SOURCE: "Force, Electricity, and the Powers of Living Matter in Newton's Mature Philosophy of Nature," in Religion, Science, and Worldview: Essays in Honor of Richard S. Westfall, edited by Margaret J. Osler and Paul Lawrence Farber, Cambridge University Press, 1985, pp. 95-117.

One of Newton's most widely quoted methodological pronouncements appears in the preface he prepared for the first (1687) edition of his Principia. "The whole burden of philosophy seems to consist in this," Newton there wrote: "from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena."¹ He made the same point in his other great work, his Opticks, towards the end of the long, final Query that he added to the Latin edition published in 1706:

To tell us that every Species of Things is endow'd with an occult specifick Quality by which it acts and produces manifest Effects, is to tell us nothing: But to derive two or three general Principles of Motion from Phaenomena, and afterwards to tell us how the Properties and Actions of all corporeal Things follow from those manifest Principles, would be a very great step in Philosophy, though the Causes of those Principles were not yet discover'd.²

The procedure thus outlined has been seen as encapsulating the very essence of the "Newtonian revolution" in science. Though Newton's world was composed, like that of mechanical philosophers such as Descartes and Gassendi, of material corpuscles in motion, no longer was Newton's science constrained, as the earlier mechanical philosophy had been, to reducing all natural phenomena to empirically inaccessible, and hence merely imagined, motions and impacts of particles. On the contrary, as the passages just quoted reveal. Newton admitted a further explanatory principle into his science, namely force. For Newton, armed as he was with a clearly understood principle of inertia, force became that which changed (or tended to change) the motions of bodies; individual forces came to be measured in terms of the changes of motion they produced; and the chief object of science became the discovery of the various forces acting in the world.³ As Newton himself demonstrated so convincingly in Principia with respect to gravity, such forces were at least in some cases empirically determinable in a way that corpuscular mechanisms were not. Elsewhere in Principia, and even more so in Opticks, Newton held out the hope that many other kinds of force might eventually become equally as well characterized as gravity now was.

At least as important, in the eyes of many Newtonian scholars, is the fact that, in Newton's hands, "force" became a quantifiable concept. His approach thus held out the prospect of a truly mathematical physics in which various natural effects would be shown to follow in a rigorously demonstrative and quantitatively exact manner from mathematically expressed force laws. This is, of course, what Newton himself achieved in Principia in relation to gravity. Though success on a similar scale in other areas of physical inquiry proved elusive, Newton, by focusing on the concept of force, had dramatically enlarged man's expectations regarding the degree of precision possible in science.

On this much, historians of science are generally agreed. In other respects, however, and especially on the question of what Newton's real attitude was to the various forces he invoked, opinion is divided. Were passages such as those quoted above merely methodological in import, or did they carry with them, as well, an ontological commitment on Newton's part? In other words, when Newton so ostentatiously refrained from offering mechanical explanations of the kind demanded by his critics for the forces of which he wrote, did he in fact do so not only because he believed that such explanations were unnecessary within the context of his inquiry, but also because he believed that these forces had a real existence of their own as true actions at a distance, independent of all mechanical explanation?

Westfall is one of those who has argued most strongly for the latter view. More than this, he has offered a reconstruction of the development of Newton's thought from which such an attitude on Newton's part toward his forces emerges as a natural consequence.⁴

Newton, Westfall has shown, was in his early years profoundly influenced in his thinking about the physical world by the ideas of the seventeenth-century mechanical philosophers—of men such as Descartes, Gassendi, Gassendi's English mouthpiece Walter Charleton, and Robert Boyle.⁵ In line with this, we find Newton as an undergraduate embracing (though with a hint of occasional reservations perhaps inspired by Henry More) the fundamental credo of the mechanical philosophy, the reducibility of natural events to the motions and impacts of particles of matter. Similarly, a decade and more later, we find him trying to account for a wide range of natural phenomena in typically mechanistic style in terms of interactions between particles of matter and an all-encompassing material aether (or aethers).⁶

It is now, however, generally recognized that by the early 1680s, Newton had abandoned his former belief in a universal dense aether and had begun to speak instead in what we now see as characteristically "Newtonian" style of forces acting between particles at a distance.⁷ Westfall claims to have found an explanation for this change of outlook on Newton's part in the alchemical investigations that engaged so much of Newton's attention at this period. In alchemy, Westfall has argued, Newton found an idea "that refused to be reconciled with the mechanical philosophy. Where that philosophy insisted on the inertness of matter, such that mechanical necessity alone determines its motion, alchemy asserted the existence of active principles in matter as the primary agents of natural phenomena."⁸ Westfall finds hints of the idea even in the generally mechanistic "Hypothesis explaining the Properties of Light" of 1675, for example in Newton's willingness to ascribe the immiscibility of oil and water to "some secret principle of unsociablenes" between them. However, he dates the full-blown transformation in Newton's thinking to a few years later when, armed with this insight drawn from alchemy, Newton turned anew to the analysis of the planetary motions:

As it appears to me, Newton's philosophy of nature underwent a profound conversion in 1679-80 under the combined influence of alchemy and the cosmic problem of orbital mechanics, two unlikely partners which made common cause on the issue of action at a distance…. Henceforth, the ultimate agent of nature would be for him a force acting between particles rather than a moving particle itself.⁹

Just as widely recognized among Newtonian scholars as the change in Newton's approach in the early 1680s is the fact that, beginning in about 1707 (that is, shortly after the publication of the Latin Optice) Newton once more began actively to consider possible causes for many of the forces he had invoked. In particular, Newton, fascinated, it seems, by various electrical experiments devised by Francis Hauksbee, began to explore the notion that many natural powers were manifestations of the activity of a subtle electric spirit that pervaded gross bodies. Hints of these speculations appear in the final paragraph of the "General Scholium" that Newton appended to the second (1713) edition of Principia, and in certain passages in the Queries published at the end of the 1717 edition of Opticks. They survive in more ample form in extensive unpublished drafts intended for these editions of Newton's two great books. By 1717, however, Newton's thinking had evolved still further, and he now attributed many of the phenomena previously accounted for in terms of the electric spirit to a new form of universal aether, the activity of which might even, he suggested, be sufficient to explain the force of gravity.¹⁰

Historians such as Westfall and McGuire, who have argued strongly for an ontological rather than a merely methodological commitment to the doctrine of forces on Newton's part in the years after 1680, have found these later vacillations something of an embarrassment. McGuire ignores the many indications that Newton provides that he sees the electric spirit as a material agency, and treats it instead as a generalized force, "an electrical arche connecting mind with matter"; and he dismisses the 1717 aether on the ground that it does not conform to his general view of Newton's philosophy of nature:

it is difficult to suppose that Newton took it seriously. It was subject to obvious conceptual inconsistencies; it was a flagrant example of the sort of intermediate entity which Newton had always tended to reject; and more significantly it repudiated his basic metaphysics of God in an empty universe.¹¹

Westfall, too, treats these later views of Newton's as aberrations, arising, he suggests, from a "growing philosophical caution" that "reflected at once the impact of Hauksbee's electrical experiments on Newton and perhaps also the effect of unrelenting criticism by mechanical philosophers on an aging man no longer able to sustain a revolutionary position in the face of general opposition."¹² He sees Newton's invocation of an electric spirit as a retreat toward a mode of explanation "which at least appeared acceptable to conventional mechanical philosophers," and concludes that "there is no satisfactory explanation of his return to such fluids…. It is hard to imagine Newton acquiescing in such a retreat at an earlier, more vigorous age."¹³ Consistent with this view, the preparation of the new edition of Opticks, with its famous Queries about the aether, is allotted a scant two pages in Westfall's 874-page biography, and these are consigned to a chapter entitled "Years of Decline."¹⁴

To be sure, there are aspects of the "aether" Queries of the 1717 Opticks that give the historian pause. Years before, in composing the concluding sections of Book II of Principia, Newton had marshaled overwhelming arguments against the possibility of any dense Cartesian-style aether filling all space. The motions of the planets in accordance with Kepler's laws were, he showed, inconsistent with the existence of such a medium. Hence the new aether of 1717 was, of necessity, of a very different character. It was both exceedingly rare, in order that it might offer no perceptible resistance to the motions of the planets through it, and exceedingly elastic, in order that it might transmit vibrations with the speed required if it were to fulfill the role in the theory of light that Newton wished to ascribe to it. Newton also supposed, however, that it was composed of particles. It followed inevitably, on Newtonian principles, that those particles exerted powerful forces of repulsion upon each other.¹⁵ Hence, even if the 1717 aether offered a satisfactory basis for explaining gravity (and there is some evidence that Newton remained less than fully convinced on this score),¹⁶ it by no means met the fundamental objection leveled by mechanical philosophers against Newton's physics: its reliance on unexplained forces acting at a distance. All the evidence we have suggests that Newton's "aether" Queries do indeed represent a defensive response of an old man to sustained criticism.

There are even indications that Newton subsequently repented their publication. Though historians have often failed to notice the fact, the new Queries were strictly inconsistent with the final paragraph of the concluding General Scholium of the 1713 Principia, because various phenomena—the reflection, refraction, inflection, and heating effects of light, and the transmission of sensations through the nerves to the brain and commands of the will from the brain to the muscles—which were there attributed to the action of "a certain most subtle spirit which pervades and lies hid in all gross bodies," were in the 1717 Queries ascribed to a universally disseminated aether. This latter was no mere transmogrification of the earlier subtle spirit, but was a second and quite distinct subtle matter. Even after reintroducing the aether, Newton continued to believe, as well, in the existence of the subtle spirit associated with matter, and to ascribe certain natural effects such as electricity to it;¹⁷ but now he transferred some of the other actions previously ascribed to the subtle matter to the aether. At the very least, therefore, some modification of the General Scholium was called for. In fact, Newton envisaged deleting the entire final paragraph dealing with the subtle spirit from the next edition of Principia.¹⁸ When that edition finally saw the light of day, however, in 1726, he did not do so; instead, the subtle matter continued to be invoked to explain the same effects as in 1713. This suggests that, at the very end of his life, Newton may have changed his mind yet again about the aether, and reverted to his previous mode of explanation of the various optical and physiological effects in terms of a subtle spirit associated with bodies.

If we may thus accept Westfall's judgment as to the relative insignificance of the 1717 aether for our general understanding of Newton's thinking about forces, the situation is, I believe, very different with respect to his writings on the subtle spirit. In the various published hints and unpublished drafts on this subject, we surely have before us no last-minute concessions by a declining 74-year-old, as the aether Queries appear to be, but the fruit of at least ten years of speculation and inquiry by a Newton still capable of prodigious feats of intellectual activity. Although much of what he had to say on the matter was consistent with views he had held at an earlier period of his career, these were now given a new lease of life by Hauksbee's remarkable experiments. There is ample evidence of the renewed interest and enthusiasm with which Newton pursued the work.¹⁹ Furthermore, this interest pre-dates the main Continental onslaught on his doctrine of forces, and so cannot be dismissed as a mere defensive response to that criticism. Rather, it should be seen as yet another constructive phase—perhaps, indeed, the last major constructive phase—in his thinking about matter and its powers. If this be accepted, it then becomes of considerable interest to discover precisely what views Newton arrived at concerning the subtle spirit and, more particularly, what implications these views might have for our broader understanding of his philosophy of nature.

I have argued elsewhere that, even at this late period of his life, Newton's ideas regarding the various modes of action of the subtle spirit depended much less than has generally been allowed on his invoking unexplained forces acting at a distance. I have suggested, indeed, that in many respects his ideas concerning the subtle spirit even then more closely resembled those of that arch-mechanist, Descartes, than they did those of a programmatic "Newtonian," systematically reducing each class of natural phenomena, in turn, to the action of some force or other.

Consider, first of all, the case of electricity, which lies at the heart of Newton's discussions of the subtle spirit. Newton sets out his ideas on this subject at length in a number of surviving documents. One of these is a long, unnumbered draft Query that predates the introduction of the aether hypothesis of 1717. "Do not all bodies abound," Newton here asks rhetorically, "with a very subtil active vibrating spirit by w^ch … the small particles of bodies cohære when continguous, agitate one another at small distances & regulate almost all their motions amongst themselves as the great bodies of the Universe regulate theirs by the power of gravity?" In particular, Newton believes that diffusion of this subtle matter into the space surrounding a rubbed body such as glass is what brings about electrical attraction, "for electric bodies could not act at a distance without a spirit reaching to that distance." Hauksbee's experiments provide, according to Newton, ample evidence that this diffusion occurs:

[B]y several experiments shewn by M^r Hawsksby before y^e R. Society it appears that a cylindrical rod of glass or hard wax strongly rubbed emitts an electric spirit or vapour w^ch pushes against the hand or face so as to be felt, & upon application of the finger to y^e electric body crackles & flashes, & that the electric spirit reaches to y^e distances of half a foot or a foot from the glass or above …; & that if a globe of glass be nimbly turned round upon an axis & in turning rub upon a man's hand to excite its electric virtue, … the glass emitts an electric vapour or spirit w^ch may be felt by the hand & w^ch in dashing upon the hand or upon white paper or a handkerchief at the distance of a quarter of an inch or half an inch from the glass or above, illuminates the hand or paper or handkerchief with a white light while the glass continues in motion, the spirit by striking upon those bodies being agitated so as to emit the light…. There is therefore an electric spirit by w^ch bodies are in some cases attracted in others repelled & this spirit is so subtile as to pervade & pass through the solid body of glass very freely …, & is capable of contraction & dilatation expanding itself to great distances from the electric body by friction. & thefore [sic] is elastic & susceptible of a vibrating motion like that of air whereby sounds are propagated. & this motion is exceeding quick so that the electric spirit can thereby emit light.²⁰

The introduction of the aether hypothesis did not in any way affect Newton's thinking about what lay behind the various phenomena of electricity. This we discover from a passage which Newton at one point numbered "Qu. 18B," intending it to be inserted between two of the early "aether" Queries of the new edition of Opticks:

Do not electric bodies by friction emit a subtile exhalation or spirit by which they perform their attractions? And is not this spirit of a very active nature & capable of emitting light by its agitations? And may not all bodies abound with such a spirit & shine by the agitations of this spirit within them when sufficiently heated? ffor if a long cylindrical piece of Ambar be rubbed nimbly it will shine in the dark & if when it is well rubbed the finger of a man be held neare it so as almost to touch it, the electric spirit will rush out of the Ambar with a soft crackling noise like that of green leaves of trees thrown into a fire, & in rushing out it will also push against the finger so as to be felt like the ends of hairs of a fine brush touching the finger. And the like happens in glass. If a long hollow tube of flint glass about a inch be rubbed nimbly with a paper held in the hand …, the electric spirit which is excited by the friction will rush out of the glass with a cracking noise & push against the skin so as to be felt, & in pushing emit light so as to make the skin shine like rotten wood or a glow worm. And if the glass was held neare pieces of leaf brass scattered upon a table the electric spirit w^ch issued out of the glass would stir them at the distance of 6, 8 or 10 inches or a foot, & put them into various brisk motions, making them sometimes leap towards the glass & stick to it, sometimes leap from it with great force, sometimes move towards it & from it several times with reciprocal motion, sometimes move in lines parall [sic] to the tube, sometimes remain suspended in the air, & sometimes move in various curve lines. Which motions shew that this spirit is agitated in various manners like a wind.²¹

The thoroughly mechanistic flavor of these passages is reinforced by yet another draft Query, numbered "Qu. 23" and apparently also intended to accompany the "aether" Queries of the published edition. Whereas in the passages quoted above, Newton focuses chiefly on the new electrical phenomena discovered by Hauksbee, he here discusses, in detail, the cause of the electrical forces of attraction and repulsion:

Qu. 23. Is not electrical attraction and repuls; performed by an exhalation which is raised out of the electrick body by friction expanded to great distances & variously agitated like a turbulent wind, & w^ch carries light bodies along with it & agitates them in various manners according to its own motions, making them go sometimes towards the electric body, sometimes from it & sometimes move with various other motions? And when this spirit looses its turbulent motions & begins to be recondensed & by condensation to return into the electrick body doth it not carry light bodies along with it towards the Electrick body & cause them to stick to it without further motion till they drop off?²²

The import of passages such as these is, in my view, unmistakable. Newton does not regard electricity as a true action at a distance. On the contrary, the various electrical effects are, in his view, brought about in a straightforwardly mechanical way following the agitation, by friction, of subtle spirit residing in the pores of electrifiable bodies such as amber and glass. (As the argument proceeds, Newton in fact concludes that the spirit resides in the pores of all bodies whatsoever.) Set in motion by the friction "like a turbulent wind," the subtle spirit simply sweeps along any light objects it encounters until, as it slows, it recondenses and carries such objects with it towards the electrified body. If it strikes against larger bodies, it may be set vibrating, and it then emits light in just the same way as a vibrating column of air generates sound. This spirit is no generalized immaterial force, but a material agency. To be sure, Newton usually referred to it as a "spirit," but on occasion he called it a "vapour" or "exhalation," terms which in Newton's vocabulary certainly implied materiality. Likewise, the word "spirit" could be used in this way, as he himself made clear in a draft intended for Principia:

Vapours and exhalations on account of their rarity lose almost all perceptible resistance, and in the common acceptance often lose even the name of bodies and are called spirits. And yet they can be called bodies in so far as they are the effluvia of bodies and have a resistance proportional to density.²³

As I have argued elsewhere, Newton's ideas about electricity, as set out in passages such as those quoted above, were in almost every respect typical of his day. The notion that electrical effects were caused by subtle matter, originally contained in the rubbed body, being excited by the friction, spreading out as effluvia into the surrounding air and sweeping along any light objects lying in the way, had been widely held at least since the time of William Gilbert. It continued to be generally espoused until the 1750s, when it began to be displaced by Benjamin Franklin's notion that the phenomena in question were brought about by static accumulations or "charges" of subtle electric fluid transferred by friction from one body to another.²⁴ In Newton's day, the main point of contention was not the correctness of the effluvial picture in general terms but the nature of the subtle matter or matters involved. In particular, whereas some maintained that the effluvia were composed of the matter of the rubbed body itself ("the finer parts of the attrahent," as Boyle put it),²⁵ others, including both Descartes and Newton, held that the stuff involved was a distinct subtle matter—whether the Cartesian "first element" or Newton's "electric and elastic spirit"—common to all bodies. Others again, most notably Niccolò Cabeo and his Jesuit followers, and also Hauksbee, thought that the air, too, was involved in bringing about the attraction.²⁶

Magnetism was a power that Newton often cited, together with electricity, as an example of a force known to act in the world. He saw it, however, as a power that was limited in its operation, being confined, so he thought, to iron and some of its ores. Accordingly, only rarely did he mention it in connection with the electric spirit; in particular, magnetism is strikingly absent from the list of effects ascribed in the 1713 General Scholium to the action of this spirit, or in the 1717 "aether" Queries to the action of the aether.²⁷ For Newton, magnetism seems to have been a separate power, subject to its own laws. Here too, however, such evidence as we have suggests that Newton did not see the magnetic force as a true action at a distance. On the contrary, it appears that, as in the case of electricity, he assumed the existence of an underlying mechanism of unmistakably Cartesian provenance.

Descartes' invention of a mechanism that could account for the archetypically occult power of the magnet had been one of the major early triumphs of the mechanical philosophy. According to his scheme, associated with any magnet were circulating streams of subtle material effluvia of a distinctive kind. These streams passed axially through the magnet, emerging from one pole and returning to the other through the surrounding air before resuming their course. The various known magnetic effects were then explained in terms of interactions between such streams of matter, or between a single stream and pieces of iron that came in its way.

This general picture quickly won wide acceptance and, indeed, remained predominant for a century and more, even though various details propounded by Descartes were challenged and in some cases largely rejected.²⁸ Newton's early adherence to at least the broad outlines of the theory is clearly displayed in an unpublished manuscript dating from 1666 or 1667, in which he actually sketches the likely patterns of flow of the subtle matter in various circumstances.²⁹ In another paper, which Westfall dates to the same period, Newton discusses magnetism at greater length than anywhere else. Throughout, he assumes the existence of peculiar "streams" in the vicinity of a magnet; indeed, he assumes (though in terms somewhat different from those used by Descartes) two separate and "unsociable" streams entering a magnet at its two opposite poles and passing through it in opposite directions.³⁰

It is tempting to see these papers as characteristic of Newton's youthful flirtation with the mechanical philosophy, and to suppose that he would later have abandoned such unrestrained hypothesizing, as he did in relation to the cause of gravity. The evidence, however, suggests otherwise, for although it is nowhere specific enough to show that Newton remained faithful to the details of his early explanatory scheme, it does unmistakably reveal that even after he had abandoned the search for a mechanical explanation for gravity, he continued to believe that magnetic effects were brought about by the action of subtle effluvia of some kind. Even in the 1690s, at the height of Newton's disillusionment with mechanism, David Gregory recorded, while visiting him, his host's view that the magnetic virtue "seems to be produced by mechanical means."³¹ A casual allusion to "magnetick Effluvia" in the first edition of Opticks, published ten years later, tells the same story.³² Finally, from the period with which we are chiefly concerned in this chapter, in one of the "aether" Queries of 1717, Newton uses the existence and activity of the magnetic effluvia, which he again takes as beyond dispute, to justify by analogy assumptions he is making about the nature of the aether: "If any one would ask how a Medium can be so rare," he suggests, "let him tell me … how the Effluvia of a Magnet can be so rare and subtile, as to pass through a Plate of Glass without any Resistance or Diminution of their Force, and yet so potent as to turn a magnetick Needle beyond the Glass."³³

Newton frequently cited electricity and magnetism as other instances, besides gravity, of forces known to act in the world, to support the suggestion that there were many other forces at work as well. Whenever he did so, however, he surrounded the argument with disclaimers, of which the one near the beginning of Query 31 is the most famous:

How these Attractions may be perform'd, I do not here consider. What I call Attraction may be perform'd by impulse, or by some other means unknown to me. I use that Word here to signify only in general any Force by which Bodies attract one another, and what are the Laws and Properties of the Attraction, before we enquire the Cause by which the Attraction is perform'd.³⁴

Historians have tended to place little weight on such remarks, and to dismiss them as little more than symptoms of Newton's habitual caution in expressing his views publicly. In the light of the evidence just presented, however, they take on a new significance. Whatever his attitude toward gravity might have been, Newton, we discover, did not at all suppose that either electricity or magnetism was an irredeemably nonmechanical power. On the contrary, he thought both that there was an underlying mechanism in each case, and that he knew what this was. The disclaimer in the passage just quoted must be taken seriously; Newton believes that some of the attractions of which he speaks are "perform'd by impulse." In other words, what is being proposed here is not a general philosophy of nature in which forces have ontological primacy, but rather a methodology only.

According to this methodology, in studying magnetism, for example, one ought above all to try, as Newton did,³⁵ to discover the mathematical law governing the magnetic force. Such a methodological priority is, however, in no way inconsistent with one's simultaneously having views about the existence and nature of a mechanism that might bring about the attraction in accordance with that law. Nor, for that matter, is having views about the existence of such a mechanism inconsistent with allowing that this may perhaps itself ultimately be found to rest upon the behavior of a fluid the particles of which exert unexplained forces on each other at a distance. Whether that be so, or whether they act on each other only by contact, remains undetermined, in the case of magnetism, at the point reached by Newton's inquiry. In the case of electricity, on the other hand, he seems rather closer to the view that the particles of the subtle matter that provides the mechanism do indeed exert forces on each other at a distance.³⁶

Conclusions such as these are further reinforced when we turn to what Newton had to say on another subject, of which the views he expressed have yet to attract systematic study: namely, the powers of living matter. Here, too, Newton's slowness to ascribe an ontological significance to the forces he discovers at work in the world becomes apparent. Once again, he appears much more inclined to assume fairly orthodox mechanistic explanations than one might have expected of a committed "Newtonian" bent on reducing natural events to the actions of independently existing forces. This is not to say, however, that Newton is here bowing to the conceptual constraints imposed by the mechanical philosophy. On the contrary, after reducing a variety of life processes to mechanisms of one kind or another, he eventually concludes that there is associated with living matter a peculiar power for which he offers no hint of an explanation, a power which appears to lie behind the various mechanisms he has described, but which itself appears to be intrinsically nonmechanical.

Newton's best-known statements about living processes comprise, first, anatomical descriptions of the optic system that offer no additional insights into his philosophy of matter³⁷ and, second, various comments about the operation of the nerves in transmitting sensations from the "organs of sense" to the brain and commands of the will from the brain to the muscles. These latter comments appear in writings from all periods of his career, the best known, however, being his late-in-life remarks in the final paragraph of the 1713 General Scholium and in the last two of the "aether" Queries of 1717 (Queries 23 and 24). In 1713, sensation and the commands of the will were held to be transmitted by vibrations of the subtle electric and elastic spirit "mutually propagated along the solid filaments of the nerves, from the outward organs of sense to the brain, and from the brain into the muscles." By 1717, the vibrations were held to occur not in the subtle spirit but in the newly introduced "Æthereal Medium": In all other respects, however, the account remained as before.³⁸ In both cases, the explanations were conceived in wholly mechanistic terms, and bore obvious affinities to the opinion, commonly held at the time, that the nerves functioned as conduits for "animal spirits" flowing between the brain and the various sensory and motor parts of the body.³⁹ Insofar as they offered scope for characteristically "Newtonian" interpretations, they did so only by admitting the possibility, noted earlier, that the fluid in question might derive the elasticity requisite to sustain the vibrations to which Newton referred, from a repulsive force operating between its particles.⁴⁰

Much more interesting from the point of view of this paper are, first, a manuscript entitled "De vita et morte vegetabile" and, second, sections of some associated draft Queries, prepared by Newton for a new edition of Opticks at about the time he composed the concluding General Scholium for the second edition of Principia, but never in fact published. In these papers, Newton confronts directly, albeit briefly, the nature of the distinction between living and nonliving matter. In doing so, he reveals yet again the extent to which his physical ideas were in tune with those held more generally in his day. At the same time, the discussion sheds new light on the place of unexplained forces in his mature philosophy of nature.

The passages in question are, in fact, linked with those discussed earlier in which Newton set out at length his ideas on electricity; indeed, the most important of them occur in the very same draft of the Queries from which most of the passages quoted above concerning electricity were drawn. Here as nowhere else in Newton's papers we see revealed the full extent to which his powerful imagination had been captivated by Hauksbee's astonishing experiments. Both the traditional electrical attraction of light objects and the striking new effects studied by Hauksbee arose, Newton had concluded, from the agitation of a very active subtle spirit associated with the particles of bodies. The role of friction in bringing about the phenomena was not, he thought, to generate the "virtue," but only to expand it, the subtle spirit under normal circumstances "reach[ing] not to any sensible distance from the particles."⁴¹ Now, however, he goes on to consider the possibility that there may be other causes, too, by which "the electric vertue is invigorated." One such cause, he says, may be the power of life:

The vegetable life may also consist in the power of this spirit supposing that this power in substances w^ch have a vegetable life is stronger then in others & reaches to a greater distance from the particles, ffor as the electric vertue is invigorated by friction so it may be by some other causes. And by being stronger in the particles of living substances then in others it may preserve them from corruption & act upon the nourishment to make it of like form & vertue w^th the living particles as a magnet turns iron to a magnet & fire turns its nourishment to fire & leaven turns past to leaven.⁴²

As Newton himself makes plain, he is concerned here not with all properties of living things but only with those that had, since ancient times, been technically distinguished as constituting the vegetative aspects of life. These amounted to the ability of living matter, plant or animal, to maintain its organized form and to organize other matter likewise, whether in the processes of nutrition and growth, whereby existing tissues or organs are renewed or enlarged through the accretion of new matter, or in reproduction, where new living individuals are generated. In these passages, Newton does not discuss the additional powers that distinguish animate from mere vegetative life, or rational animals (i.e., human kind) from the beasts. For Newton, the subtle spirit, the agitation of which through friction gives rise to the phenomenon of electricity, also sustains the organic character of living matter by activity brought about in another way. When, at death, that activity ceases, putrefaction sets in; that is, the organized structures that characterized the matter in question during life now fall into decay.

In order to uphold this general position, Newton in these documents sets out at some length his general views on nutrition, growth, and generation, and contrasts these essentially constructive processes with the destruction and dissolution that constitute putrefaction. The ideas he displays are, in fact, very typical ones for his day.

So far as the reproduction of living forms is concerned, Newton reveals himself as an "ovist," that is, an upholder of that version of the preformation theory that held that the primordium or embryo of the young was present, already formed, in the female egg, and that the role of the male parent was merely to provide an indirect stimulus for its growth.⁴³ On this view, the process of generation reduces to one of growth of the embryo from suitable nutriment and, at an appropriate stage, a budding off of the embryo from the mother's body. Indeed, Newton draws the analogy with budding explicitly:

Generation is nothing else then separating a branch from the tree and giving it better nourishment. If a separated branch takes root in the earth or a separated twigg or bud by grafting or inoculation is nourished from y^e root of a young stock, it grows into a new tree as big as the tree from which it was separated being better nourished from a young root than from an old one. The seed of a tree has the nature of a branch or twig or bud. While it grows upon y^e tree it is a part of the tree: but if separated and set in the earth to be better nourished, the embryo or young tree contained in it takes root & grows into a new tree. [In] like manner the egg of a female w^lh the embryo formed inside while it grows in the ovarium is a branch of y^e mothers body & partakes of her life, yet the Embryo is as capable of being separated from the mother & growing great by due nourishment as a branch or twigg or bud or seed of a tree is of being separated from the tree & growing into a new tree.⁴⁴

Newton's adherence to the preformation theory, and to the ovist version in particular, is made plain by his reference to "the egg of a female w^th the embryo formed inside," growing in the ovarium prior to the intervention of the male parent. This theory was widely held during the last decades of the seventeenth century, though it was being challenged at the end of the period by the animalculist alternative according to which the preformed individual derived not from the female but from the male parent, being present in the spermatozoa newly discovered in the male semen by Leeuwenhoek. Newton does not mention the spermatozoa, which suggests that he adhered to the fairly common opinion that, far from being the essential part of the semen, they were in fact parasites. For him, the male semen played an entirely different role, namely the essentially chemical one of reacting with the female juices to produce the nourishment required by the embryo if it were to grow:

by the act of generation nothing more is done then to ferment the sperm of the female by y^e sperm of y^e male that it may thereby become fit nourishment for y^e Embryo, ffor y^e nourishment of all animals is prepared by ferment & the ferment is taken from animals of the same kind, & makes the nourishment subtile & spiritual. In adult animals the nourishm' is fermented by the choler and pancreatic juice both w^ch come from the blood. The Embryo not being able to ferment its own nourishment w^ch comes from the mothers blood has it fermented by the sperm w^ch comes from y^e fathers blood, & by this nourishment it swells, drops off from y^e Ovarium & begins to grow with a life distinct from that of y^e mother.⁴⁵

According to this line of thinking, nutrition and growth become the fundamental biological phenomena. Nutrition, in turn, was for Newton atwo-stage affair. First, a fermentation occurs, brought about by the digestive juices or, in the act of generation, by the male semen. This chemical process breaks down the nutriment into minute, extremely subtle parts:

Now in all fermentation w^ch generates spirits, the ferment abounds w^th a supprest acid w^ch being more attracted by the other body forsakes its own to rush upon & dissolve y^e other & by the violence of the action breaks both its own particles & the particles of y^e other body into smaller particles & these by their great subtilty volatility & continual digestion resolve y^e whole mass into as subtile parts as it can be resolved by putrefaction.

Then, these separated parts reassemble under the organizing influence of the body being nourished:

And when the nourishment is thus prepared by dissolution & subtillation, the particles of the body to be nourished draw to themselves out of the nourishment the particles of the same density & nature w"' themselves, ffor particles of one & the same nature draw one another more strongly then particles of different natures do.⁴⁶

Newton alludes at this point to the formation of mineral deposits in the earth as a known instance in which the "like attracts like" principle operates in nature. Yet, such a principle remains insufficient for his purposes because it does not explain that assembling of the new materials into appropriate structures, which manifestly occurs in the nourishment and growth of living creatures. Newton argues that the growth of crystals provides a model here:

And when many particles of the same kind are drawn together out of y^e nourishment they will be apt to coalesce in such textures as the particles w^ch drew them did before because they are of the same nature as we see in the particles of salts w^ch if they be of the same kind always crystallize in the same figures.

The analogy is one that was commonly drawn upon by mechanical philosophers. Newton, however, is intellectually rigorous enough to recognize that it remains but an analogy, and does not in any sense constitute an explanation. It is precisely at this point, in order to provide an explanation for the remarkable power that living forms possess of organizing new matter into existing structures, that he invokes the force of electricity:

And for faciliating [sic] this assimilation of y^e nourishment & preserving the nourished bodies from corruption it may be presumed that as electric attraction is excited by friction so it may be invigorated also by some other causes & particularly by some agitation caused in the electric spirit by the vegetable life of the particles of living substances: & the ceasing of this vigour upon death may be the reason why y^e death of animals is accompanied by putrefaction.⁴⁷

Throughout the different versions of this long draft Query, whenever Newton mentions electricity, he presents it as an effect produced by the agitation of a subtle spirit that pervades the pores of ordinary matter. By contrast, in the document "De vita et morte vegetabile," the electric spirit is not mentioned. Instead, Newton refers repeatedly to the "electrical force" associated with vegetating matter, without any further hint as to its ontological status. It is therefore tempting to date this paper to the period before Hauksbee's spectacular experiments rekindled Newton's belief in the existence of the subtle spirit and his faith in its explanatory possibilities. Other features of the paper seem firmly to link it, however, with the "subtle matter" Queries. Not only are there close parallels between the arguments presented in the two documents, "De vita et morte vegetabile" displays the same striking conviction—which must surely postdate Hauksbee's work—that we find in the "subtle matter" Queries, that electricity is a power universally associated with matter that lies behind many well-known effects not normally thought of as electrical. In this case, the document would stand as a suggestive instance of Newton confining his discussion to forces and offering no explanation for these, even though he actually had an elaborate explanation worked out at the time.

Whether or not this be allowed, however, these discussions by Newton of the vegetative power of living matter are revealing in other ways. In particular, as indicated earlier, they show that here, too, as with (or so I have argued) electricity and magnetism, Newton preferred to adopt a mechanistic (or at least pseudomechanistic) theory current in his day, the preformation theory, rather than invoking yet another category of unexplained forces to explain the effects in each case.

Newton, I would suggest, is in fact much more unwilling than has often been supposed to invoke a specific kind of in-principle-inexplicable force to account for each different category of natural event. Unexplained forces remain full of mystery for him; indeed, his conviction that the most famous of them, gravity, brought him close to the activity of God himself is well known,⁴⁸ and there is no reason to suppose that others would have had less profound implications.

Prima facie, then, one might expect that Newton would have recourse to entities of this kind only for the most universal of natural powers; and this does, indeed, seem to be the pattern that emerges. Neither the "amber effect"—the drawing of little light objects to rubbed amber or glass—nor the attraction of iron to a magnet, is an effect of sufficient universality; but a force of repulsion acting between the particles of an all-pervading subtle matter and ultimately responsible not just for the macroscopic electrical attraction but for the wide range of natural phenomena listed in the 1713 General Scholium, might be. Similarly, the various specific forces associated with living forms are unlikely to be the end point of the analysis, whereas a very general power of activating the subtle electric spirit even without friction might be what universally distinguishes living from nonliving matter. Something presumably does; and whatever it be, it is likely to be closely linked to the activity of God as the Creator of life and to be irreducibly nonmechanistic in character.

Meanwhile, Newton's scientific method leads him to invoke forces of all kinds in studying natural processes, and encourages him to focus on discovering the mathematical laws according to which those forces act rather than on explaining their action. In these cases, I believe, as I have indicated above, that we ought to take more seriously than has usually been done Newton's oft-expressed caveat about the causes lying behind the forces of which he spoke. Some of these forces might, he suggested, be "perform'd by impulse."⁴⁹ If the interpretation I am suggesting is correct, Newton would in fact have thought that a mechanism was involved in most instances. I have presented evidence, indeed, that he did think so in certain leading cases. He would also not have been surprised, however, if, as these mechanisms were investigated, they were found ultimately to depend upon a small number of irreducibly nonmechanical powers of quasi-universal application. At that point, he would have said—but only at that point—we are indeed approaching a knowledge of the first cause, which is God.

Notes

¹ Isaac Newton, Mathematical Principles of Natural Philosophy, trans. Andrew Motte, rev. Florian Cajori (Berkeley: University of California Press, 1934), pp. vii-viii.

² Newton, Opticks, or a Treatise of the Reflections, Refractions, Inflections and Colours of Light (New York: Dover, 1952), pp. 401-2.

³ The evolution of the concept of force during the seventeenth century is described in R. S. Westfall, Force in Newton's Physics: The Science of Dynamics in the Seventeenth Century (London: MacDonald, 1971).

⁴ Richard S. Westfall, Never at Rest: A Biography of Isaac Newton (Cambridge: Cambridge University Press, 1980), chaps. 7-9.

⁵ Richard S. Westfall, "The Foundations of Newton's Philosophy of Nature," British Journal for the History of Science, I (1962), 171-82; idem, Never at Rest, pp. 83-93. Cf. also A. R. Hall, "Sir Isaac Newton's Notebook, 1661-65," Cambridge Historical Journal, 9 (1948), 239-50; and J. E. McGuire and Martin Tamny, eds., Certain Philosophical Questions: Newton's Trin ity Notebook (Cambridge: Cambridge University Press, 1983).

⁶ Newton, "An Hypothesis explaining the Properties of Light," in H. W. Turnbull et al., eds., The Correspondence of Isaac Newton (Cambridge: Cambridge University Press, 1959-77), 1, 362-86; De Aere et Aethere," in A. R. and M. B. Hall, eds., Unpublished Scientific Papers of Isaac Newton (Cambridge: Cambridge University Press, 1962), pp. 214-20 (English trans., pp. 221-8); Newton to Robert Boyle, 28 February 1679, in Correspondence, II, 288-95.

⁷ Henry Guerlac, "Newton's Optical Aether: His Draft of a Proposed Addition to His Opticks, " Notes and Records of the Royal Society of London, 22 (1967), 45-57; J. E. McGuire, "Force, Active Principles, and Newton's Invisible Realm," Ambix, 15 (1968), 154-208.

⁸ Westfall, Never at Rest, p. 299.

⁹ Ibid., p. 390. Cf. also Westfall, Force in Newton's Physics, chap. 7. McGuire has likewise seen a definite ontological commitment in Newton's attitude towards his forces, arguing, indeed, that these become the chief components of the universe as Newton conceived it in the years immediately after the first publication of Principia: "Not only did Newton enrich his ontology by including forces, but in the nineties they became essential to his natural philosophy. By 1706, he seemed to consider them, rather than matter, to be the primordials of nature" (McGuire, "Force, Active Principles, and Newton's Invisible Realm," p. 161).

¹⁰ Newton, Opticks, Queries 17-24 (pp. 347-54). The "classic" papers on Newton's flirtation with the electric spirit are by Henry Guerlac: "Francis Hauksbee: expérimentateur au profit de Newton," Archives internationales d'histoire des sciences, 16 (1963), 113-28; "Sir Isaac and the Ingenious Mr. Hauksbee," in I. B. Cohen and R. Taton, eds., Mélanges Alexandre Koyré (Paris, 1964), I, 228-253; Guerlac, "Newton's Optical Aether" (note 7). See also Joan L. Hawes, "Newton and the 'Electrical Attraction Unexcited.'" Annuals of Science, 24 (1968), 121-130 and "Newton's Two Electricities," Annals of Science, 27 (1971), 95-103; and, more recently, R. W. Home, "Newton on Electricity and the Aether," in Z. Bechler, ed., Contemporary Newtonian Research (Dordrecht: D. Reidel, 1982), pp. 191-213.

¹¹ McGuire, "Force, Active Principles, and Newton's Invisible Realm" [see note 7], pp. 176, 187.

¹² Westfall, Never at Rest, pp. 644, 793.

¹³ Ibid., pp. 793, 747.

¹⁴ Ibid., pp. 792-4.

¹⁵ Newton, Opticks, p. 352.

¹⁶ J. T. Desaguliers to Sir Hans Sloane, 4 March 1730/31, quoted by Guerlac, "Newton's Optical Aether" [see note 7], p. 51.

¹⁷ Home, "Newton on Electricity and the Aether" [see note 10], passim.

¹⁸ A. Koyré and I. B. Cohen, eds., Isaac Newton's Philosophiae Naturalis Principia Mathematica: The Third Edition (1726) with Variant Readings (Cambridge: Cambridge University Press, 1972), II, 764.

¹⁹ Guerlac, "Francis Hauksbee: expérimentateur au profit de Newton" [see note 10], passim.

²⁰ University Library, Cambridge, Add. MS. 3970, fols. 241^v-241^r.

²¹ Ibid., fol. 295.

²² Ibid., fol. 293^v.

²³ University Library Cambridge, Add. MS. 3965. 13, fol. 437^V; quoted by J. E. McGuire, "Body and Void in Newton's De Mundi Systemate: Some New Sources," Archive for History of Exact Sciences, 3 (1966), 206-48; p. 219 (Latin original, p. 245).

²⁴ Home, "Newton on Electricity and the Aether" [see note 10], pp. 204-7. Cf. idem, The Effluvial Theory of Electricity (New York: Arno Press, 1981), and J. L. Heilbron, Electricity in the 17th and 18th Centuries: A Study of Early Modern Physics (Berkeley: University of California Press, 1979).

²⁵ Robert Boyle, Experiments and Notes about the Mechanical Origine or Production of Electricity (London, 1675; reprinted Oxford University Press, 1927), p. 6.

²⁶ On Hauksbee's ideas, see R. W. Home, "Francis Hauksbee's Theory of Electricity," Archive for History of Exact Sciences, 4 (1967), 203-17.

²⁷ On at least one occasion, however, Newton does inexplicably include magnetism in such a list (University Library Cambridge, Add. MS. 3970, fol. 241; quoted by Joan L. Hawes, "Newton's Two Electricities" [see note 10], p. 97). I find it difficult to understand this except as a slip of the pen.

²⁸ R. W. Home, introduction to Aepinus's Essay on the Theory of Electricity and Magnetism (Princeton, N.J.: Princeton University Press, 1979), chap. 4, "Magnetism."

²⁹ University Library Cambridge, Add. MS. 3974, fols. 1-3.

³⁰ University Library Cambridge, Add. MS. 3970, fols. 473-4. Cf. Westfall, Force in Newton's Physics [note 3], p. 332.

³¹ Newton, Correspondence, III, 335 (English trans., p. 338). The entry in Gregory's diary is dated 5-7 May 1694.

³² Newton, Opticks (1st ed., 1704), Book II, p. 69. Cf. reprint ed. (New York, 1952), p. 267.

³³ Newton, Opticks (reprint cd.), p. 353. For further discussion of Newtonian views on magnetism, see R. W. Home, "'Newtonianism' and the Theory of the Magnet," History of Science, 15 (1977), 252-66.

³⁴Opticks, p. 376.

³⁵ Newton, Mathematical Principles of Natural Philosophy, p. 414.

³⁶ Home, "Newton on Electricity and the Aether" [see note 10], p. 209.

³⁷Opticks, pp. 15-17. Cf. also three letters of Newton's to William Briggs, 1682-5, in his Correspondence, II, 377-8, 381-5, 417-9; and his "Description of the Optic Nerves and their Juncture in the Brain," published in D. Brewster, Memoirs of the Life, Writings and Discoveries of Sir Isaac Newton (Edinburgh, 1855), /, 432-6.

³⁸ Newton, Mathematical Principles of Natural Philosophy, p. 547; Opticks, pp. 353-4.

³⁹ Cf. René Descartes, Treatise of Man, trans. T. S. Hall (Cambridge, Mass.: Harvard University Press, 1972), pp. 21 ff.

⁴⁰ In Newton's earliest surviving discussion of the operation of the nerves, in the "Hypothesis explaining the Properties of Light" of 1675, he invokes the additional nonmechanical concepts of sociableness and unsociableness to explain how impulses could be confined to the nerves and how, once arrived at a muscle, they could cause this to contract (Newton, Correspondence, I, 368-9).

⁴¹ University Library Cambridge, Add. MS. 3970, fol. 235; quoted by Westfall, Force in Newton's Physics [see note 3], p. 394, and Home, "Newton on Electricity and the Aether" [see note 10], p. 197.

⁴² University Library Cambridge, Add. MS. 3970, fol. 241; quoted by Home, "Newton on Electricity and the Aether" [see note 10], p. 199. Folios 235 and 241 are the two halves of a single, folded sheet. A long unnumbered Query that begins on fol. 241^v was drafted first. This carries over to fol. 241^r and then, briefly, to fol. 235^V. At this stage, however, Newton evidently turned the sheet over and started again on fol. 235^r, writing out new Queries which he numbered 24 and 25, intending them to be inserted into the middle of what is now Q. 31 but in the Latin Optice of 1706 was Q. 23. A series of dashes in the proposed Q. 25 indicate that the long first paragraph of the unnumbered Query on fol. 241^v is to be taken in at this point. The manuscript headed "De vita et morte vegetabile" is now marked as fol. 237 in the same bundle, with drafts on fols. 238 and 240. It deals with the same subject matter but, as we shall see, in a rather different way. The first few paragraphs and the concluding sentences of this paper have been published by Westfall, Force in Newton's Physics (note 3), pp. 417-8.

⁴³ For a lively account of the preformation theory and its intellectual milieu, see Elizabeth Gasking, Investigations into Generation: 1651-1828 (London: Hutchinson, 1967), Chapters 2-5. Also see Jacques Roger, Les sciences de la vie dans la pensée française du XVIII^esiècle (Paris: Armand Colin, 1963).

⁴⁴ University Library Cambridge, Add. MS. 3970, fol. 235.

⁴⁵ Ibid.

⁴⁶ Ibid., fols. 235-235.^v

⁴⁷ Ibid., fol. 235.^v

⁴⁸ Westfall, Force in Newton's Physics [see note 3], pp. 395ff.

⁴⁹Opticks, p. 376.