One of the main oddities of Newton's Principia, is that Book 3 has a preface that tells the reader that Newton (1643-1727) composed an earlier version of Book 3 that he had discarded. This work, A Treatise of the System of the World (hereafter A Treatise) saw the light of day (first in Latin then in translation) shortly after his death. That brief preface to Book 3 also tells us that while most of the first two books of the Principia are strictly mathematical, they include some philosophical scholiums that are not mathematical but most fundamental to philosophy. One of these is presumably the one attached to proposition 69 of book 1, where Newton explains what he means by "attraction" (that is, any endeavour of bodies to approach one another); Newton is very explicit on a number of possible mechanism that could be the cause of this endeavour and remains agnostic among them. Even so, in the Principia's Third Rule of Reasoning (added to second edition), Newton insists that gravity is a universal quality of bodies, but denies that he is asserting that it is an essential quality.
This has led to one of the enduring (and recently very lively) controversies in Newton scholarship: how to interpret Newtonian attraction. Is (a) the whole mathematical framework just a useful calculating device to predict the behavior of moving bodies [this is Berkeley's view--McMullin more recently]; a related interpretation is (b) that the force of gravity is nothing but a projection of the human mind [i.e., as Yoram Hazoney has taught me this is Hume's view in the Treatise]; others affirm (c) that attraction is something real, but we don't know what it is except that we can discern its consequences [this is Clarke's view in his debate with Leibniz--it is also one defended by Andrew Janiak]; for much of his life Newton himself inclined to (d) that attraction is the consequence of the workings of an ether [as Newton suggests in the queries to the Optics]; but sometimes Newton encouraged his readers to think (e) that attraction is the product of God's constant will [a view defended by Westfall] or (f) that it is a quality superadded to matter by God at creation [Newton's contemporaries read Newton's correspondence with Bentley this way--John Henry has more recently defended this view]; on the other hand, the able editor of the second edition of the Principia, Cotes, argued in his preface (h) that gravity is an essential/primary quality of body [Kant endorsed this--and so does Howard Stein]; based on a reading of the A Treatise I have argued (and here in exchange with Hylarie Kochiras) that when Newton drafted the first edition of the Principia (when he most doubted the ether explanation), he thought that (i) gravity is a relational, non-intrinsic quality of matter that is the product (now quoting A Treatise) of "the conspiring nature" of both bodies in the interaction. (This interpretation allows one to account for a lot of other things Newton says about gravity in his published writings.) Given that my interpretation does not seem to have had a historical precedent in the reception of the Newton, it can easily seem like a modern rational reconstruction. Consider now this passage of William Gilbert's (posthumous, 1651) De mundo nostro sublunari philosophia nova:
I learned of this passage, which was originally written English and translated into Latin by Gilbert's brother, from Duhem's majestic The Aim and Structure of Physical Science, where it is used to contextualize Keplerian action at a distance (In the translation it can be found on p. 235, but that is blocked on Books.Google, so I linked to the French edition.) In context, Gilbert is explaining the behavior of the tides in terms of the rotation of the Earth and the mutual action of the conspiring natures of the bodies of the Moon and Earth. In order to explain this mutual action Gilbert illustrates this by way of analogy of the workings of the magnet (on which he was the acknowledged world expert). In On the Magnet, magnetic force is treated as something active and Gilbert had appealed to conspiring natures (see chap. 12 & 15, among others) to explain the mutual action. In chapter 4, Gilbert had distinguished his explanation of the conjoined action responsible for magnetic force from the Stoic/Platonic account that appealed to sympathy as promoted by, say, Fracastorio.
In chapter 16 of On the Magnet, Gilbert hints toward his explanation of the tides elsewhere. But here, too, he emphasizes the shared nature of the matter of the Moon and Earth as the explanation. Interestingly and confusingly enough, he immediately illustrates it again by way of the magnet, but now in terms of how the "loadstone and iron show sympathy with a loadstone though solid bodies be interposed." In De Mundo Nostro the conspiring nature of the bodies of the Moon and Earth explains the distant (and mutual) action of the Moon on the Earth's sea. It is the conspiring nature that elicits sympathy. (I learned from Eyjólfur Emilsson, René Brouwer, and Ann Moyer that in classical times sympathy presupposes a unified nature in which like can act on like at a distance.)
Now, there is no evidence that Newton read Gilbert, but it is hard to imagine he would have been genuinely unfamiliar with On the Magnet in particular. (Bacon and Descartes repeatedly called attention to Gilbert's work on the magnet.) Newton did have a tendency to explain the Sun's action at a distance with reference to the magnet (see here [Principia, Book 3, Proposition 7, Cor. 1] and here [proposition 7 of Part 3 of Book 2] in the Opticks). Now, in query 22 of the Optics, Newton suggests that the action of the magnet can be explained in terms of effluvia (invisible emanation of little spirits)--it is worth mentioning that this is, in fact, the first possible hypothesis that Newton lists in the scholium to proposition 69 of Book 1 of the Principia (let's call it "d*"). The theory of magnetic effluvia comes straight out of Lucretius (as this quote in Gilbert's On the Magnet testifies). As far as I know no modern commentator has suggested that this was ever Newton's favorite explanation of action at a distance, although it is worth mentioning that Bentley (a Classicist by training) initially repeatedly attributed Epicurean gravity to Newton (something Newton adamently denied, knowing full well that it was not a good thing to be associated with Epicureanism after the Glorious Revolution--Bentley was Stillingfleet's sidekick at the time, and these were not people to messed around with on such matters [see the full story here]). Of course, Newton also emphasizes a crucial difference between magnetic force and gravity; magnetic force intends and remits and is not a universal quality of matter.
So, let me return to Newton's A Treatise. Newton never disowned the piece and he kept calling attention to its existence through two carefully edited new editions of the Principia as debates over his methods and metaphysics fired up. The manuscript surfaced quickly after his death in print. In A Treatise, Newton explains atraction in terms that are nearly identical to Gilbert's. The planets have a disposition to act on each other in virtue of their conspiring natures. In order to make this intelligible Newton turns to the magnet as illustration.That is to say, when Newton wrote the first edition of the Principia he was toying with ideas that were fashionable in the generation of Gilbert and Kepler. As is well known, Hooke had revived interest in these and that had stimulated the famous wager of Wren to Halley and Hooke, which Halley was smart enough to take to Newton, etc.
One very smart reader of the first edition of the Principia, guessed as much in responding to the first edition of the Principia: in a critical letter to Huygens (March 20, 1693), Leibniz ridiculed Newtonian attraction as an inexplicable process, not unlike sympathy. Of course, by the time Newton completed the first edition of the Principia, he had started to develop a methodology that told him he did not have enough evidence to attribute the cause of gravity to any particular mechanism.
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