Written in the Flesh

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    Written in the flesh: Isaac Newton on themind–body relation

    Liam Dempsey

    Department of Philosophy, Dalhousie University, Halifax, Nova Scotia, B3H 4P9, Canada

    Received 17 July 2005; received in revised form 17 October 2005

    Abstract

    Isaac Newton’s views on the mind–body relation are of interest not only because of their some-what unique departure from popular early modern conceptions of mind and its relation to body, butalso because of their connections with other aspects of Newton’s thought. In this paper I argue that

    (1) Newton accepted an interesting sort of mind–body monism, one which defies neat categorization,but which clearly departs from Cartesian substance dualism, and (2) Newton took the power bywhich we move our bodies by thought alone to be a member of the family of forces that includesgravity and electricity. Time and again, Newton draws an analogy between the ultimate cause andnature of the volitional powers of mind and the ultimate cause and nature of these other forces.  2006 Elsevier Ltd. All rights reserved.

    Keywords:  Isaac newton; René descartes; John locke; Monism; Mortalism; Mental causation

    Fantasy is helped by good air, fasting, and moderate wine. But spoiled by drunken-

    ness, gluttony, too much study (whence, and from extreme passion comes madness),

    dizziness, and commotions of the spirits.1

    0039-3681/$ - see front matter    2006 Elsevier Ltd. All rights reserved.

    doi:10.1016/j.shpsa.2006.06.004

    E-mail address:  [email protected] (L. Dempsey).1 This remark is taken from Newton’s student notebooks (McGuire & Tamny, 1983, p. 395). In the first

    sentence, Newton is apparently paraphrasing Henry More who, while discussing imagination, writes: ‘For

    Fasting, fresh Aire, moderate Wine, and all things that tend to an handsome supply and depuration of the  Spirits,make our thoughts more free, subtle, and clear’ (More, 1978a, p. 106).

    Stud. Hist. Phil. Sci. 37 (2006) 420–441

    www.elsevier.com/locate/shpsa

    Studies in Historyand Philosophyof Science

    mailto:[email protected]:[email protected]

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    1. Introduction

    In his student notebooks (McGuire & Tamny, 1983), Newton briefly considered boththe materialist program of Thomas Hobbes and Cambridge colleague Henry More’s

    defence of an immaterial and naturally immortal soul. Although he expressed an appreci-ation for Hobbes’s emphasis on the role of motion in perception and thought, Newton wasnot prepared to reduce mental phenomena to motions in the brain. Instead, he acceptedMore’s argument that the  agency  of mind is required for memory, imagination, and vol-untary motion.2 Nevertheless, in Section   2, I demonstrate that Newton did   not   acceptCartesian substance dualism; he rejected the Cartesian conceptions of space, body, andmind, contended that minds are extended and locally present, and embraced both the cau-sal and ontological interdependence of mind and body. In Section   3, I consider threeaspects of Newton’s rather unorthodox theology which evince sympathy for an ancientmonist anthropology; he accepted the heresy of mortalism, rejected the literal existenceof ghosts and demons, and expressed an appreciation for a monist conception of mindqua life according to which persons are identified not with immaterial souls but with livingbodies. In Section 4, I return to Newton’s conception of mind–body causation. His studentnotebooks demonstrate an early interest in the causal interdependence of mind and body;3

    the challenge was demarcating those functions which belong to body and brain, and thosewhich belong to mind.4 Interestingly, throughout his published works, his unpublished

    2 On these points see McGuire & Tamny (1983), pp. 216–240. For example, in agreement with More, Newtonargued that memory requires the re-initiation of motion of the image in question, and this requires the agency of the mind. In Newton’s words: ‘Were the soul nothing but modified matter and did memory consist in action (for it

    can thus consist in nothing else) we could never call things into our memory, for so long as that action continueswe must think of and remember that phantasm, and when that action ceases, and not before then, we may ceaseto think of and remember that phantasm: but how shall we call this thing into memory the action being done andwe having no principle within us to begin such a motion again within us’ (ibid., p. 449).3 Newton was clearly impressed with the extent to which memory and the physiology of the body are connected.

    For instance, many bodily ailments are accompanied by deficits in memory. Newton remarked, ‘MessalaCorvinus forgot his own name. One, by a blow with a stone, forgot all his learning. Another, by a fall from ahorse, forgot his mother’s name and kinfolk. A young student of Montpellier, by a wound, lost his memory, sothat he was fain to be taught the letters of the alphabet again. The like befell a Franciscan friar after a fever.Thucydides writes of some who, after their recovery from the great pestilence at Athens, forgot the names andpersons of their friends and themselves too; not knowing who they were or by what names they were called’.Further, citing More who cites Lucretius, Newton writes: ‘And certain others fell into forgetfulness of all things,so that they could not even know themselves’ (ibid., p. 393).4 Consider perception. The results of Newton’s earliest inquiries place great importance on the contributions of 

    the physiology of the organs of sense. As McGuire and Tamny put it, Newton’s experiments on perception show‘how much of vision is determined by the physiology of the eye and the optic nerves. For Newton, the eye is notthe simple  camera obscura  that leaves everything to the observer’s (the soul’s) interpretation. Colors are mixed,images are drowned out, double images become one, and so forth, all before the final ‘‘motional pictures’ arepresented to the soul’’ (ibid., p. 237). According to Newton’s account of perception, ‘motional pictures’ areconveyed through the eyes and the optic nerves to the brain’s sensorium. Consider a 1682 letter to William Briggsin which Newton addresses the phenomenon of double vision experienced when one presses one’s thumb upon thebase of one’s eye. Newton proffers an explanation in terms of a sensorium account of perception: ‘When we lookwith two eyes distorted so as to see the same object doubled, if it be asked why those objects appear in this or that

    situation and distance one from another, the answer should be because through the two eyes are transmitted intothe sensorium two motional pictures by whose situation and distance then from one another the soul judges shesees two things so situate and distant. And if this be true, then the reason why, when the distortion ceases and theeyes return to their natural posture, the doubled object grows a single one is that the motional pictures in thesensorium come together and become coincident’ (Newton, 1850, p. 265; cited in Newton, 1953, p. 102).

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    manuscripts, and his correspondence, Newton related the volitional powers of mind toGod’s creation and movement of corporeal bodies; time and again, Newton draws ananalogy between the ultimate cause and nature of mental causation and the ultimate causeand nature of gravity, electricity, and corporeality. We consider some of Newton’s many

    references to this relation, paying particular attention to the implication that, for Newton,mental phenomena are akin in nature to gravitational and electrical phenomena.

    2. Newton contra Cartesian substance dualism

    Newton’s views on the nature of the mind–body relation are greatly informed by hishypothetical account of God’s creation of corporeal bodies and his critique of Cartesianmind–body metaphysics found in the undated manuscript  De gravitatione.5 The primaryfocus of   De gravitati one   is Descartes’s   Principles of philosophy, specifically, his accountof space and motion.6 According to Descartes, space   is   extended substance, and hence,a material  plenum. To account for the orbits of the planets, he proffered a vortex theoryof planetary motion. The spinning of the sun causes vortices in the thick aether of inter-planetary fluid and these vortices sweep the planets around the sun in their orbits. Most of De gravitatione   is an extended critical examination of Descartes’s theory of space andmotion.7 Newton stressed his departure from the Cartesian account of space and motionprimarily by distinguishing between bodies and extension, and by defining motion notwith respect to the positions of other bodies but with respect to the parts of extensionitself. As Howard Stein (2002, p. 264) points out, it is this second point that leads Newtoninto the metaphysical digression that will concern us.

    Space, Newton tells us, is a necessary consequent of any being; if anything exists, spaceexists. And since God exists necessarily, there is no time at which space did not exist.

    Space is an affection of a being just as a being. No being exists or can exist which is

    not related to space in some way. God is everywhere, created minds are somewhere,

    and body is in the space that it occupies; and whatever is neither everywhere nor any-

    where does not exist. And hence it follows that space is an emanative effect of the

    first existing being, for if any being whatsoever is posited space is posited. ( Newton,

    2004a, p. 25)

    5 This manuscript is untitled and has come to be known by the opening words of the text. Although the date of the composition of   De gravitatione   is unclear,  Andrew Janiak (2004), pp. xix–xx, puts it before 1685. On themetaphysical import of  De gravitatione, see Howard Stein (2002).6 Janiak (2004), p. xvii n. 13, points out that in his personal library Newton possessed a 1656 Amsterdam

    edition of Descartes’s  Principles of philosophy and a 1664 London edition of the  Meditations.7 According to Robert Disalle, Descartes’s theory of motion is entirely unsuited for the sort of dynamical

    account of motion Newton had in mind. For Newton, ‘if physics is to understand the real causal connections inthe world, then physics must define space, time, and motion so as to make those connections intelligible’ (Disalle,2002, p. 38). Descartes distinguishes two senses of motion: according to the philosophical sense of motion— motion relative to the interplanetary fluid—the Earth does not move. According to the non-philosophical sense of motion—motion relative to the fixed stars—the Earth does move. But it is precisely Descartes’s common or non-

    philosophical sense of motion that is relevant in understanding the real causal connections of the planetarymotions. In Newton’s words, to understand the tendency of the planets to recede from the sun we don’t need toconsider the so-called ‘true and philosophical rest of the planets’ but their ‘common and non-philosophicalmotion’. In this case, ‘surely motion ought to be acknowledged in the common sense, rather than thephilosophical’ (Newton, 2004a, p. 15; on this point, see also Stein, 2002, pp. 263–265).

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    Compare these sentiments to very similar ones expressed in the General Scholium to hisPrincipia:

    God endures always and is present everywhere, and by existing always and every-

    where, he constitutes duration and space. Since each and every particle of space isalways, and each and every moment of duration is  everywhere, certainly the maker

    and lord of all things will not be  never or  nowhere . . . He is omnipresent not only  vi r-

    tually  but also  substantially; for action requires substance. (Newton, 1999, p. 941)8

    Similarly, the Scottish mathematician David Gregory, who kept extensive notes of hismeetings with Newton, recorded that

    [Newton] believes God to be omnipresent in the literal sense; and that as we are sen-

    sible of Objects when their images are brought home to the brain, so God must be

    sensible of every thing, being intimately present with everything: for he supposes that

    God is present in space where there is no body, he is present in space where a body isalso present. (Cohen & Westfall, 1995, p. 329)

    God’s literal omnipresence is an important aspect of Newton’s hypothetical account of God’s creation of corporeal bodies.9 The objective was to describe something indistin-guishable from bodies which ‘display all their actions and exhibit all of their phenomena’(Newton, 2004a, p. 27) but which require only ‘Extension and an act of divine will’ (ibid.,p. 29) for their existence.10 Three conditions must be met in order for these bodies to exhi-bit all the phenomena of actual bodies:

    (1) that they be mobile . . . (2) that two of this kind cannot coincide anywhere . . . and

    hence that oppositions obstruct their mutual motions and they are reflected in accordwith certain laws; (3) that they can excite various perceptions of the senses and imag-

    ination in created minds, and conversely be moved by them . . .  (Ibid., pp. 28–29)

    To recover all of the phenomena of actual bodies, they must be mobile and they must beable to fill space such that they can exclude other bodies (in accord with certain laws).Moreover, they must have the power to affect minds, and, in turn, be affected by them. Hav-ing met these conditions, such a body would have all the attributes of an actual body. It

    would have shape, be tangible and mobile, and be capable of reflecting and being

    reflected, and constitute no less a part of the structure of things than any other cor-

    puscle, and I do not see that it would not equally operate upon our minds and in turn

    8 Janiak (2004), pp. xix–xx, compares these two passages.9 Although this account was only recently published, Newton had apparently discussed it with several friends,

    including John Locke (Stein, 2002, p. 272–273, discusses this connection). In the second edition of the Essay (Bk.IV, Ch. X, §18) Locke alludes to an account of the creation of matter . As Alexander Campbell Fraser relates,Pierre Coste, who translated the   Essay  into French under Locke’s supervision, received many requests for anexplanation of what Locke had in mind here(Locke, 1894, Vol. II, pp. 321–322 n. 2). Indeed, Leibniz sent a letterto Locke’s friend Lady Masham containing an urgent request for Locke to elucidate, but it did not reach her untilafter Locke’s death in 1704. At the time, Coste had no answer; after Locke’s death, however, he chanced to

    broach the subject with Newton who told him it was he who had suggested to Locke this way of explaining thecreation of matter. The account related by Coste is essentially the one found in  De gravitatione. See note 40.10 In deference to God’s creative powers, Newton does not wish to say definitively what the actual nature of 

    body is, ‘but I would rather describe a certain kind of being similar in every way to bodies, and whose creation wecannot deny to be within the power of God, so that we can hardly say that it is not body’ (ibid., p. 27).

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    be operated upon, because it would be nothing other than the effect of the divine

    mind produced in a definite quantity of space. (Ibid., p. 28)

    This account of body ‘clearly involves the principle truths of metaphysics and thor-

    oughly confirms and explains them’ (ibid., p. 31).

    11

    For one thing, it eliminates unintelli-gible substances—it makes no appeal to   prime matter.12 And since bodies do not existindependent of God’s will, this account leaves no room for atheism. Given that the exis-tence of bodies (and minds) is derivative and incomplete, the widely held ‘prejudice’ thatcorporeal bodies have an existence independent of God’s will must be ‘laid aside, and sub-stantial reality is rather to be ascribed to these kinds of attributes which are real and intel-ligible things in themselves and do not need to be inherent in a subject, rather than to thesubject which we cannot conceive as dependent, much less form any idea of it’ ( ibid., p.32). We should abandon our preconceptions of body as having an independent existencebased in an unintelligible substrate. Instead, we should view both mind and body as

    directly dependent on God for their existence and reality. Indeed, Newton likened the rela-tionship between God and his creation to the relationship between a body and its acci-dents: ‘God does not sustain his creatures any less than they sustain their accidents, sothat created substance . . .   is of an intermediate nature between God and accident’ (ibid.,p. 32).13

    Newton took this account of body to have significant implications for the Cartesianconception of the mind–body relation. He asked the reader to ‘abstract from body (as[Descartes] commands) gravity, hardness, and all sensible qualities, so that nothingremains except what pertains to its essence’. Will only extension remain? No, for ‘whatwe cannot eliminate from body is that faculty or power by which they [the qualities of 

    body] stimulate the perceptions of thinking beings’ (ibid., p. 34). A defining characteristicof bodies, but not  extension, is the power to affect minds; likewise, a defining characteristicof minds is the power to cause motion in bodies, specifically the physiological bodies towhich they are united.

    Descartes’s identification of extension and body, coupled with his dualism, also impliesthat minds lack extension; Newton rejected this position as untenable. First, it seems tolead to the absurd consequence that minds do not even exist, since they are not ‘substan-tially present’ in any extension. Second, it renders the mind–body union—and mind–bodycausation—unintelligible, if not impossible. On Descartes’s view we must say that ‘mindhas no extension at all, and so is not substantially present in any extension, that is, existsnowhere; which seems the same as if we were to say that it does not exist, or at least ren-ders its union with body totally unintelligible and impossible’ (ibid., p. 31). To put thepoint in modern terms, the metaphysical heterogeneity of the Cartesian mind and body

    11 Newton also alludes to the creation of matter in the 31st Query of his  Opticks: ‘since space is divisible   ininfinitum, and Matter is not necessarily in all places, it may be also alow’d that God is able to create Particles of Matter of several Sizes and Figures, and in several Proportions to Space, and perhaps of different Densities andForces, and thereby to vary the Laws of nature, and make Worlds of several sorts in several Parts of the Universe’(Newton, 1952, pp. 403–404).12 In his words, ‘for the existence of these beings it is not necessary that we suppose some unintelligible substance

    to exist. . .

    . Extension and an act of divine will are enough’ (Newton, 2004a, p. 29). See Stein (2002), pp. 280–282,for a discussion of Newton’s rejection of unintelligible substances.13 Newton continues, ‘And hence the idea of it no less involves the concept of God than the idea of accident

    involves the concept of created substance. And so it ought to embrace no other reality in itself than a derivativeand incomplete reality’ (ibid., p. 32).

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    raises the specter of  epiphenomenalism, the (absurd) view that the mind has no causal influ-ence over the operations of the body.

    In fact, Newton’s insistence that the mind is necessary for, and co-extensive with, activ-ities in the body14 represents a radical departure from Cartesian substance dualism. Body

    is not the metaphysical antithesis of mind; indeed, it is far better that ‘the distinctionbetween these ideas [extension and thinking] will not be so great but that both may fitthe   same  created substance, that is, but that a   body may think, and a thinking being beextended ’  (ibid., p. 31; my emphasis). This is an intriguing and prescient gesture towardmind–body substance   monism. In his   Principles of philosophy   (Part I, §51)   Descartes(1912, p. 184)  defines a substance as ‘nothing else than a thing which exists in such away as to stand in need of nothing beyond itself’ for its existence except God. I take it thatNewton is here using a similar conception of ‘substance’. For Newton, then, mind andbody do  not  possess independent existence. In other words, mind and body are not onlycausally, but also ontologically, coupled. Human minds exist only in living human bodies,and human bodies owe their life and motion to the minds with which they are coupled. Buthow are we to reconcile such a view with Newton’s profound belief in God? After all, if minds lack independent existence, they cannot survive the death of their bodies. Isn’t sucha view a clear ‘path to atheism’?15

    3. Hebraic monism and the mortality of the mind

    The ontological interdependence of mind and body has important theological conse-quences. That the mind is not incorruptible, immutable, and necessarily conscious, that

    it is not, therefore, immortal, is both a challenge to orthodoxy and an apparent path toatheism, at least according to many of Newton’s contemporaries. But while he was quiteunorthodox in his Christology, Newton was no atheist. The answer to this puzzle is t heChristian heresy of mortalism, a heresy that recent scholarship has attributed to Newton.16

    According to the doctrine of mortalism, we are, as Richard Overton put it in his 1644Mans mortalitie, ‘a compound wholly mortal’ (Overton, 1968, p. 7). For our purposes,two types of Christian mortalist should be distinguished: (1)  psychopannychists  hold thatthe soul ‘sleeps’ after the death of the body until the final judgment when the soul is‘reawakened’ with the resurrection of the body, and (2)   thnetopsychists hold that the souldies and is destroyed with the death of the body but is recreated with the resurrection

    of the body. According to either view, a naturally immortal soul capable of independent

    14 To exactly what extent the mind is co-extensive with the body—whether, for example, the mind is co-extensivewith the entire body, or just the sensorium—is not clear.15 This is the phrase Newton used to describe what he took to be the atheistic consequences of the Cartesian

    identification of extension and body, which follow because, being a presupposition of God’s eternal existence,‘extension was not created but has existed eternally’ (Newton, 2004a, p. 31); hence, if body  is  extension, we canconceive of it as independent of God in the sense that it does not depend on God for its creation and continuedexistence.

    Although More had some sympathies for Descartes’s substance dualism, he apparently shared many of Newton’s reservations. For example, More rejected both the identification of body and extension, and the non-

    extensionality of mind. And like Newton, More came to see Descartes’s metaphysics as a path to atheism. AsJohn   Cottingham (1986), p. 104, puts it, More ‘began by cautiously welcoming Descartes’s philosophy (asproviding, for example, a defence of the immortality of the soul), but gradually came to see Cartesian Cosmologyas providing the royal road to atheism’. See More (1978a).16 James E. Force (1994,1999); Reiner Smolinski (1999); see also Stephen D. Snobelen (2004), pp. 262–264.

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    existence is superfluous and foreign to scripture, and, indeed, the monist anthropology of ancient Judaism and primitive Christianity (Snobelen, 2004, pp. 162–164). Materialistmortalist Joseph Preistley puts the point nicely in his 1777  Disquisitions relating to matterand spirit:

    According to the uniform system of revelation, all of our future hopes are built

    upon . . .   the   resurrection   of . . .   the   body   which is always considered   the man. This

    doctrine is manifestly superfluous on the idea of a soul being a substance so distinct

    from the body as to be unaffected by death, and able to subsist, and even to be more

    free and happy, without the body. This opinion . . .   [was] not . . .  known to the  Jews,

    and . . .  must have had its source in  heathenism.  (Priestley, 1975, pp. 167–168)

    A number of seventeenth-century philosophers were attracted to some form of mortal-ism.17 For example, both Locke and Hobbes were apparently thnetopsychists.Hobbes setout his mortalist position in Chapters 38 and 44 of his  Leviathan. According to Hobbes,‘immortal life (and soul and life, in the Scripture, do usually signify the same thing) begin-neth not in man till the resurrection and day of judgment’ (Hobbes, 1994, p. 305). For theview that ‘the soul of man is in its own nature eternal, and a living creature independent of the body; or that any mere man is immortal, otherwise than by the resurrection in the lastday . . .   is a doctrine not apparent in scripture’ (ibid, p. 304).18

    Locke, who was one of the few colleagues with whom Newton shared his Christology,19

    also accepted a mortalist interpretation of death. There are hints of Locke’s mortalism inhis Essay. To begin with, his account of personal identity in Book II (Chapter 27) does  notidentify persons with immaterial and naturally immortal souls. Personal identity, accord-

    ing to Locke, is founded in memory and self-consciousness; hence, if a human had oneimmaterial soul but two distinct and independent conscious lives, say, one consciousnessby day and another by night, we would have ‘two persons with the same immaterial spirit’(Locke, 1894, Vol. I, p. 464).20

    The immateriality of the mind was taken by many to imply its natural immortality.Locke challenges this line of reasoning in both his   Essay   and in his personal journal.Indeed, he was apparently not even convinced that minds are necessarily immaterial. InBook IV (Chapter 3, §6) Locke expressed agnosticism concerning the question of whethermatter can think, it being impossible for us ‘to discover whether Omnipotency has notgiven to some systems of matter, fitly disposed, a power to perceive and think, or else

     joined and fixed to matter, so disposed, a thinking immaterial substance’. For it is no less

    17 See  Norman T. Burns (1972). As he suggests, perhaps the most powerful seventeenth-century defense of mortalism was propounded by Overton (1968).18 Hobbes here cites Job 14.12. For more on Hobbes’s denial of the natural immortality of the soul, see the first

    (Sections 41–56) and third (Sections 19–20) chapters of the appendix to the Latin edition of the  Leviathan(Hobbes, 1994, pp. 506–511, 544). On Hobbes’s mortalism see  Burns (1972). As the editor of  Mans mortalitie,Harold Fisch, emphasizes, Hobbes ‘revised Epicureanism’ can be seen as a reaction against the sprit world of More and the Platonists (Overton, 1968, p. xxiii). So like Priestley, but apparently unlike Newton and Locke (seebelow), Hobbes advances an explicitly materialist interpretation of mortalism.19 Newton began a theological dialogue with Locke sometime in 1689 sending Locke his letters on the

    Trinitarian corruptions of scripture—his ‘Two notable corruptions’—intended for anonymous publication on thecontinent. See Westfall (1981), pp. 488–493.  On Locke’s mortalism see John Marshall (2000), pp. 158–161.20 It is for these sorts of reasons that Locke rejects the possibility of reincarnation or the transmigration of souls

    (Bk. II, Ch. 27, §14). In so far as one’s memory of a past life is irrecoverable, one is, on Locke’s view, a differentperson. In other words, on Locke’s view, reincarnation only makes sense if memory of the past life is preserved.

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    conceivable that ‘God can, if he pleases, superadd to matter  a faculty of thinking , than thathe should superadd it to  another substance . . . For I see no contradiction in it, that [God]should, if he pleased, give to certain systems of created senseless matter, put together as hethinks fit, some degrees of sense, perception and thought’ (ibid. Vol. II, pp. 192–193).

    What’s more, such a view is no threat to morality or religion:

    All the great ends of morality and religion are well enough secured,   without   philo-

    sophical 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 sensibil-

    ity   in another world, and make us capable there to receive the retribution he has

    designed to men, according to their doings in this life. (Ibid., p. 195; my emphasis.)

    In his personal journal, Locke (1858, pp. 128–130)  considered the following argumentfor the immortality of the mind. If matter cannot think, then the mind is immaterial, andsince nothing can destroy an immaterial thing, the mind is immortal. On the other hand,this line of reasoning seems to lead to the objectionable consequence that, in so far as ani-mals think and feel, they too have naturally immortal minds (souls). Given this, some‘have rather thought it fit to conclude all beasts perfect machines, rather than allow theirsouls immortality’.21 However, these positions ‘perfectly mistake immortality’. If we focusonly on ‘bare substantial existence and duration’, the same argument applies to matter‘since nothing can really destroy a material substance more than immaterial’. What isreally important, according to Locke, and what the above argument does not address,is the persistence of a ‘state of sensibility’. The argument assumes that because an imma-terial soul is immutable, it cannot cease to ‘think and perceive’. ‘But this is manifestly

    false’, Locke tells us, since we see just such a loss of sensibility every night in a dreamlesssleep. Hence the above argument goes no way to proving what is really important, the per-sistence of consciousness, which, if sleep is any indication, depends, at least in part, on thecurrent state of the body. From here Locke concluded:

    Let, therefore, spirit be in its own nature as durable as matter, that no power can

    destroy it but the Omnipotence that at first created it; they may   both lie dead and 

    inactive, the one without thought, the other without motion, a minute, an hour, or

    to eternity, which wholly depends upon the will and good pleasure of the first

    Author. (ibid. pp. 129–130; my emphasis.)

    Locke’s mortalist inclinations are made even more explicit in his   Reasonableness of Christianity. Commenting on Adam’s punishment— his loss of immortality —Locke tellsus that by the word ‘death’ he ‘understands nothing but a   ceasin g to be, the losing of all actions of Life and Sense’ (Locke, 1999, p. 8; my emphasis).22 According to Locke,

    21 On the Cartesian view, thinking substance is indivisible, and thus, incorruptible. Hence, if animals think andhave minds, they too have naturally immortal souls. According to mortalism, on the other hand, there is noreason to deny mentality to animals since immortality does not follow from mentality. For animals, there is(presumably) no prospect of bodily resurrection, and so, their deaths are permanent.22 For many mortalists, while the punishment for Adam’s sin is mortality, the promise of Christ’s crucifixion is

    immortality via (bodily) resurrection. As Hobbes put it, ‘as Adam lost eternal life by his sin, and yet lived after itfor a time, so the faithful Christian hath recovered eternal Life by Christ’s passion, though he die a natural death,and remain dead for a time (namely, till the resurrection)’ (Hobbes, 1994, p. 303). See also, for example, Overton(1968), Ch. 1, and Priestley (1975), Ch. 10. Note well that for one who takes mortality to be Adam’s punishment,the idea that Adam nevertheless has a soul that is, by its very nature, immortal, makes little sense.

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    death is the complete cessation of thought and sensation; eternal life is a gift given to therighteous at the Second Coming through bodily resurrection. In notes added to a copyof  Reasonableness of Christianity  Locke wrote: ‘That  Death  (i .e. a cessation of sense andperception) shall at last . . .  be a punishment of the unrighteous is plain from   Gal . VI. 8.

    where  corruption   is set in opposition to   life everl asting , the one the fruit of righteousnessand the other of unrighteousness’ (ibid., p.14).23 For Locke, life-after-death in hellfire isstill life, and so, the idea that death for the wicked, rather than being a cessation of life(that is, a permanent cessation of consciousness), is an eternity of hellfire is untenable,even bizarre: ‘it seems a strange way of understanding a Law, which requires the plainestand directest words, that by   Death  should be meant Eternal Life in Misery. Could any-one be supposed by a Law, that says,   For Felony you shall die, not that he should losehis Life, but be kept alive in perpetual exquisite Torments? And would any one thinkhimself fairly dealt with, that was so used?’ (ibid., pp. 7–8). Compare this to Overton’sremarks that if the soul is naturally immortal, and the body its mere instrument, itmakes little sense to punish (that is, kill) the body of a criminal, ‘as if a Magistrateshould hang the   Hatchet, and spare the Man that beate a mans braines out with it’(Overton, 1968, p. 11).

    Newton also accepted a mortalist interpretation of death.24 Consider first that Newton(like Locke) had intellectual sympathies with the antitrinitarian and   thnetopsychist  Soci-nians. As Snobelen (2005, p. 263) puts it, ‘Both the Socinians and Newton were mortalistswho saw the teaching of the immortal soul, like the Trinity, as an unwarranted andunscriptural obtrusion upon primitive Christianity’. So, for example, Newton tells us inone theological tract that ‘resurrection from the dead is called living again & therefore

    between death & the resurrection men do not live’.

    25

    Similarly, meeting with Newton inMay of 1694, David Gregory recorded in his notebook Newton’s position that, ‘Not a sep-arate existence of the soul, but a resurrection with a continuation of memory is the require-ment of religion’ (Newton, 1961, pp. 336, 339). In agreement with Locke, then, Newtonheld that the sort of preservation of personhood necessary for religion and morality doesnot require a soul with a separate and independent existence, but rather a resu rrection of the body and a continuation (or re-constitution) of consciousness and memory.26 Thus theheresy of mortalism coheres nicely with Newton’s insistence that mind and body are bothcausally and ontologically coupled.

    23 As Snobelen (2001) points out in his review of the 1999 edition, Locke’s mortalism is greatly informed byemendations made to an annotated and interleaved copy of a first edition now held by Harvard University.Concerning the death of Adam, Locke (1999), p. 8, notes a number of biblical sources and cites Justin Martyr as adefender of the primitive Christian belief in the mortality of the soul.24 Whether or not Newton is best characterized as a psychopannychist or a thnetopsychist is open to debate.

    Force (1994) argues for the former, Snobelen (in conversation) defends the latter. Snobelen points out that (1) aperennial problem in untangling these two forms of mortalism is that thnetopsychists often use the   metaphor of sleep to describe death, (2) psychopannychism, taken literally, contradicts what he takes to be the Hebraic monistanthropology in so far as a sleeping soul, although it may depend on a living body for its consciousness, continuesto exist while the body disintegrates, and (3) Newton’s mortalism may not have been sufficiently developed toallow him to articulate the difference. In any case, it is clear that Newton accepted some form of mortalism and

    thus, some form of mind–body interdependence.25 Jerusalem, Jewish National and University Library, MS Yahuda 7.2e, fol. 4v; cited with permission.26 Nevertheless, as Burns emphasizes, many seventeenth-century orthodox Christians ‘could not conceive of a

    Christian faith that did not include belief in an immortal, incorporeal substance called soul; thus they naturallythought of soul sleepers as pagan rationalists’ (Burns, 1972, p. 157).

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    At least two other aspects of Newton’s theology evince an appreciation for an ancientmonist anthropology. First, given Newton’s mortalism and substance monism, weshould expect a skeptical view concerning the literal existence of ghosts and demons,in so far as they are taken to be the  disembodied  spirits of the dead who haunt and tempt

    the living. On the other hand, such skepticism might be thought to represent a sure pathto atheism. As   Snobelen (2004, pp. 155–157)  notes, in his   A blow at modern Sadducism(1668), Royal Society Fellow Joseph Glanvill argued that the denial of the literal exis-tence of evil spirits was tantamount to atheism. In a similar vein,   Henry More1978a,b   advocated an inductive methodology for demonstrating and documenting theexistence of evil spirits. Such documentations were taken by some to represent empiricalproof of the divine.27

    However,  Snobelen (2004)  has recently demonstrated that Newton rejected the literalexistence of disembodied spirits (ghosts), evil demons, witchcraft, and a personaldevil.28 As he puts it, on Newton’s ‘mortalist system, there was no place for ghosts,whether good or evil’ (ibid., p. 171). In his   Leviathan, Hobbes maintained that demonswere metaphors for psychotic disorders: ‘I see nothing at all in the Scripture, that req-uireth a belief, that demoniacs were any other thing but madmen’ ( Hobbes, 1994, p.46). Similarly, in a 1692 letter to Locke, Newton argued that the language of castingout demons ‘was ye language of ye ancients for curing Lunatics’ (Newton, 1961, p.214). Rather than identifying demons and evil spirits with fallen angels, as was com-monplace, Newton was apparently influenced by Joseph Mede’s   Apostasy of the lattertimes   (1641), which identified one of the chief apostasies of the Roman Church asthe invocation of saints. The demons of the Gentiles, Mede argued, were taken to be

    the ‘deified soules of men after death’ (Mede, 1641, p. 14; cited in  Snobelen, 2004, p.164) and this (pagan) view led to the notion of saintly intermediaries. For Newton,the belief that people can ‘really divine, charm, inchant, bewitch or converse with spiritsis a superstition of the same nature’ as the belief   ‘that the idols of the gentils were notvanities but had spirits really seated in them’.29 Rejecting disembodied spirits, evildemons, and witchcraft is thus directly related to rejecting false gods and idolatry.As Snobelen puts it, ‘For Newton belief in activity by evil spirits is equivalent to theconviction that the false gods or idols of the pagans were real independent beings; bothpositions are equally untrue’ (Snobelen, 2004, p. 163). This pagan doctrine of demons,like the doctrines of the Trinity, and the naturally immortal soul, follow, on Newton’s

    view, from the false teachings of Athanasius, the champion of Trinitarian orthodoxy.30

    In fact, Athanasius’s belief in the existence of a conscious soul between death andresurrection is directly linked to this misguided notion of demons. ‘Athanasius by

    27 As Snobelen (2004), p. 157, points out, at the end of his  Antidote against Atheism, More (1978b) tells us that if there is no spirit, there is no God. According to Smolinski (1999, p. 288) for many orthodox immortalists, ‘storiesof walking ghosts, poltergeists, deathbed narratives of parishioners with out-of-the-body-experiences, andmanifestations of witchcraft were precisely the type of tangible and extrabiblical evidence they needed to rebut theHobbesian Leviathan’.28

    See also Smolinski (1999), pp. 259–290. Snobelen is quick to emphasize that Newton did accept the existenceof spiritual beings, such as angels and perhaps ‘spectral saints’ (Force, 1999, 2004).29 Geneva, Fondation Martin Bodmer, MS 5A, fol. 8v; cited in  Snobelen (2004), p. 163.30 Overton identifies Athanasius and Augustine as progenitors of the orthodox view of the soul as ‘invisible,

    immortall, [and] incorporeall like the Angels’ (Overton, 1968, p. 19).

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    making . . .   the soul of Ammon ascend . . .   up to heaven, laid the foundation for intro-ducing . . .   into the greek Churches . . .   this heathen . . .   doctrine of Daemons . . .  togetherwith that Popish one of Purgatory’.31

    To understand fully Newton’s demonology, it is necessary, Snobelen insists, to consider

    pre-modern rabbinic Judaism; rather than taking Satan to be a fallen angel, Satan is seenas a metaphor for the Hebrew notion yetzer ha-ra —the evil inclinations within one’s heart.This difference represents ‘a shift from an ontology of Satan to a psychology of tempta-tion, a reorientation from the external to the internal’ (ibid., p. 181). From this perspec-tive, then, the source of one’s own sin is not some external devil, but the lust and vicewithin one’s own heart. For Newton, those who claim to be tempted by a personal Devilare, in Snobelen’s words, ‘deluded and provoked by their own fleshly imagination’ (ibid.,p. 165).

    A third important connection with Newton’s theology concerns a short theologicaltract entitled ‘The Question stated about absteining from blood’ in which he consideredthe ancient Hebraic prohibition against eating the blood of an animal.32 According toGenesis (9.4–5), Newton argued, the prohibition stems from a view according to whichthe blood of an animal is its  anima   or ‘nephesh’. So, while often translated ‘soul’ in theKing James Bible, ‘nephesh’, in this case, denotes a corporeal substance. The identificationof the nephesh of an animal with its blood represents a decidedly  organic  conception of mind  qua life. According to advocates of Christian mortalism, a more organic and vital-istic conception of mind is an important aspect of Hebraic monism. As Priestley put itin his discussion of God’s creation of Adam:

    Nothing but the circumstance of   breathing  made the difference between the unani-

    mated earth, and the   living soul . It is not said that when one constituent part of the man was made, another necessary constituent part, of a very different nature,

    was superadded to it; and that the two, united, constituted the man; but only that

    that substance that formed of  the dust of the earth became a living soul , that it became

    alive, by being made to  breathe.  (Priestley, 1975, p. 115).33

    31 Los Angeles, William Andrews Clark Memorial Library, UCLA, MS**N563M3 P222, fol. 55r; cited inSnobelen, (2004), p. 164.   Smolinski (1999), pp. 285–289, comes to a similar conclusion concerning Newton’sviews, arguing that Newton’s mortalism led him to reject the influences of Greek demonology and the Catholicdoctrine of purgatory.32 Private collection, Sotheby’s Lot 232, New York, 3 December 2004, f. 2r; cited in  Snobelen (2005), p. 264. Cf.

    Hobbes’s (1994), p. 419, discussion of this prohibition in Chapter 44 of his  Leviathan.33 It is worth noting that in his personal notes, Locke (1999), pp. 216–219, discusses the relation between death

    and biblical allusions to breath. In Job (34.14–15), for example, it is said that ‘if god gather unto him self his spiritand his breath all flesh shall perish together and man shall turn again unto dust’ (cited in  Locke, 1999, p. 216).And in Psalms (146.4) it is written that ‘His breath goeith forth he returneth to his earth, in that very day histhoughts perish’ (cited in ibid.). We might add to this Acts 17.25, Daniel 12.2–3, Gen 1.2, and Psalm 104 (thanks

    to James E. Force for pointing out these biblical allusions to breath).  Overton (1968), Ch. II, also reviews thescriptural evidence for mortalism (noting, e.g., Job 3.11–20, Job 4.19–21, and 2Cor. 5.1–4). Likewise, Hobbesexpresses a sympathy for ancient conceptions of mind  qua  life and breath when he writes: ‘The  soul  in scripture,signifieth always either the life or the living creature; and the body and soul jointly, the  body alive . . .. God made[man] of the dust of the earth, and breathed in his face the breath of life’ ( Hobbes, 1994, p. 419).

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    An organic conception of mind was not alien to the rest of the ancient world. One isreminded of Aristotle’s contention that the common sensorium is the blood around theheart.34 It may very well have been Aristotle Newton had in mind when he included theheart as one of the ten potential locations of the common sensorium in his student note-

    books.35 Consider, moreover, that the Latin  ani ma, the Latin  spiritus, the Greek  psyche,like the Hebrew   nephesh  and the Hebrew   ruach,36 all carry connotations of life, breath,and mind. According to Priestley a corporeal and physiological conception of mind wasthe received view for much of the ancient world. For the ancients the mind was ‘only a

     finer kind  of what we should now call  matter; something like  air  or  breath . . .  or else like fire  or  flame’, given the ‘warmth  of the living body’. ‘Consequently’, Priestley concludes,‘the ancients did not exclude from the mind the property of  extension, and local presence’(ibid., p. 222).

    4. Mental causation and the philosophy of animal motion

    Recall Newton’s suggestion that we lay aside the prejudice that bodies have a ‘complete,absolute and independent [reality] in themselves’.

    And this we can manage without difficulty if (besides the idea of body expounded

    above) we reflect that we can conceive of space existing without any subject, when

    we think of a vacuum . . .. In the same way, if we should have an idea of that attri-

    bute or power by which God, through the action of his will alone, can create

    beings, we should readily conceive that attribute as subsisting of itself, without

    any substantial subject, and [thus as] involving the rest of his attributes. (Newton,

    2004a, pp. 32–33)

    Just as we can conceive of the attributes of corporeal bodies as existing without an unin-telligible substrate, requiring only extension and an act of divine will, so too, we can con-ceive of the power, by which God, through the sole action of his will, creates, maintains,and moves bodies, as existing, not as an attribute of an unintelligible substance, but withhis other attributes alone. However, one should hesitate in concluding too much concern-ing the ultimate nature of God or created minds. Given that we have very little under-standing of the power by which God moves bodies, ‘nor even of our own proper powerby which we move our bodies, it would be rash to say what be the substantial basis of 

    mind’ (ibid., p. 33).Howard Stein (2002, p. 282)  maintains that this line of thought bears directly on the

    ‘so-called ‘‘mind–body problem’’’. According to Stein, Newton is saying that since our

    34 See, for example, Aristotle’s  On sleep 455a13–456a28 and Generation of animals   738b16–18(Aristotle, 1984).35 In his student notebooks, Newton considered ten possible locations for the ‘common sensorium’ including the

    whole body, the heart, the brain, the fourth ventricle of the brain, and the animal spirits contained within thefourth ventricle (McGuire & Tamny, 1983, p. 383). It seems clear from his views on perception that Newton was

    inclined to locate the common sensorium somewhere within the skull.36 Although ‘ruach’ is not generally translated ‘soul’, consider that the humanities are called in Hebrew ‘madaeiharouch’ which means the ‘sciences of the mind’. ‘Ruach’ can also be used as a synonym for ‘spirit’ (as in ‘goodspirits’), and is related to ‘ruchani’ which is translated ‘spiritual’. Thanks to Itay Shani for discussions of Hebrewnotions of mind and spirit.

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    knowledge of the relationship between mental phenomena and bodily phenomena isextremely limited, we are not warranted in claiming to know the ultimate nature of the mental. What is required for a Newtonian investigation into the mental is an inves-tigation into the lawful relationships between mental and bodily phenomena. Until such

    an investigation is completed, it would be premature to speculate on the metaphysics of mind. In Stein’s words:

    Just as in the theological case, the suggestion sets aside the distinction of ‘kinds of 

    substance’: mind–body dualism/mind–body monism, in favor of the program (or

    problem): to understand mental attributes and their relation to corporeal ones. When

    these relations are sufficiently understood, Newton implies, we may expect to know

    all that there is to know about the ‘substantial foundation of minds’;  before they are

    sufficiently understood, ‘it would be rash to say what is the substantial foundation of 

    minds’. (Ibid.; Stein’s translation)

    According to this reading, questions concerning the metaphysics of the mind should be setaside until the lawful relationships between mental and corporeal phenomena are estab-lished. It is only in the maturity of this endeavour that we will find ourselves in a positionto speak to the metaphysics and ontology of mind; only then will we be warranted in con-sidering the merits of the debate between dualists and monists.

    Stein is right, I think, that for Newton, the ideal approach to the study of the mind andthe resolution of the mind–body problem will involve the study of the lawful relationshipsbetween all the phenomena in question, including not only stimulus, behaviour, and phys-iology, but also mental phenomena as accessed via introspection. What is more, Newton is

    clearly uncertain both about the ultimate nature of the volitional powers of mind, and theultimate nature of corporeality, electricity, and gravity. However, as we have seen, Newtondoes not  suspend judgment on questions concerning the metaphysics of mind or the mind– body relation. Rather than setting aside the monism/dualism debate, Newton weighs inheavily against Cartesian substance dualism, rejecting Descartes’s conception of space,body, and mind. In short, Newton does not take the mind–body problem or the debatebetween dualists and monists to be a pseudo-problem or an area of disquisition that mustbe indefinitely shelved.

    Stein is also right that this passage presents an example of Newton’s more generalrejection of unintelligible substances; Newton is tentatively suggesting that even the attri-

    butes of God do not require an unintelligible substrate in which to inhere. Unintelligiblesubstances are metaphysically gratuitous and unnecessary. In deference to God’s omnip-otence, he could not rule out God’s possible use of unintelligible substances. Neverthe-less he is confident that they are, to say the least, unnecessary. To understand fully theabove passage, however, we must understand a relation to which Newton referred, notonly in   De gravitatione, but also in his published work and his correspondence. In allthese places, he related the mystery of how God creates, maintains, and moves bodiesby divine will to how   we   move our   own  bodies by thought alone. One solution to boththese mysteries appeals to the  mechanical philosophy, which requires force-by-contact.37

    37 On Newton and the mechanical philosophy see Alan Gabbey (2002).

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    On this view, both gravity and the volitional   powers of mind are explained in terms of the mechanical activities of a material aether.38 On the other hand, the mechanical phi-losophy may be wrong in both cases; perhaps forces require an   immaterial   medium, orno medium at all. In what remains, I will argue that it is just this question—the question

    concerning the ultimate nature of forces—that Newton proposed to set aside,   not   theentire mind–body problem.39

    Recall Newton’s account of corporeal bodies. Regarding the power of bodies to interactwith minds, he argued that since bodies are just the realization of the divine mind in def-inite quantities of space, and since ‘it is certain that God can stimulate our perception bymeans of his own will’, it is, in principle, unproblematic for him to ‘apply such power tothe effects of his will’ (Newton, 2004a, p. 28). Concerning the impenetrability and mobilityof corporeal bodies, Newton related our willful control of our own bodies, and God’s abil-ity, through the ‘sole action of thinking and willing’, to make regions of space impenetra-ble. All of us, he maintained, ‘enjoy the same power of similarly moving [our] bodies bythought alone’, and hence, ‘the free power of moving bodies at will can by no means bedenied to God . . .   And . . .   it must be agreed that God, by the sole action of thinkingand willing, can prevent a body from penetrating any space defined by certain limits’(ibid., p. 27). So, as we move our bodies through individual acts of volition, God, whois literally co-extensive with everything that exists, creates and maintains bodies throughacts of divine volition. In fact, the problem of how God creates and maintains bodiesreduces, for Newton, to the problem of explaining the willful movement of our ownbodies:

    I have deduced a description of this corporeal nature from our faculty of moving our

    bodies, so that all the difficulties of this conception may at length be reduced to that;and further, so that God may appear (to our innermost consciousness) to have cre-

    ated the world solely by the act of will, just as we move our bodies by an act of will

    alone; and, moreover, so that I might show that the analogy between the divine

    38 Newton considers the notion of an aethereal medium in several places including the second edition of hisOpticks. In the twenty-fourth query of his  Opticks, Newton (1952), pp. 353–354, considered the possible role of anaethereal medium in the production of ‘animal motions’. But as I. Bernard Cohen (1999), p. 286, points out, thisis a somewhat different entity from the dense aether of the Cartesians.39 One might wonder what motivates Newton’s suspension of judgment, his epistemic prudence, concerning the

    ultimate nature of forces. Part of the answer may lie in Newton’s voluntaristic conception of divine power. As wesaw with his account of body, Newton held that all of creation depends on the will of God. Force has recentlyargued that Newton’s epistemology is strongly influenced by this ‘awe before the power of the free exercise of theLord God’s will’ (Force, 2004, p. 83). According to Newton, ‘We cannot rule out the possibility of futuremiraculous interpositions of [God’s] will sequentially timed to drive home God’s moral lessons to fallen mankind’(ibid., p. 92). In agreement with Robert Boyle, Newton held that the axioms of natural philosophy, no matterhow well established by experiment, are susceptible to invalidation by God. For, as Boyle put it, ‘the mostoptimistic investigator must acknowledge that if God be the author of the universe, and the free establisher of the

    laws of motion, whose general concourse is necessary to the conservation and efficacy of every particular physicalagent, God can certainly invalidate all experimentalism by withholding His concourse, or changing those laws of motion, which depend perfectly upon his will’ (Boyle, 1966, p. 161; cited in Force, 2004, p. 87). Force argues thatNewton’s fourth rule of reasoning, which was added to the second edition of the  Principia   (1713), effectivelycodifies Boyle’s sentiments (Force, 2004, pp. 83–88).

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    laws. Without some   mechanical  explanation for why the planets do not fly from theirorbits, God’s constant miraculous intervention is required. Newton responded to this lineof criticism first by pointing out that, if adherence to the mechanical philosophy isrequired to avoid miraculous interventions in the case of gravity, then the same can be

    ‘said of . . .

      the extension, the duration and mobility of bodies, and yet no man everattempted to explain these qualities mechanically, or took them for miracles or supernat-ural things or fictions or occult qualities’ (ibid., p. 116). So, if an aethereal medium isrequired to avoid miraculous interventions, then this will apply both to gravity and corpo-reality. But the qualities of corporeality are the ‘natural . . . qualities of all bodies seated inthem by the will of God’ and are ‘perfectly incapable of being explained mechanically’(ibid., p. 116). In fact, according to Leibniz’s line of criticism, he must admit, contraryto the theoretical commitments of his doctrine of pre-established harmony, that think-ing—the ebb and flow of thought and feeling—can only be explained mechanically. How-ever, ‘Mr. Leibniz himself will scarce say that thinking is mechanical as it must be if toexplain it otherwise be to make a miracle, and an occult quality, and a fiction’ ( ibid., p.116).42

    For Newton, the lack of a mechanical explanation of gravity is no objection to his con-clusions on gravity, ‘as if it were a crime to content [oneself] with certainties and let uncer-tainties alone’ (Newton, 2004c, p. 124). Nor does the lack of a mechanical explanationimply a supernatural one, at least in so far as ‘supernatural’ is taken to mean beyond nat-ure or beyond the laws of nature.

    [Leibniz] . . .   tells us that God   could not create planets   that should move round of 

    themselves without any cause that should prevent their removing through the tan-

    gent. For a miracle at least must keep   the planet in. But certainly God could createplanets that should move round of themselves without any other cause than gravity

    that should prevent their removing through the tangent. For gravity without a mir-

    acle may keep the planets in. (Newton, 2004b, p. 117)

    The forces that frame nature are the vehicles through which God expresses his will, justas the volitional powers of mind are the vehicles through which we express our wills.The exact natures of all these forces were not yet determined; for Newton, these werequestions requiring further research. And although he had not discovered the cause of gravity, nor provided the sort of explanation of gravity that would satisfy Leibniz

    and other mechanists, he nevertheless determined that gravity was, itself, causal. As An-drew Janiak puts it:

    from Newton’s point of view, gravity had been successfully identified as the cause of 

    the celestial phenomena in question, particularly the planetary orbits . . . . It should

    be of little surprise to find Newton warning Leibniz against inferring that gravity

    42 Consider Leibniz’s well known appeal to the metaphor of a mill to explain the non-mechanical nature of thought and perception. ‘Perception. . .   cannot be explained by mechanical principles, i.e. by shapes and

    movements. If we pretend that there is a machine whose structure makes it think, sense, and have perception, thenwe can conceive it enlarged, but keeping to the same proportions, so that we might go inside it as into a mill.Suppose that we do: then if we inspect the interior, we shall find there nothing but parts which push one another,and never anything that would explain a perception’ (Leibniz, 1875–1890, Vol. 6, p. 609; cited in Bennett, 1994, p.101; Bennett’s translation).

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    itself is not a cause from the f act that Newton had failed to uncover gravity’s cause.

    (Janiak, 2004, pp. xxviii–xxix)43

    To understand the laws of planetary motion, Newton insisted,

    without knowing the cause of gravity, is as good a progress in philosophy as tounderstand the frame of a clock and the dependence of the wheels upon one another

    without knowing the cause of the gravity of the weight which moves the machine is

    in the philosophy of clockwork; or the understanding of the frame of the bones and

    muscles and their connection in the body of an animal and how the bones are moved

    by the contracting or dilating of the muscles   without knowing how   the muscles are

    contracted or dilated   by the power of the mind , is [in] the philosophy of animal

    motion. (Newton, 2004b, p. 117; my emphasis)

    It is an open question whether forces require an aethereal medium—as seventeenth-cen-

    tury conventional wisdom required—or perhaps an as yet undiscovered immaterial med-ium, or any medium at all.44 But as in the case of gravity, we can study the powers of themind by studying their effects, even if we have yet to understand fully their cause or ulti-mate nature.

    In 1715 Newton published ‘The account of the book entitled   Commercium epistolicum’in the Royal Society’s  Philosophical Transactions. This was his ‘supposedly anonymousresponse to a supposedly impartial review of the calculus affair’ (Janiak, 2004, p. xxix),Commercium epistolicum, published by the Royal Society in 1713. In it he distinguisheshimself from Leibniz in some interesting ways:

    It must be allowed that these two gentlemen differ very much in philosophy. . .

     Theone for want of experiments to decide the question doth not affirm whether the cause

    of gravity be mechanical or not mechanical: the other that it is a perpetual miracle if 

    it be not mechanical. The one (by way of enquiry) attributes it to the power of the

    creator that the least particles of matter are hard: the other attributes the hardness

    of matter to conspiring motions, and calls it a perpetual miracle if the cause of this

    43 Similarly, consider this remark from the General Scholium to the  Principia, ‘it is enough that gravity reallyexists and acts according to the laws that we have set forth and is sufficient to explain all the motions of theheavenly bodies and of our sea’ (Newton, 1999, p. 943).44 Many of Newton’s continental critics rejected his claim that the Principia was without hypotheses and accused

    him of introducing occult qualities—that is, qualities that are hidden from sensory experience and observation— into his account. As L. T. More puts it, ‘attraction at a distance was to be an essential property of matter; if so,then they argued, he had introduced an occult quality, and so his philosophy was as hypothetical as the Cartesianhypothesis of vortices in an occult medium’ (More, 1934, p. 552; cited in Thayer, 1953, p. 184). The view thatgravity is an essential property of matter is one way to cash out force-at-a-distance. However, in a series of lettersto Richard Bentley, Newton, while claiming agnosticism, expressed a deep skepticism concerning the idea thatgravity is inherent in matter. ‘Pray do not ascribe that notion to me, for the cause of gravity is what I do notpretend to know and therefore would take more time to consider’ (Newton, 1953a, p. 53). Newton went further ina subsequent letter in which he argued that it is ‘inconceivable that inanimate matter’ should affect other matterwithout mutual contact or an immaterial medium, ‘as it must be if gravitation . . . be essential and inherent in it’.He continues: ‘That gravity should be innate, inherent, and essential to matter, so that one body may act upon

    another at a distance through a  vacuum, without the mediation of anything else. . .

     is to me so great an absurditythat I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it.Gravity must be caused by an agent acting constantly according to certain laws, but whether this agent bematerial or immaterial I have left to the consideration of my readers’ (ibid. p. 54). On Newton’s complex viewconcerning the cause and nature of gravity, see  John Henry (1994).

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    hardness be other than mechanical. The one doth not affirm that animal motion in

    man is purely mechanical: the other teaches that it is purely mechanical, the soul or

    mind (according to the hypothesis of pre-established harmony) never acting on the

    body so as to alter or influence its motions. (Newton, 2004c, p. 125)

    While Newton, for lack of experimental evidence, remained agnostic concerning the truthof the mechanical philosophy, Leibniz argued  a priori  that it must be true. While Newtonattributed the hardness of particles to the will of God and refrained from further spec-ulation, Leibniz endorsed a mechanical account in terms of ‘conspiring motions’. Mostimportantly, while Newton accepted the   possibility   of a non-mechanical (immaterial)component to animal motions, because of his metaphysical commitments, specificallythe doctrine of pre-established harmony, Leibniz was theoretically precluded fromexplaining any aspect of animal motion non-mechanically. Newton, on the other hand,was committed to a robust causal and ontological coupling of mind and body; he was

    undecided, however, whether or not the volitional powers of mind require an aetherealor immaterial medium.

    Finally, in the concluding paragraph of the General Scholium Newton introduced thenotion of ‘spirit’ to help explain attractive and repulsive forces like electricity and magne-tism.45 This discussion is related to the problem of mental causation in interesting ways.‘[A]ll sensation is excited, and the limbs of animals move at the command of the will,namely, the vibrations of this spirit being propagated through the solid fibers of the nervesfrom the extended organs of the senses to the brain and from the brain into the muscles’(Newton, 1999, p. 944). According to I. Bernard Cohen (1999, pp. 280–292), to understandwhat Newton means by ‘spirit’ it is useful to consider drafts of his ‘The account of the

    book entitled  Commercium epistolicum’. One of the charges Newton was responding toin ‘The account’ was the charge that, in this final paragraph of the General Scholium,he was introducing a   hypothesis   concerning the ultimate nature of attractive forces.Indeed, it was suggested that he was actually proffering Henry More’s notion of a hylarchic

     principle   (More, 1978b) which posited a type of immaterial spirit that controls materialbodies, ‘and this spirit’, Newton wrote, ‘is represented of less value then [sic] Hypothesesare unless it be the Aether or subtle matter of the Cartesians’.46 On the contrary, Newtonasserted that he ‘has no where denyed that the cause of gravity is Mechanical nor con-firmed whether the subtle spirit be material or immaterial nor declared any opinions abouttheir causes’ (cited in Cohen, 1999, p. 281). Clearly, ‘spirit’ ought not be read ‘immaterialspirit’ as it would in the case of More’s hylarchic principle; on the other hand, Newtonwould not rule out the possibility that forces require an immaterial medium.

    According to Cohen, the final paragraph of the General Scholium should be regardedas comparable to the queries at the end of Newton’s   Opticks, in so far as it suggests aresearch Pprogram. In fact, Newton had specific experiments in mind. In drafts of ‘TheAccount’, as well as in a draft conclusion to his   Principia, he considered the electricalexperiments of a Royal Society member, Francis Hauksbee who had shown ‘that bodiesdo attract one another at very small distances’ (ibid). Furthermore, Hauksbee ‘suspectsthat this . . .   electrical attraction may be performed by one & the same agent . . . & this

    45 In several other places, Newton qualifies this use of ‘spirit’ with the adjective ‘electrical and elastic’. See Cohen(1999), p. 282.46 Cambridge, University Library, MS Add. 3968, sec. 41, fol. 26; cited in Cohen (1999), p. 281.

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    Agent he calls a subtle spirit’. However, ‘what is this Agent or spirit and what are the lawsby which it acts he leaves to be decided by experiment’ (ibid).

    Of particular interest for our purposes is a passage in which Newton discusses Mr. Hau-ksbee’s considerations concerning the role of this subtle spirit in animal motions. On the

    basis of his experiments, Hauksbee ‘has represented. . .

      that light & this spirit may actmutually upon one another . . . & if this spirit may receive impressions from light and con-vey them into the sensiorium [sic] & there act upon that substance which sees &  thinks,that substance may mutually act upon this spirit for causing animal motions’ (ibid).47 Herewe see gestures toward an account of how attractive forces may be  implicated in mentalcausation; the same subtle spirit that underpins electrical attractions48 may also underpinboth our perception of the external world and the free movement of our bodies. But again,the ultimate nature of this subtle spirit—that is, whether or not its nature is consistent withthe mechanical hypothesis—is an open question. Moreover, it is unclear to what extentNewton considered the ‘substance which sees & thinks’ to be comparable in nature to thiselectrical spirit. I take it, however, that Newton was not here endorsing the independentexistence of the mind, but rather is distinguishing it from the organs of sense and the brain.What is more, one straightforward explanation of the natural interaction of this spirit andthe mind is precisely that the mind and the spirit with which it interacts are quite similar innature. But again, Newton refrained from speculating on their ultimate natures given thelack of experimental evidence.

    5. Conclusions

    Although Newton clearly distinguished created minds from corporeal bodies andrejected the Hobbesean reduction of mental phenomena to motions in the brain, hewas, nevertheless, a mind–body substance monist. He rejected the independent existenceof mind and body—minds and bodies are (1) (to some degree) literally co-extensive,and (2) both causally and ontologically coupled and interdependent. Indeed, he showedgreat sympathy for the monist anthropology of Christian mortalism—contra traditionand orthodoxy, persons are not naturally immortal souls but living, conscious bodies— and expressed a corresponding skepticism concerning the existence of ghosts, demons,witchcraft, and a personal devil. Essential to Newton’s conception of mind is an analogybetween our own volitional powers and God’s creation and movement of bodies; he took

    the volitional power of mind to be a force akin to gravity and electricity. In other words,for Newton, gravity, electricity, and the volitional power of mind are all determinates of the same determinable. Put theologically, the free movement of our bodies by thoughtalone is a dim and perhaps pale reflection of that power by which an omnipresent God,one who is literally co-extensive with everything that exists, creates bodies   ex nihilo  andframes their lawful movement and interaction. However, leaving it to future experimenta-tion to determine whether or not a material or immaterial medium is required to explain

    47

    As Cohen notes, Newton had crossed this passage out. Newton’s reasons for deletion are unclear, but, in anycase, we once again see Newton’s tendency to relate the source of animal motions to the other forces in nature.48 It is worth noting that in a draft conclusion to the  Principia   Newton clearly distinguishes electricity from

    gravity given differences in the phenomena they exhibit. On the other hand, they are both similar in importantrespects. On this point, see  Cohen (1999), pp. 287–292.

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    them, Newton remained agnostic concerning the ultimate nature of forces, including theultimate nature of mental causation.

    Acknowledgements

    I would like to thank the Social Sciences and Humanities Research Council of Canadafor funding the postdoctoral research that led to this paper. I am especially grateful to Ste-phen D. Snobelen for his help with this research. I would also like to thank Tom Vinci andWilliam Harper for their advice and encouragement, as well as the participants of the Dal-housie Department of Philosophy’s colloquium series, and the referees at this journal, fortheir critical feedback. Finally, I gratefully acknowledge the Jewish National and Univer-sity Library for permission to quote directly from manuscripts in their archives.

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