itsnature and yet may be considered substance dualists. Although Newtonindicates inDe gravitatione that he does not pretend to knowthe substantial foundation of minds, he consistently presents minds asimmaterial, lacking certain characteristics of body, namely, hardness,impenetrability and resistance. Similarly, while Locke explicitlyallows the possibility of thinking matter in hisEssay anddiscusses it at length with Stillingfleet, he emphasizes that in allprobability, the soul is immaterial (E IV.iii.6, pp. 540–541).Does Locke share Newton’s view that even immaterial spirits arespatially extended, such that a mind can co-occupy place with a body?A Cartesian interpretation does not seem impossible givenLocke’s remarks about the possibility of thinking matter, whichcontrast matter against an immaterial soul that is unextended.^[33] Still, at several passages in theEssay, Locke seems toagree with Newton. At E II xxvii.2 (p. 329), he locates mindsspatially: “Finite spirits having had each its determinate timeand place of beginning to exist, the relation to that time and placewill always determine to each of them its identity, as long as itexists”.

Finally, Locke seems to share Newton’s belief in God’ssubstantial ubiquity and its implications. As the above-quotedpassage’s discussion of finite spirits continues, he remarksmore generally on the possibility of co-presence: “These threesorts of substances, as we term them, do not exclude one another outof the same place” (ibid.). His later remarks about the“infinite Space … possessed by God’s infiniteOmnipresence” (E II.xvii.20, p. 222) are similar, but by framingthem in terms of third parties’ ideas, Locke leaves more roomthere for interpretive controversy.

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