2. ‘Struck at ye root of morality’

In the summer of 1693, Newton suffered a mental breakdown, resulting in, among other things, him sending some angry letters to Samuel Pepys, apparently having to do with Newton’s attempts to secure a position in London.Footnote ¹ In September of the same year, he sent a letter to Locke apologizing for accusing him of having ‘endeavoured to embroil me with woemen & by other means I was so much affected with it as that when one told me you were sickly and would not live I answered twere better you were dead.’ And further, he begs Locke’s pardon ‘for representing that you struck at ye root of morality in a principle you laid down in your book of Ideas & designed to pursue in another book & that I took you for a Hobbist.’Footnote ² Locke’s reply is magnanimous, but he does ask Newton to ‘point out to me the places that gave occasion for censure’ so that he might ‘avoid being mistaken by others.’ In a subsequent letter, Newton replies that his illness was the result of insomnia and that when he had written to Locke, he had, ‘for five days together not a wink.’ And although he remembered having written Locke, ‘what I said of your book I remember not’ (Brewster Reference Brewster[1855] 1965, 150–151).

Admittedly, Newton was not in his right mind when he leveled these insults against his friend, making the question of what precisely he had in mind difficult if not impossible to settle. Nevertheless, before laying out some of the ways a man like Newton could view Locke’s positions as unduly materialist, I briefly consider two interpretations. Brewster (Reference Brewster[1855] 1965, 148) suggests that the offending principle is Locke’s rejection of innate ideas in Book I of his Essay (Reference Locke and Nidditch[1700] 1979). On the face of it, however, this is an odd reason to accuse Locke of being a ‘Hobbist’ since Hobbes was notorious, not for his rejection innate ideas, but for his materialism. G. A. J. Rogers argues that the offending principle lies elsewhere in the Essay. He concedes that for someone influenced by the Cambridge Platonists, Locke’s wholesale rejection of innate ideas would likely be unpalatable (Rogers Reference Rogers1979, 199). But as Rogers emphasizes (ibid., 197), Locke’s entire Essay (Reference Locke and Nidditch[1700] 1979) is predicated on the rejection of innate knowledge and is hardly an aspect of the Essay (Reference Locke and Nidditch[1700] 1979) one could forget. Interestingly, however, Newton had a habit of dog-earing pages of interest in his books. One dog-eared page in his personal copy of the Essay (Reference Locke and Nidditch[1700] 1979) includes Locke’s assertion that ‘Moral good and evil … are nothing but Pleasure and Pain’ (2.28.5; cf. 2.20.2). Such a view, Rogers contends, might be read by Newton as holding that ‘all pleasure is good without qualification, a view which Newton’s puritanism would certainly have found impossible to accept’ (Rogers Reference Rogers1979, 200).

But again, it is not clear why this would provoke the ‘Hobbist’ epithet since Locke is not here advocating psychological egoism but, arguably, is gesturing toward an ethical theory based on psychological hedonism,Footnote ³ a view that is not obviously Hobbesian. Ultimately, I favor what is perhaps a more straightforward reading according to which Newton is using ‘Hobbist’ simply in the sense of ‘materialist,’ that is, as a general term of opprobrium, one which was considered a serious insult at the time implying not only materialism but also some level of religious heterodoxy.Footnote ⁴ Newton may simply have been brought to use the term of abuse as a result of his illness coupled with the failed attempts of his friends, including Locke, to secure him a plum position in London. Underneath all this, however, Newton may very well have suspected Locke of inclining too far toward materialism, with its irreligious consequences. Rogers demurs, concluding that, while the Essay does provide prima facie grounds for thinking that Locke has materialist sympathies, ‘they are ultimately ungrounded’ (Rogers Reference Rogers1979, 199). Against Rogers, I argue that with respect to human beings and other animals, the suspicion of materialist sympathies is well-grounded, representing an account of the mind–body relation which, from Newton’s perspective, would strike at ‘ye root of morality.’

3. Hobbesian Soul-Death: ‘all are of the dust, and all turn to dust again’Footnote ⁵

In the most general terms, Christian mortalism is the heretical view that we die with our bodies only to live again when our bodies are resurrected at the Final Judgement.Footnote ⁶ English leveller and General Baptist pamphleteer Richard Overton offers one of the most powerful seventeenth-century defences of mortalism in his Reference Overton and Fisch[1643] 1968 tract Mans Mortalitie. He defends the doctrine known as ‘thnetopsychism,’ adducing both philosophical and scriptural evidence for soul-death. Philosophically speaking, surviving the final dissolution of one’s body is belied by the apparent dependence of one’s mind on the health and proper functioning of one’s body.Footnote ⁷ In this respect, we are no different from other animals, a consequence which is also supported by scripture. Consider the motto of Mans Mortalitie, Ecclesiastes 3:19: ‘For that which befalleth the sons of men befalleth beasts; even one thing befalleth them: as the one dieth, so dieth the other; yea, they have all one breath; so that a man hath no preeminence above a beast: for all is vanity.’Footnote ⁸ When it comes to death, we differ from animals only in that we have been promised life after death through bodily resurrection.

Clearly, soul-death mortalism would be attractive to someone like Hobbes, for his unremitting materialism precludes even the possibility of an immaterial soul. ‘[S]ubstance and body,’ he tells us, ‘signify the same thing,’ rendering ‘incorporeal substance’ a contradiction in terms, ‘as if a man should say an incorporeal body’ (Hobbes Reference Hobbes and Curley1994, XXXIV, 2, 262).⁹ Consequently, the substance of the soul must be corporeal or else ‘nothing’ at all, for ‘to say that “an angel or spirit is … an incorporeal substance” is to say in effect “there is no angel nor spirit at all”’ (Leviathan, XXXIV, 24, 270). All veridical uses of ‘spirit,’ Hobbes insists, refer either to corporeal phenomena like wind or breath or are metaphors for ‘inclinations of the mind,’ where, for example, we might say of an argumentative person that he has ‘a spirit of contradiction’ (ibid., XXXIV, 3, 262). To be clear, Hobbes finds ‘in Scripture that there be angels and spirits … but not that they are incorporeal (as are the apparitions men see in the dark, or in a dream, or vision)’ (ibid., XLV, 8, 440–441).

Accordingly, Hobbes provides a thoroughly embodied account of sense, memory, and imagination. Utilizing Harvey’s (Reference Harvey[1628] 1889) discovery and explanation of the circulatory system, Hobbes explicates mental phenomena in terms of the ‘vital motion’ of the blood. ‘The subject of sense,’ he writes, ‘is the sentient itself, namely, some living creature’ (Hobbes Reference Hobbes and Molesworth[1839] 1962, 391) and the heart is the active principle of the sentient; not only is it the ‘original of life’ (Elements, 406) but also the ‘fountain of all sense’ (ibid., 392). We learn by experience how objects affect the vital motion of the blood, some generating pleasure, others, displeasure, and from these experiences we develop ‘appetites’ and ‘aversions.’Footnote ¹⁰ When we deliberate, ‘the last act of it, if it be appetite, is called will; if aversion, unwillingness’ (ibid., 409). Appetites and aversions, Hobbes contends, are tightly connected with memories and metal imagery. ‘[T]he motions of the heart,’ he writes, ‘are appetites and aversion’ which ‘are generated by phantasms’ (ibid., 401). When we sleep, on the other hand, ‘phantasms are generated by appetites and aversions’ (ibid.). Hobbes gives some examples including love and beauty which ‘stir up heat in certain organs.’ But when we sleep, ‘the heat in the same organs … often causeth desire and the image of an unresisting beauty’ (ibid.). The heart, then, causes us to dream, for when the motion from the heart reaches the brain the ‘predominant motion in the brain makes the phantasm’ (ibid.). Ultimately, it is our dreams that are the source of the confused and misguided notion of immaterial substances; cold generates fear and fear, Hobbes explains, causes men to ‘dream of ghosts’ (ibid.). To some, ‘especially to guilty men, and in the night, and in hallowed places, fear alone … hath raised in their minds terrible phantasms, which have been and are still deceitfully received for things really true, under the names of ghosts and incorporeal substances’ (ibid., 402; cf. Leviathan, XII, 7, 65). Hobbes, it would seem, is one of those few men who are not beguiled by their dreams.

Like Overton, Hobbes does not see any of this as a threat to religion. Hobbes interprets Adam and Eve’s punishment for their original sin in strictly mortalist terms. ‘Adam,’ Hobbes tells us, ‘was not created immortal by virtue of his nature, but by the adventitious grace of God, i.e. by virtue of the tree of life; while he had an abundance of its fruit to eat, he could not die’ (Leviathan XXXVIII, 2, 301, n. 1). Once ‘barred from approaching the tree of life … they [Adam and Eve] and their posterity’ became mortal (ibid., Appendix III, 19, 544). ‘Dying they remained dead, until … the general resurrection of the dead’ (ibid). Furthermore, Hobbes emphasizes what he believes the ancient Hebrews actually meant by ‘nephesh,’ the word usually translated ‘soul’ in the English Bible. Rather than denoting an immaterial substance, it ‘signifieth always either the life or the living creature … the body alive’ (ibid., XLIV, 15, 419). By ‘soul,’ the ancient Hebrews did not mean a thing capable of ‘an existence separated from the body,’ for in that case ‘it might as well be inferred of any other living creatures’ (ibid). Like Overton, Hobbes sees humans and animals as being in the same position with respect to death. When God created man, he did not create a body to which he then superadded an immaterial soul; rather he made man ‘of the dust of the earth, and breathed in his face the breath of life,’ that is, he made man ‘a living creature’ (ibid).Footnote ¹¹ Upon bodily death man therefore returns to the dust from which he was formed, a consequence only ‘remitted by Christ’s death,’ and the promise that the dead ‘shall come to life again … in the general resurrection of the dead’ but not before ‘the day of judgment’ (ibid., Appendix III, 19, 544).

As mentioned, mortalism also comes in a dualistic form. According to ‘psychopannychism,’ souls are immaterial, and thus distinct from their bodies, but nevertheless sleep between bodily death and resurrection. A young Martin Luther was attracted to this latter, less radical, view, holding that the soul, being an immaterial substance, persists in an unconscious state when separated from the body (Burns, 28; Almond, 38–39). On my view, Newton adopts this version of mortalism as part of a broader commitment to a dualist anthropology.

4. Newtonian dualism

I have argued elsewhere that, with respect to the mind–body relation, Newton defends a kind of substance monism, at least in one important sense of the word ‘substance’ (Dempsey Reference Dempsey2006). I have also advanced the view that Newton’s account of God’s creation of bodies (‘corpuscles’) in his De gravitatione manuscript,Footnote ¹² coupled with his sensorium account of perception alluded to in his Opticks and elsewhere, imply a sort of idealism (Dempsey Reference Dempsey2014). Here I wish to emphasize the aspects of his view that are dualistic in nature, aspects that follow from his idiosyncratic idealism. According to Rogers (Reference Rogers1979, 198), Newton follows More in both his theism and his dualism.Footnote ¹³ This is only partially true; although their views on the nature of mind align in two important respects, there is at least one significant point of disagreement. First, they agree that minds are extended in space. More criticizes Descartes – ‘the Prince of the Nullibists’ – for, among other things,Footnote ¹⁴ affirming ‘that a Spirit is Nullibi, that is to say, no where.’Footnote ¹⁵ In De gravitatione, Newton presents his account of minds and bodies in explicit opposition to Descartes’s,Footnote ¹⁶ utterly rejecting nullibism, insisting that ‘[n]o being … can exist which is not related to space in some way’ (Reference Newton and Janiak2004a, 25). Descartes’s nullibism, Newton worries, challenges the very reality of ‘created minds,’ for if the ‘mind has no extension at all … it exists nowhere, which seems the same as if we were to say that it does not exist’ (ibid., 31). Even God is extended throughout objective space. In 1705, Newton’s student, David Gregory, records that Newton was unsure whether to append the following query to his Opticks: ‘what the space that is empty of body is filled with?’ ‘The plain truth,’ Gregory writes, ‘is that he believes God to be omnipresent in the literal sense’ being both ‘present in space where there is no body’ and ‘present in space where a body is also present’ (cited in Hall and Hall Reference Hall, Hall, Cohen and Westfall1995, 87). Likewise, in the General Scholium to his Principia (Reference Newton, Cohen and Whitman1999), Newton insists that God is ‘present everywhere,’ not just virtually omnipresent ‘but also substantially.’Footnote ¹⁷ In his Opticks, Newton likens objective space to God’s ‘boundless uniform sensorium’ (Newton Reference Newton and Janiak2004b, 138).Footnote ¹⁸ Importantly, in De gravitatione, Newton conceives of ‘created minds’ as finite images of the Divine Mind and he explicitly draws an ‘analogy between the divine faculties and our own’ (Reference Newton and Janiak2004a, 30).Footnote ¹⁹ Thus since ‘God is everywhere, created minds are somewhere’ (ibid., 25), namely in ‘that place to which the sensitive substance is present’ (Reference Newton and Janiak2004b, 130), the brain’s sensorium.Footnote ²⁰

Second, Newton agrees with More that, while minds are extended in space, they are nevertheless immaterial. As I see it, Newton believes that created minds are both metaphysically and morally superior to their biological bodies. The matter that composes the body exists because omnipresent God directly wills corporeal form to empty space.Footnote ²¹ Corpuscles ‘borrow existence from the will’ of God, for ‘they are beings of the divine reason’ (Reference Newton and Janiak2004a, 31). On this account, empty space replaces matter-substance. ‘Extension,’ Newton tells us, ‘takes the place of the substantial subject,’Footnote ²² while the properties of corpuscles are ‘conserved by the divine will’ (ibid., 29). Created minds, on the other hand, do not depend on continuous acts of Divine Volition but rather provide humans with finite volition, God having given us ‘the power of creation in the same degree as his other attributes’ (ibid., 30). In this respect, created minds are more substantial than bodies since the latter’s movement and very existence depend on continuous acts of mind, whether Divine or created.

The analogy Newton draws between created minds and the Divine Mind also suggests mind’s moral superiority over body. A created mind’s causal interaction with images in its sensorium, Newton insists, is comparable to omnipresent God’s immediate contact with bodies in His ‘boundless’ sensorium. In the Opticks, Newton tells us that omnipresent God is ‘more able by his will to move … bodies … and thereby to form and reform the parts of the universe, than we are by our will to move the parts of our own bodies.’ Bodies are ‘his creatures, subordinate to his Will’ which he perceives by their ‘immediate Presence’ (Reference Newton and Janiak2004b, 138), just as created minds, to use David Gregory’s characterization of Newton’s view, ‘are sensible of Objects when their images are brought home to the brain’ (Reference Newton, Cohen and Westfall1995, 329; cf. Principia, Reference Newton, Cohen and Whitman1999, 940). Of course, ‘we only simulate the power of creation’ for we cannot create bodies as God does ‘but … only move … our own bodies, to which we are united … by divine constitution’ (Reference Newton and Janiak2004a, 30). And while we have a very dim conception of how God wills corporeal form to empty space, Newton believes we would have a better understanding if only we understood human volition ‘since the same question arises with regard to the way we move our own bodies’ (ibid., 29). Morally speaking, then, ‘since it is in the image of God’ a created mind ‘is of a far more noble nature than body’ (ibid., 31; emphasis added). Being finite images of the Divine Mind, created minds are therefore more God-like than their bodies.

However, in an interesting twist that marks a significant disagreement with More, Newton does not believe in disembodied spirits, at least not ones that continue to think and feel. As Snobelen demonstrates, Newton rejects the existence of ghosts and demon possession at a time ‘when leading members of the Royal Society,’ including More, ‘catalogued case histories of witches, demons and ghosts as evidence for the reality of spirits’ (Reference Snobelen, Force and Hutton2004, 157).Footnote ²³ To David Gregory Newton explains that ‘the requirement of religion’ is not ‘a separate existence of the soul, but a resurrection with a continuation of memory’ (The Correspondence of Isaac Newton, 336, 339). In other words, piety and faith do not demand that created minds continue to think and feel apart from the bodies to which they are united by ‘divine constitution,’ but only that both are resurrected and that the memory of one’s actions in this life be restored so that one can be judged accordingly. With the death and dissolution of our bodies, then, created minds enter a dreamless sleep to await the resurrection, a view which Newton believes is thoroughly supported by scripture. For example, Newton cites Daniel 12:2 which declares that ‘many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.’Footnote ²⁴ Theologically speaking, the belief that one’s mind continues to think and feel between bodily death and resurrection implies, on Newton’s view, the absurdity of a ‘double judgment.’Footnote ²⁵ The dominant, and as it were, catholic, view of immortality trivializes the resurrection of the body and the significance of the Final Judgment by implying that men are ‘rewarded according to [their] works before Christ comes to’ judge them (cited in Force 2004, 191; emphasis added). ‘[T]he dead,’ Newton writes, ‘are not sentenced to reward or punishment in heaven purgatory or Hell before they be judged nor judged before the day of judgment.’Footnote ²⁶ The doctrine that souls continue to think and feel after bodily death is a corruption introduced into the early Christian church by ‘the Egyptians, Platonists, & other heathens [who] placed the souls of the better sort of dead men about their sepuchers [sic] & statues & temples & in y ^e air & in heaven & so filled all places w ^th ghosts or Daemons’ (Force 2004, 191). From this corrupting influence comes the ‘heathen doctrine of Daemons, together w ^th that Popish one of Purgatory’ (ibid.).Footnote ²⁷

5. Locke, crypto-materialist

To be sure, Locke does not adopt Hobbes’s thoroughgoing materialism, for Locke never doubts the existence of immaterial, spiritual entities like angels and God.Footnote ²⁸ Matthew Stuart (Reference Stuart2013, 250–264) has recently advanced ‘a case for dualism’ on the basis of Locke’s version of the cosmological argument for God’s existence (IV.x.1–19). I agree with Stuart that, insofar as Locke accepts immaterial beings, Locke is a substance dualist. And it is also true, as Stuart emphasizes, that Locke denies the possibility that ‘bare incogitative Matter should produce a thinking intelligent Being’ (IV.x.10), a possibility Locke’s protégé, Anthony Collins, would later defend.Footnote ²⁹ Nevertheless, I believe that with respect to humans and other animals, Locke leans towards a substance monism and the substance in question is matter.Footnote ³⁰

Certainly, the materialist implications of Locke’s account of persons coupled with his speculations about thinking matter did not escape the notice of some critics, including Gottfried Leibniz.Footnote ³¹ Locke, Leibniz writes, ‘undermines the nature of the immaterial soul by digging rabbit holes. He inclined towards the Socinians … whose philosophy of God and mind was always poor.’Footnote ³² The Socinians – Polish Protestants who followed the heretical teachings of Faustus Socinus (1539–1604) – maintained, among other things, that the soul is mortal. Snobelen notes that Locke’s library contained 43 Socinian books, confirming his interest in their heterodox views (Snobelen Reference Snobelen, Rohls and Mulsow2005, 247). According to Nicholas Jolley, Leibniz looks with suspicion on Locke’s appreciation of Socinian theology. Locke’s crypto-materialism, as Leibniz sees it, serves only to ‘abase us’ unlike the ‘generous philosophy of the Platonists.’Footnote ³³ Even Locke’s many official avowals of the soul’s probable immateriality are seen as a threat to immortality. It is from this perspective, then, that Leibniz interprets Locke’s account of persons as an attempt to square personal immortality with his skepticism over the soul’s immateriality, indicating Locke’s (covert) sympathy for a materialist anthropology (Jolley Reference Jolley1984, 25).Footnote ³⁴

I think Leibniz’s suspicions are on the mark for several reasons. We begin with Locke’s seminal contributions to our understanding of personhood and personal identity in Book II of the Essay (Reference Locke and Nidditch[1700] 1979).Footnote ³⁵ An important implication of Locke’s account is that the existence of an immaterial soul is irrelevant with respect to personhood, personal identity over time, and even personal immortality. According to Udo Thiel, one reason that Locke’s consciousness-based account of personal identity was so influential in the eighteenth century was precisely because ‘it remains neutral with respect to the debate between materialist and immaterialist philosophers of mind’ effectively making one’s ‘view about the nature of the thinking substance … irrelevant’ (Thiel Reference Thiel2011, 144). Despite the neutrality of his account, there is, I believe, good reason to think that Locke actually doubts the existence of immaterial souls, at least in the case of humans.

It will be useful to very briefly sketch Hobbes’s work on the nature of identity as well as his legalistic or political conception of personhood. In the chapter entitled ‘Of Identity and Difference,’ of his Elements of Philosophy Concerning Body Hobbes considers questions which are central to the issue of identity over time, including personal identity, like ‘whether a man grown old be the same man as he was whilst he was young, or another man’ (Elements, 135). In fact, Hobbes frames part of his discussion in terms of reward and punishment. Against the view that the sameness of a man is based in the sameness of the matter of which he is composed, Hobbes writes, ‘he that sins and he that is punished, should not be the same man, by reason of the perpetual flux and change of man’s body’ (ibid., 136). Hobbes goes on to distinguish sameness of body and sameness of ‘man’ – for Socrates’s body ‘when he has grown old, cannot be the same as it was when he was an infant … yet, nevertheless, he may be the same man’ (ibid., 137). Ultimately, for Locke, Hobbes’s distinction is inadequate; not only should we distinguish a mass of atoms from an ‘organized body’ to which atoms are ‘vitally united’ (II.xxvii.6), in the case of humans, we must distinguish the man from the person.Footnote ³⁶

Thiel notes that in the seventeenth century, while commonly used to refer to an ‘individual human being,’ in at least some philosophical discussions ‘a person is regarded as someone with rights and obligations, to whom we attribute actions and, whom we hold responsible for those actions’ (Thiel Reference Thiel, Garber and Ayers1998a, 868–869). Hobbes, for one, uses ‘person’ in this legalistic sense. George MacDonald Ross writes that ‘when Hobbes talks of “person”’ in his discussion of the original contract in chapter XVII of his Leviathan, ‘he means this in the sense of a legal person,’ that is, one ‘legally entitled to own property, make contracts and so on’ (MacDonald Ross Reference MacDonald Ross2009, 115). Both Thiel (Reference Thiel, Garber and Ayers1998a, 869, n. 4; 881–882) and Strawson (Reference Strawson2011, 18)Footnote ³⁷ concur. In De Homine, Hobbes characterizes persons as, among other things, those entities to which we attribute actions, writing that ‘a person is he to whom the words and actions of men are attributed, either his own or another’s: if his own, the person is natural; if another’s, it is artificial’ (Cited in Copp Reference Copp1980, 591).Footnote ³⁸ In his Leviathan, during his discussion of natural and artificial persons, Hobbes contends that young children and the insane are not the ‘authors’ of their actions, ‘for children, fools, and madmen … have no use of reason’ and so, ‘may be personated by guardians and curators, but can be no authors (during that time) of any action done by them’ (Leviathan, XVI, 103). Given their lack of reason, they are not responsible for their actions, although such responsibility could fall to their guardians.

Luc Foisneau contends that Locke’s account of ‘person as a juridical term owes a lot to the definition that Hobbes gives of the natural person’ (Reference Foisneau, Hutton and Schuurman2008, 95). With Hobbes, Locke sees the question of personhood as crucially the question of moral and legal responsibility. Concerning the madman, Locke would come to make a similar point writing that he was ‘not himself, or is beside himself’ (II.xxvii. 20) and thus, not responsible for the actions committed during the episode of madness. Ultimately, Locke has an eye toward the resurrection, that ‘great Day, wherein the Secrets of all Hearts shall be laid open … [where] no one is made to answer for what he knows not of; but shall receive his Doom, his Conscience accusing or excusing him’ (II.xxvii.22). According to Mackie, Locke’s forensic account of persons ‘might be best described as a theory of action appropriation’ (Reference Mackie1976, 183). Personhood Locke writes is a ‘Forensic Term appropriating Actions and their Merit; so belongs only to intelligent Agents capable of a Law, and Happiness and Misery’ (II.xxvii.26).

On Locke’s view, a person’s identity, and thus her responsibility, extends into the past via consciousness. Strawson makes a compelling case that by ‘consciousness’ Locke does not simply mean ‘memory’ (Reference Strawson2011, (in particular) chapter 5).Footnote ³⁹ Far from it. It is consciousness ‘whereby [self] becomes concerned and accountable; owns and imputes to itself past Actions, just upon the same ground … as it does the present’ (II.xxvii.26, emphasis added). The ‘ground’ on which self becomes concerned for itself at ‘present’ is self-consciousness, the consciousness that attends every thought, which, as he says repeatedly is ‘inseparable from thinking, and … essential to it’ for it is ‘impossible for anyone to perceive, without perceiving that he does perceive’ (II.xxvii.9). ‘[T]hinking,’ Locke writes, ‘consists in being conscious that one thinks’ (II.i.19). As he reiterates in Book IV, ‘[i]n every Act of Sensation, Reasoning or Thinking, we are conscious to ourselves of our own Being’ (IV.ix.3). It is this reflexive awareness of our selves which accompanies thinking that is the basis of personhood.Footnote ⁴⁰ Importantly, ‘[s]elf depends on consciousness, not on substance’ (II.xxvii.17). ‘Self is that conscious thinking thing, whatever substance made up of, (whether Spiritual or Material, Simple or Compounded, it matters not) which is sensible, or conscious of Pleasure and Pain, capable of Happiness or Misery, and so is concerned for itself, as far as that consciousness extends’ (ibid.). Consciousness, as Strawson emphasizes, does require some substance (subject) or other, yet it need not be the same substance, either in principle (e.g. the prince and the cobbler thought experiment (II.xxvii.15)) or in practice (e.g. if we receive a new body on the Day of Judgment).

Dualists will here insist that that the subject of consciousness must be an immaterial substance. In keeping with this commonly held view concerning the subject of consciousness, it would not be implausible to infer that while Locke thinks persons conceivably move from soul to soul and so on, they need some soul or other. And indeed, in his account of personal identity, Locke asserts that it is ‘probable’ that ‘consciousness is annexed to … one individual immaterial Substance’ (II.xxvii.25). However, Locke’s notorious speculations about thinking matter in Book IVFootnote ⁴¹ challenge this commonly held intuition and cast doubt on the sincerity of his claim that the subject of consciousness is probably an immaterial substance. We ‘possibly shall never be able to know,’ Locke writes, ‘whether any mere material Being thinks’ (IV.iii.6). It is, Locke suggests, conceivable that ‘Omnipotency’ has ‘given some Systems of Matter fitly disposed, a power to perceive or think’ (ibid; emphasis added). ‘What certainty of knowledge can any one have,’ Locke asks, ‘that some perceptions, such as, e.g. pleasure and pain, should not be in some bodies themselves, after a certain manner modified and moved’ (ibid). After all, making matter think is surely within omnipotent God’s power. Furthermore, animals perceive, remember, and experience pleasure and pain, points to which we return shortly. Hence, it is surely within God’s omnipotence to superadd active powers, like ‘sense and spontaneous motion,’ to mere matter; as he puts it in his third letter to Edward Stillingfleet, it would be an undue limitation on God’s omnipotence to deny that He could also go ‘one step further’ and ‘give to matter thought, reason, and volition’ (Locke Reference Locke1823, vol. 4, 460). In his Essay (Reference Locke and Nidditch[1700] 1979), Locke is quick to qualify this claim, asserting that he is ‘not here speaking of Probability’ (IV.iii.6) but only possibility, namely, that it is within God’s power to do so. Locke’s official position, here as in Book II is that we probably have an immaterial soul.

However, Locke does, in effect, offer several reasons for thinking that God actually did superadd thought, reason, and volition directly to the living body. First, the thinking matter hypothesis is the simpler hypothesis, for on the immaterial soul view, God must perform two acts of superaddition. It is just as conceivable that God should ‘superadd to Matter a Faculty of Thinking, than that he should superadd to it another Substance, with a faculty of thinking’ (IV.iii.6). Notice that the second option requires an extra step. God must first superadd a faculty of thinking to an immaterial substance which he then must superadd to the systems of matter that compose our bodies. Since both possibilities are conceivable, why shouldn’t God opt for the first option? Several reasons might be proffered in favor of the second option. On the one hand, it might be argued that it is not in fact more complex, that God does not need to superadd consciousness to an immaterial substance, Locke’s above characterization notwithstanding, for such a substance thinks by its very nature. But against the Cartesians, Locke insists that souls, if we have them, can and do lose consciousness, indeed, every night: ‘every drowsy Nod shakes their Doctrine, who teach that the Soul is always thinking’ (II.i.13). To those who argue that the soul continues to think even in a dreamless sleep without our remembering it, Locke replies that a man in a sound sleep is not ‘capable of Happiness or Misery,’ at least, ‘no more than the Bed … he lies on’ (II.i.11). Or as he puts it in a 1682 Journal entry, ‘one may, with as much certainty and evidence, say that the bed-post thinks and perceives too all the while, but remembers it not’ (Reference Locke and King1858, 128). And, in any case, if the soul were conscious every night in a sound sleep, the sleeping and waking man would make ‘two Persons’ (II.i.12; emphasis added). Further, as his account of personal identity makes clear, not only is it conceivable that a single consciousness can move from one body to another, there could possibly be more than one consciousness in a single human being (II.i.12; II.xvii.19), a possibility one would not expect if it were ‘annexed’ to an individual immaterial substance. In short, ‘the perception of Ideas’ is ‘to the Soul what motion is to the Body; not its Essence, but one of its Operations’ (II.i.10).

On the other hand, one might argue that God opts for the complexity of two superadditions because (1) thinking requires a simple – i.e. indivisible – subject or (2) immaterial souls are required for immortality. But Locke explicitly denies both of these requirements. First, if thinking matter is conceivable, then the subject of thinking need not be simple. That is, if a system of matter, say the brain, could possibly be the subject of thought and feeling – and Locke clearly believes that this is at least a possibility – and if we concede that the brain, rather than being simple, is exceedingly complex, then thought and feeling do not entail a simple subject. Second, personal immortality does not require a immaterial soul, as is apparent when Locke concludes that ‘[a]ll the great Ends of Morality and Religion, are well enough secured, without philosophical Proofs of the Soul’s Immateriality; since it is evident, that he who made us … sensible intelligent Beings … can and will restore us to the like state of Sensibility in another World’ such that we may ‘receive the Retribution he has deigned to Men, according to their doings in this Life’ (IV.iii.6). In other words, the promise of resurrection secures our hope of an afterlife irrespective of the soul’s immateriality.

In his Journal, Locke is just as clear when he states that an immaterial soul is not sufficient for personal immortality either since it does not guarantee the preservation of consciousness. Those who think otherwise, ‘perfectly mistake immortality; whereby is not meant a state of bare substantial existence and duration, but a state of sensibility’ (Reference Locke and King1858, 128). To demonstrate that persons persist after bodily death, one must demonstrate that ‘sensibility’ persists. But again we know from ‘experience of what we find daily in sleep’ that the soul, if we have one, can and does lose sensibility, for example, ‘during two or three hours of sound sleep without dreaming’ (ibid., 129). This daily deliquium is apparently the result of changes in the disposition of the brain and body, rather than changes in a simple soul substance. With Overton, the physician Locke is aware of the proportionality between changes in the brain and body and changes in one’s mental life. Thus, whether or not we have an immaterial soul, Locke is cognizant that thought and feeling apparently depend on a properly functioning, living body. In fact, Locke notes other more radical changes in the body such as ‘swooning and apoplexy, &c., [which] put it past doubt that the soul may subsist in a state of insensibility, without partaking in the least degree of happiness, misery, or any perception whatsoever’ (ibid.). In the case of the body’s complete ‘dissolution,’ (ibid., 130) then, we have every reason to expect that that all thought and sense will cease. Thus, Locke reasons, the soul – if we have one – is like an atom in that it may be indestructible. But this is irrelevant when it comes to personal immortality. For to prove ‘eternal duration, which may be without any perception, is to prove no other immortality of the soul than what belongs to one of Epicurus’s atoms, viz. that it perpetually exists, but has no sense either of happiness or misery’ (ibid., 129). Hence, even granting ‘that no power can destroy it but that Omnipotence that at first created it,’ the soul may cease to think after the death of the body. In this case, soul and body ‘may both lie dead and inactive, the [soul] without thought, the [body] without motion, a minute, and hour, or to eternity, which wholly depends upon the will … of the first Author’ (ibid., 129–130; emphasis added).

The dependence of thought and sense on the proper functioning of the body is at least implicit in the Essay. Recall Locke’s important qualification regarding the possibility of thinking matter: God would be superadding thought and sense, not to matter per se, but to ‘Systems’ of matter that are ‘fitly disposed’ and ‘put together as he thinks fit’ (IV.iii.6), that is, systems of matter properly organized and ‘vitally united’ in one organism.Footnote ⁴² That a system of matter can be more or less fitly disposed to be the subject of thought and sense, implies some level of dependence. In that case, we should expect that an entirely corrupted body would be an entirely unfit subject. The dependence of sensibility on the organization of our bodies is also implied by Locke’s treatment of the possibility of an inverted spectrum. Logical positivists of the early twentieth century took the question of the possibility of an inverted spectrum as a paradigmatic example of a ‘pseudo-problem.’ In Sydney Shoemaker’s words, it was ‘a favorite target for applications of the verificationist theory of meaning’ (Reference Shoemaker1982, 357). Yet Locke is not speculating on the possibility of physiological identical humans having inverted spectra. Rather, he is suggesting that differences in the structure of different people’s eyes and brain could result in an inversion of their color ideas. It is possible, he writes, that by ‘different Structure of our Organs … the same Object should produce in several Men’s Minds different ideas at the same time,’ for example, ‘that if the Idea that a Violet produced in one Man’s Mind by his Eyes were the same that a marygold produced in another man’s’ (II.xxxii.15; emphasis added; cf. II.xxxii.13). In other words, if two men’s spectra were inverted, it would be due to physiological differences, again implying a dependence on the physiology of the brain and body.Footnote ⁴³

That we lose sensibility upon bodily death is, on Locke’s view, also evident from Scripture. In his Reasonableness of Christianity, Locke explicitly defines death in starkly mortalist terms. ‘I must confess by Death here [in the New Testament] I can understand nothing but a ceasing to be, the losing of all actions of Life and Sense’ (Reference Locke and Higgins-Biddle[1696] 1999, 8; emphasis added). With Hobbes, Locke interprets Adam’s punishment for his disobedience in paradise as precisely the loss of immortality, that is, he was turned away from ‘the Tree of Life’ which ‘shews that the state of Paradise was a state of Immortality … which he lost that very day’ (ibid., 6). Thus, with Adam’s ejection from paradise, ‘all men must die’ (ibid., 11). Thanks to Adam’s disobedience, we are all now mortal and subject to death. However, ‘[f]rom this estate of Death Jesus Christ restores all mankind to Life … which they receive at the Resurrection’ (ibid., 11–12). We are not, therefore, naturally immortal, even if we have indestructible souls. Rather, the promise of personal immortality depends solely on the resurrection. Even then we are subject to the possibility of a second death for our sins in this life. ‘That Death (i.e. a cessation of sense and perception) shall at last … be the punishment of the unrighteous is plain from Gal. VI. 8. where corruption is set in opposition to life everlasting’ (ibid., 14; emphasis added).

A final line of argument for Locke’s skepticism over immaterial souls concerns the nature of animal minds, a topic that was vigorously debated in the seventeenth and eighteenth century.Footnote ⁴⁴ While animals may not have a capacity for abstract thought and language, they are, on Locke’s view, capable of sense and perception, of happiness and misery, all capacities Locke mentions in his discussion of the possibility of thinking matter. Moreover, ‘brutes have memory,’ that is, ‘several other Animals seem to have to a great degree, as well as Man’ the capacity for ‘laying up and retaining the Ideas that are brought into the Mind’ (II.x.10; emphasis added). In his Journal, Locke considers those who worry that if we concede that animals have minds we must concede that they have immaterial souls as well. ‘[F]or, say they, beasts feel and think, and therefore their souls [must accordingly be] immaterial, and consequently, immortal’ (Reference Locke and King1858, 128). This un-theistic position is, for many, implausible, so much so that ‘some men … have rather thought fit to conclude all beasts perfect machines, rather than allow their souls immortality’ (ibid). In other words, if we deny immaterial souls to animals and yet maintain that minds require immaterial souls, we must deny minds to animals. To defend such a position, one might appeal to those capacities that appear to be uniquely human – e.g. rationality and language – identifying them as the definitive indicators of an immaterial soul. With respect to uniquely human mental faculties, Locke focuses on the capacity for abstraction which, he believes, ‘puts a perfect distinction betwixt Man and Brutes’ (II.xi.10). Yet he immediately goes on to insist that while ‘Brutes abstract not’ neither are they ‘bare machines.’ For it ‘seems as evident … that they do some of them in certain instances reason, as that they have sense’ (II.xi.11; emphasis added). Jolley argues that from Locke’s perspective, the addition of an extra mental faculty is a rather weak difference on which to base ‘such a sharp ontological divide between human beings and animals’ (Reference Jolley2015, 46).Footnote ⁴⁵ For this very reason, Ann Thomson maintains, the question of animal souls was particularly vexing for those like Ralph Cudworth and Henry More who sought to defend the immateriality of the human soul. ‘An immaterial [animal] soul entailed the danger of making it immortal, while a material animal soul could be extended to the human soul as well’ (Thomson Reference Thomson2010, 11). As Cudworth puts the worry, a material animal soul ‘leaves us also in an absolute impossibility of proving the immortality of the rational soul’ (cited in Thomson Reference Thomson2010, 14).

Importantly, if animals lack an immaterial soul and yet are still capable of happiness and misery, of sense, memory, and simple reasoning, they represent an actual case of thinking matter. For Locke, Jolley writes, ‘it is a virtue of the thinking matter hypothesis that it can easily accommodate what he takes to be the facts about human and animal mentality’ (Reference Jolley2015, 45). When God creates animals, he must superadd mental capacities directly to the animal’s brain and body. The only difference, then, between humans and animals, is that God superadds greater mental capacities to humans. In his exchange with Stillingfleet, Locke writes that ‘if Omnipotency can give thought to any solid substance, it is not hard to conceive that God may give that faculty in a higher or lower degree, as it pleases him, who knows what disposition of the subject is suited to such a way or degree of thinking’ (cited in Jolley Reference Jolley2015, 45).