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ABSTRACT
As well as the mathematically-supported celestial mechanics that Newton developed in his Principia, Newton also proposed a more speculative natural philosophy of interparticulate forces of attraction and repulsion. Although this speculative philosophy was not made public before the ‘Queries’ which Newton appended to the Opticks, it originated far earlier in Newton’s career. This article makes the case that Newton’s short, unfinished manuscript, entitled ‘De Aere et Aethere’, should be seen as an important landmark in Newton’s intellectual development, being the first work in which Newton assumed there are repulsive forces operating at a distance between the particles of bodies. The article offers an account of how Newton came to write ‘De Aere et Aethere’ and why. It also outlines its relationship to the ‘Conclusio’, with which Newton briefly intended to finish the Principia, and to the ‘Queries’ in the Opticks. The date of the manuscript is disputed, and the article also aims to settle this dispute. Claims that the ‘De Aere et Aethere’ must have been written before the ‘Hypothesis... of Light’ of 1675 are dismissed, and it is suggested, following R. S. Westfall, that it was written after a well-known letter Newton wrote to Boyle early in 1679.
Acknowledgements
I am extremely grateful to Dmitri Levitin, whose fair-minded and penetrating criticisms helped me to greatly improve upon earlier versions of this article.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Notes
1 Isaac Newton, The Principia: Mathematical Principles of Natural Philosophy, translated by I. B. Cohen and Anne Whitman (Berkeley: University of California Press, 1999). ‘The System of the World’ is the sub-title of the culminating third book of the Principia. Hereafter cited simply as Newton, Principia. See also I. B. Cohen, ‘A Guide to Newton’s Principia’, included in his and Whitman’s translation, pp. 1-370, at p. 11.
2 Isaac Newton, Opticks, or a Treatise of the Reflections, Refractions, Inflections, & Colours of Light (London: Royal Society, 1704); Optice … libri tres (London: Royal Society, 1706); Opticks (London: Royal Society, 1717). Except where necessary, I have used the Dover edition: (New York: Dover, 1979), hereafter cited as Newton, Opticks. There were 16 Queries appended to the first edition. Seven more were added in the Optice, Finally, a group of 8 more were interpolated after the original 16, in the second English edition.
3 Newton, Opticks, pp. 375-76.
4 Newton, Opticks, p. 339.
5 On eighteenth-century speculative Newtonianism two classic studies have still not been superseded: Robert E. Schofield, Mechanism and Materialism: British Natural Philosophy in an Age of Reason (Princeton: Princeton University Press, 1970); and Arnold Thackray, Atoms and Powers: An Essay on Newtonian Matter-Theory and the Development of Chemistry (Cambridge, Mass.: Harvard University Press, 1970).
6 Isaac Newton, ‘De Aere et Aethere’, in A. R. Hall and M. Boas Hall, Unpublished Scientific Papers of Isaac Newton (Cambridge: Cambridge University Press, 1962), pp. 223-24, original Latin, pp. 216-17. Hereafter cited simply as ‘De Aere’, with the page reference to the Halls’ edition. It is difficult to imagine how repulsive forces between particles could act other than at a distance. The repulsion between two particles would make them move away from one another. At that point we would either have to assume the particles would immediately cease to act on one another, or we have to assume the repelling force continues across the distance between the particles. Action at a distance, therefore, seems to be even more bound up with the assumption of repulsive force than it does with the assumption of attraction. Alexandre Koyré showed long ago that for early modern thinkers a force of ‘attraction’ implied action at a distance. See his, ‘Gravity an Essential Property of Matter?’ in Newtonian Studies (London: Chapman & Hall, 1965), pp. 149-69.
7 Newton, Principia, pp. 697-99.
8 Hall and Hall, Unpublished Scientific Papers of Isaac Newton, pp. 321-47; p. 345; original Latin, p. 331-32. Hereafter cited as ‘Conclusio’.
9 ‘Conclusio’, p. 345; Latin, p. 332.
10 Hall and Hall, Unpublished Scientific Papers of Isaac Newton, pp. 302–08 (‘Partial Draft of the Preface’).
11 Newton, Principia, pp. 382-83. This appeared in the first edition of 1687 and remained unchanged in subsequent editions. Newton, Philosophiae naturalis principia mathematica (London: Royal Society, 1687), sig. A3v.
12 Readers may note the overlap between a speculative physics based on interparticulate forces of attraction and repulsion, and the Principia, which establishes attractive forces between all bodies, including invisibly small ones. The existence of the ‘Conclusio’ indicates that Newton came to see it too. In spite of this obvious overlap, however, I wish to make it clear that my focus in this paper is Newton’s speculative physics of interparticulate forces, as most clearly seen in the ‘Queries’ appended to the Opticks, and only briefly considered as a possible addition to the Principia. I am not making any would-be revisionist claims about the historical development of the Principia. I fully endorse the histories of the writing of the Principia as told by John Herivel and others, up to George E. Smith, which is a story of mathematics, mechanics, and dynamics, not a story of speculative matter theory. Literature along these lines is extensive and well established; see, for example, John Herivel, The Background to Newton’s Principia: A Study of Newton’s Dynamical Researches in the Years 1664–84 (Oxford: Clarendon Press, 1965); and George E. Smith, ‘The Principia: From Conception to Publication’, in Eric Schliesser and Chris Smeenk (eds), Oxford Handbook of Isaac Newton (Oxford: Oxford University Press, 2019), pp. 1-30, published online: https://doi.org/10.1093/oxfordhb/9780199930418.001.0001
13 ‘De Aere’, p. 223; Latin, p. 217.
14 R. S. Westfall, Never at Rest: A Biography of Isaac Newton (Cambridge: Cambridge University Press, 1980), p. 374n. Westfall declines to defend his interpretation, preferring to let it ‘stand by itself’. This paper is an attempt to defend his interpretation, and his dating of ‘De Aere et Aethere’, to some time after the letter Newton wrote to Robert Boyle, dated February 28, 1678/79.
15 Hall and Hall, Unpublished Scientific Papers, pp. 187-89.
16 William R. Newman, Newton the Alchemist: Science, Enigma, and the Quest for Nature’s Secret Fire (Princeton: Princeton University Press, 2019); Dmitri Levitin, The Kingdom of Darkness: Bayle, Newton, and the Emancipation of the European Mind from Philosophy (Cambridge: Cambridge University Press, 2022). Both of these works are models of excellent scholarship and their importance is hardly affected by my criticisms of this small matter, which has no impact on the power of their main theses. My aim is not to expose their errors but to better understand Newton’s intellectual development.
17 Robert Boyle, New Experiments to Make Fire and Flame Stable and Ponderable (London: M. Pitt, 1673) in Boyle, Works, ed. M. Hunter and E. B. Davis (London: Pickering & Chatto, 1999), vol. vii, pp. 299-322. See Hall and Hall, pp. 187-88.
18 Letter from Newton to Boyle, February 28, 1678-9, in I. B. Cohen (ed.), Isaac Newton’s Papers and Letters in Natural Philosophy, (Cambridge: Cambridge University Press, 1958), p. 251. Hall and Hall, p. 188. These other two documents are published in Isaac Newton’s Papers and Letters in Natural Philosophy, pp. 178–99 (‘An Hypothesis explaining the Properties of Light’), and pp. 250–53 (Letter from Newton to Boyle, February 28, 1678-9). These are also available online, thanks to the Newton Project, at https://www.newtonproject.ox.ac.uk/view/texts/diplomatic/NATP00002 (‘Hypothesis’), and https://www.newtonproject.ox.ac.uk/view/texts/diplomatic/NATP00275 (Letter). Hereafter cited as ‘Hypothesis’, and Letter respectively, followed by page reference in the Newton Project and, secondly, the page reference in Cohen (ed.), Newton’s Papers and Letters.
19 Hall and Hall, p. 188.
20 Hall and Hall, p. 188.
21 Hall and Hall, pp. 188-89.
22 ‘Hypothesis’, p. 257, p. 187. The reference to the ‘black’ is to a dark centre spot which appears where the lenses should be at their closest (but, according to Newton, do not quite touch). See also, ‘Hypothesis’, p. 264, p. 194.
23 ‘De Aere’, p. 222, Latin, p. 215.
24 ‘De Aere’, p. 223; Latin, p. 216. There is more to say about this comment in ‘De Aere’, which was in fact crossed out. We shall return to it.
25 Letter, fol. 62r, p. 250.
26 ‘Hypothesis’, p. 252, p. 182.
27 Letter, fol. 62v, p. 251.
28 Letter, fol. 63r, p. 251.
29 The same goes for the other mention of ‘particles at a distance from one another’, Letter, fol. 64r, p. 253.
30 Older historical scholarship on Newton, even by leading experts such as Alexandre Koyré and I. B. Cohen, has tended to deny that Newton believed in the possibility of action at a distance, in spite of the fact that he explicitly invokes it not just in ‘De Aere’, the draft preface for the Principia, and the Opticks. Evidently, Huygens and Leibniz were not so easily fooled even though they only had access to the Principia, which deliberately avoids any explicit mention of action at a distance. For a survey of the role of actions at a distance in Newton’s work, see John Henry, ‘Newton and Action at a Distance’, in Eric Schliesser and Chris Smeenk (eds), Oxford Handbook of Isaac Newton (Oxford: Oxford University Press, 2019), pp. 1-30, published online: https://doi.org/10.1093/oxfordhb/9780199930418.013.17
31 ‘De Aere et Aethere’, p. 224, Latin, p. 217. The first part of ‘De Aere et Aethere’ strongly suggests that Newton is thinking like an atomist – supposing that individual particles of air are separated from one another with void space in between. Certainly, in the examples he provides, the particles are said to be separated and to interact by means of a repulsive force operating between them: there is no suggestion that really their interaction is mediated by an intervening aether. Indeed, the superfluousness of an aether which was implicit in the first section of the manuscript, entitled ‘De Aere’, might be one reason why Newton, almost immediately after beginning, stopped writing the following section, ‘De Aethere’. This is necessarily speculative, but we shall discuss it further in due course.
32 Hall and Hall, p. 189.
33 Hall and Hall, p. 191.
34 See Koyré, Newtonian Studies, pp. 115-38; and I. B. Cohen, The Newtonian Revolution (Cambridge: Cambridge University Press, 1981), pp. 79-83.
35 Some readers might object at this point that Newton used aether-language in the ‘Queries’ appended to the Opticks, as this passage from the Halls’ introduction makes clear, and that it is therefore wrong to suggest a chronological divide with aether-language confined only to his early career. As a matter of historical fact, however, Newton rejected the use of aethers in his physics at least from the Principia, only to re-introduce them in a group of Queries added to the second English edition of the Opticks in 1717. This is the one and only appearance of this aether, however. It was introduced into the Opticks for specific restricted purposes, but never developed any further by Newton, and certainly never made the basis of a generalised physics to replace the physics developed in the Principia; or even the more conjectural physics promoted elsewhere in the Opticks. It was not introduced into the third edition of the Principia in 1726, for example, which remained as resolutely opposed to the concept of an aether as the earlier editions. Accordingly, these aether Queries of 1717 had no impact on wider aspects of his physics. To all intents and purposes, Newton’s speculative physics (as opposed to the more rigorous and mathematical physics of the Principia) continued to depend upon interparticulate forces operating at distances, and not on an aether. Indeed, even this aether of 1717 was held to consist of particles separated from one another by repulsive forces (see Henry, ‘Newton and Action at a Distance’). Newton did not revert back to ‘aether-language’ in 1717 except within the confines of these eight new Queries; nor did he rescind any of the objections to the concept of aethers which appeared elsewhere in the Queries (for example, Query 28), or anywhere else in his post-Principia works. In short, Newton’s physics continued to be radically different from the physics of his contemporaries, by virtue of its emphasis upon interparticulate forces, and it is wrong (and does Newton a great wrong) – the aether Queries notwithstanding – to imagine him reverting in 1717 to the kind of aetherist physics professed by Cartesians, Leibniz, and others.
36 Hall and Hall, p. 187. In the letter, Newton wrote: ‘had not your encouragement moved me to it, I should never, I think, have thus far set pen to paper about them.’ Letter, fol. 65r, p. 253.
37 Letter, fol. 62r, p. 250. For Westfall’s view, see his Never at Rest, p. 374. See also, Westfall, Force in Newton’s Physics (New York: American Elsevier, 1971), pp. 377-78, and 409-10. Westfall’s date has been accepted in Henry, ‘Newton and Action at a Distance’.
38 See Hall and Hall, pp. 188-89.,
39 Newman, Newton the Alchemist, p. 437; Dmitri Levitin, The Kingdom of Darkness, p. 556n.
40 ‘De Aere’, p. 221, Latin, p. 214.
41 ‘De Aere’, p. 221, Latin, pp. 214-15.
42 Newman, Newton the Alchemist, p. 438n.
43 Robert Boyle, A Continuation of New Experiments Physico-Mechanical Touching the Spring and Weight of the Air (1669), Experiment XXVII, in Boyle, Works, edited by M. Hunter and E. B. Davis (London: Pickering & Chatto, 1999), vol. vi, p. 105.
44 ‘Hypothesis’, p. 252, p. 182.
45 Newman, Newton the Alchemist, p. 438n.
46 Letter to Boyle, fol. 62r, p. 250.
47 Hall and Hall, p. 188.
48 Hall and Hall, ‘Conclusio’, p. 339, Latin, p. 326. The Halls suggest that the ‘certain person’ might be Boyle, but this seems doubtful, especially in view of the fact that Boyle is mentioned by name, in connection with another matter, just a few lines before this. It might be Robert Hooke who discusses capillarity in ‘small canes’ in his Micrographia (London: Royal Society, 1665), pp. 11-12. Hooke’s account, in which differential pressure within the cane and outside it, is explained in terms of Hooke’s concept of incongruity, is rather similar to Newton’s account. On Hooke’s concept of incongruity, see John Henry, ‘Robert Hooke, The Incongruous Mechanist’, in Robert Hooke: New Studies, edited by Michael Hunter and Simon Schaffer (The Boydell Press, Woodbridge, Suffolk, 1989), pp. 149-80.
49 Newton, Opticks, p. 391.
50 Newton, Opticks, p. 393. See Henry Guerlac, “Francis Hauksbee: Expérimentateur au profit de Newton,” in Archives internationales d’histoire des sciences, 16 (1963), 124–127; and A. Rupert Hall, All Was Light: An Introduction to Newton’s Opticks (Oxford: Clarendon Press, 1993), pp. 146-47.
51 Hall and Hall, p. 354, Latin, p. 351.
52 Newman, Newton the Alchemist, p. 438n.
53 The text of ‘Of Nature’s Obvious Laws & Processes in Vegetation’ (Washington DC, Dibner MS. 1031 B, The Dibner Library of the History of Science and Technology, Smithsonian Institution Libraries, Smithsonian Institution) is available online via The Chymistry of Isaac Newton website: https://webapp1.dlib.indiana.edu/newton/mss/norm/ALCH00081/query/field1=text&text1=Of%20Nature's%20obvious%20laws. In older works on Newton, this is sometimes referred to as ‘The Vegetation of Metals’. On its date, see Newman, Newton the Alchemist, p. 139n.
54 Newman, Newton the Alchemist, p. 438.
55 Newman, Newton the Alchemist, p. 439.
56 ‘De Aere’, p. 227, Latin, p. 220.
57 Newman, Newton the Alchemist, p. 439, quoting from Dibner 1031B, fol. 6r.
58 Newman, Newton the Alchemist, p. 440.
59 ‘De Aere’, p. 226, Latin, p. 219.
60 Newman, Newton the Alchemist, p. 438.
61 ‘Of Nature’s obvious laws’, fol. 3v.
62 ‘Hypothesis’, p. 250, p. 180.
63 Hall and Hall, p. 191.
64 ‘De Aere’, pp. 227-28, Latin, p. 220.
65 Westfall, Never at Rest, p. 374n.
66 See Hall and Hall, pp. 305, 306 (draft Preface), Latin, p. 303; ‘Conclusio’, pp. 336, 338, Latin, pp. 324, 325. We will return to these. Newton also hinted at interparticulate forces in some of the first sixteen ‘Queries’ appended to the first edition of the Opticks in 1704 (e.g. Newton, Opticks, pp. 339, 343). He developed his physics of interparticulate forces in more detail in the seven Quaestiones he added to the Optice in 1706, which became Queries 25 to 31 in the Opticks of 1717. For the English text, see Newton, Opticks, pp. 339-406.
67 Many of the features of Newton’s aetherist speculations in the ‘Hypothesis’ can be seen to have derived from earlier ideas propounded by Robert Hooke, as even Newton admitted: ‘Hypothesis’, pp. 248-49, pp. 178-79. See also, Newton’s letter to Henry Oldenburg, of December 21, 1675, in Papers and Letters, pp. 208-09. It might also be added that Newton’s aether theory was not only unoriginal, it was also far from convincing. Consider, for example his somewhat risible theory of gravity in the ‘Hypothesis’, p. 251, p. 181.
68 Westfall, Never at Rest, 374.
69 Letter, fol. 62r, p. 250; ‘Hypothesis’, pp. 250-51, 180-81.
70 ‘Hypothesis’, p. 250, p. 180.
71 Letter, ff. 62r-62v, pp. 250-51.
72 Letter, fol. 63r, p. 251.
73 Letter, fol. 63v, p. 252.
74 ‘Hypothesis’, pp. 251, 181; 252, 182.
75 Letter, fol. 62r, p. 250.
76 Letter, fols. 62r, and 65r, pp. 250, 253.
77 Letter, fol. 64r, p. 252.
78 Letter, fol. 64r, p. 252.
79 Aristotle, Physics, IV, 9, 216b, 30-35.
80 Clearly, they made allowance for windy days and other situations where air was put in motion, but there was nothing remotely like a kinetic theory of gases among early modern thinkers. On the importance of vibrating matter in early modern natural philosophy, see Xiaona Wang, Handling ‘Occult Qualities’ in the Scientific Revolution (Leiden: Brill, 2023), Section 4.2.
81 Francis Bacon, ‘Cupid, or the Atom’ [1609], in The Works of Francis Bacon, edited by J. Spedding, R. L. Ellis, and D. D. Heath (Cambridge: Cambridge University Press, 2011), vol. vi, p. 731.
82 Letter to Boyle, fol. 62r, p. 250.
83 Westfall, Never at Rest, p. 374.
84 ‘De Aere’, p. 221, Latin, p. 214.
85 ‘De Aere’, p. 221, Latin, p. 214.
86 ‘De Aere’, p. 222, Latin, p. 215.
87 Newton was no Antoine Lavoisier, and no John Dalton. He did not think in terms of qualitatively different particles of gold, mercury, etc. The particles constituting gold were materially the same as particles constituting water, differing only in size, shape, and how they were packed together – the matter of which both were made was the same. As the densest substance, gold might be presumed to consist of particles so closely packed together as to be touching; while the particles constituting water must be assumed to be spread further apart, allowing for bigger intervening pores. This way of conceiving things already raised problems for early modern thinkers – could air particles be in contact with one another, air being so much rarer than gold, or must there be self-determined voids between them? See, Newton’s discussions in ‘De Aere’, ‘Conclusio’, pp. 337 and 341; and in Queries 21 and 22, Newton, Opticks, pp. 350-53.
88 ‘De Aere’, p. 223, Latin, p. 216. See Robert Hooke, Micrographia (London, 1665), pp. 226-27.
89 ‘De Aere’, p. 223, Latin, p. 217.
90 ‘De Aere’, p. 223-24, Latin, p. 217.
91 Robert Boyle, Works, vol. vi, p. 387.
92 Boyle, Works, vol. i, p. 166.
93 Newton, Principia, p. 588. It is not clear to me how a surrounding medium could bring about an attraction, and if it did, whether the attracting bodies would obey the inverse square law. A number of Newton scholars have taken this and similar comments by Newton at face value and have therefore suggested he did not really believe in actions at a distance. I have never seen any of these scholars show how this could work mathematically. Newton never showed it either – it is almost as if he did not really take this comment seriously.
94 ‘De Aere’, p. 223, Latin, p. 216. Interestingly, the only one of these opinions that allows action at a distance involves God. When Newton defended interparticulate forces acting at a distance in the Queries appended to the Opticks, he also relied on God. See, for example, Query 28, and Query 31, Newton, Opticks, pp. 369-70, and pp. 400-403. See also, John Henry, ‘Newton, the Sensorium of God, and the Cause of Gravity’, Science in Context, 33 (2020): 329-51.
95 Newton, The Principia, p. 588. See also ‘Definition 8’, p. 408, where he also says the opposite of what he says throughout the rest of the Principia. For discussion, see Cohen, Newtonian Revolution, pp. 68-78, and passim.
96 ‘De Aere’, p. 223; Latin, p. 216. It is not obvious why Newton chose to put this comment in square brackets. Perhaps this was because it was saying the opposite of what had just been said before, and he wished therefore to signal that he was now making a contrasting (and contradicting) point? Be that as it may, it is of little consequence, since he struck out the foregoing passage and this contradiction of it.
97 ‘De Aere’, p. 224; Latin, p. 217. Again, it should be remembered that there was no equivalent of our conception of the kinetic theory of gases. So, heat did not simply increase the motion of already moving particles, for Newton. On the contrary, he seems to have envisaged air particles standing stationary apart from one another. Heat would cause the particles to vibrate, more or less vigorously, but always around the point where they were originally standing stationary.
98 ‘De Aere’, p. 226, Latin, p. 219.
99 ‘De Aere’, p. 227, Latin, p. 220.
100 ‘De Aere’, p. 227, Latin, p. 220.
101 ‘De Aere’, pp. 227-28, Latin, p. 220.
102 This view is largely associated with the Cartesian insistence that empty space is impossible, and therefore, there must be a very subtle form of matter which can, and does, fill all the gaps in nature between larger particles of matter (for example, the gaps unavoidably left between close-packed spherical particles). Since there was only one kind of matter in Cartesianism (as there was in atomism), different substances were distinguished in terms of size, shape, and arrangement of their particles. So, gold differed from iron, or air, by virtue of these differences. In this scheme, aether was therefore different from air, in the same way air was different from water, even though only one kind of matter was involved. There was some originality, therefore, in Newton’s claim that aether was simply ‘spirit of air’; although the distinction between this and Cartesianism was certainly a fine one.
103 ‘De Aere’, p. 228, Latin p. 220.
104 I say that this seems plausible. In fact, I am inclined to add that, in the light of what Newton had just written in ‘De Aere’, he ought to have quickly abandoned ‘De Aethere’ – which he did.
105 It was only nearly unanimously rejected because of the revival of natural magical traditions, inaugurated largely by Marsilio Ficino and other Renaissance thinkers enthralled by occult sciences. See Henry, ‘Newton and Action at a Distance’.
106 The quoted words are Roger Cotes’s in his Preface to the second edition (1713), Newton, Principia, p. 388. Literature on the composition of the Principia is already vast and still growing. See, for example, Herivel, The Background to Newton’s Principia; Cohen, The Newtonian Revolution; and George E. Smith, ‘The Principia: From Conception to Publication.’
107 For discussion of this point, see, for example, Alexandre Koyré, ‘Huygens and Leibniz on Universal Attraction’, in Newtonian Studies, pp. 115-38; and Cohen, Newtonian Revolution, pp. 79-83.
108 See, for example, Herivel, The Background to Newton’s Principia, pp. 96-102; Cohen, ‘Guide to Newton’s Principia’, in Newton, The Principia, p. 180-81; and George E. Smith, ‘Newton’s Numerator in 1685: A Year of Gestation’, Studies in History and Philosophy of Modern Physics, 68 (2019), 163–77.
109 I have slightly modified Cohen’s and Whitman’s translation in Newton, Principia, p. 697. As Cohen points out, the order of these two sentences are reversed in the second edition, which they quote. I have presented them as they appear in the first edition, Newton, Principia (London: The Royal Society, 1687), p. 301.
110 Cohen’s ‘Guide’ appears in Newton, Principia, pp. 1-370, see p. 164.
111 ‘De Aere’, pp. 223-24; Latin, p. 217.
112 Newton, Principia, p. 698.
113 Newton, Principia, p. 699.
114 Newton, Principia, p. 588.
115 Edited and translated in Hall and Hall, pp. 333-47; Latin, pp. 321-33. Hereafter cited simply as ‘Conclusio’.
116 ‘Conclusio’, p. 333, Latin, p. 321.
117 ‘Conclusio’, p. 341, Latin, p. 327.
118 ‘Conclusio’, p. 345, Latin, p. 331.
119 Hall and Hall, ‘Partial Draft of the Preface’, p. 307, Latin, p. 304.
120 As we have seen, the interparticulate forces hypothesised in ‘De Aere’ seem to have arisen as a way of understanding the extreme rarefactions developed in the air-pump. The force of gravity, by contrast, was arrived at through the attempt to provide a mathematical proof that Kepler’s laws of planetary motion were the result of the inertial tangential motion of planets being attracted to the sun by an attractive force varying inversely as the distance between them. During the course of the latter enterprise, Newton found himself rejecting the concept of an aether, and implicitly establishing gravity as a force acting at a distance. Remarkably, therefore, Newton recurred to actions at a distance twice, but through completely different routes. Small wonder that he should be led to link them together, as he seemed to be trying to do in the ‘Conclusio’.
121 Newton, Principia, pp. 382-83.
122 Newton, Principia, p. 943.
123 ‘De Aere’, p. 223, Latin, pp. 216-17; Newton, Opticks, pp. 395-96.
124 See ‘Of Nature’s obvious laws’, fol. 3v, and cf. ‘Hypothesis’, p. 250, p. 180.
125 In the ‘De Aethere’, p. 228 (Latin, p. 220), Newton wrote glibly about magnetism and electrical attraction as evidence for the existence of aether. It was at this point, however, that he stopped dead in his tracks. We can only speculate, but it may have been at this point that he realised he had further evidence for interparticulate forces, but this time attractive, rather than repelling. At the very least, it might have been the mention of attractions which brought repulsions back to the forefront of his mind, and caused him to realise aethers were therefore redundant.
126 Newton, Opticks, pp. 375-76.