On the Epicurean Conception of Gravity, and Newton

[T]here is a second aspect of Newton’s thought that is relevant: there are numerous instances in which both Newton and his followers reject action at a distance in strong terms. Newton’s apparent rejection of distant action in the correspondence with Bentley has been subject to considerable debate, but Newton makes similar points elsewhere, even in unpublished works that cannot be read as involving a hedging of his bets. For instance, in the unpublished corollaries to propositions 4 through 9 of Book III to the Principia, written in the early 1690s, when Newton thought a second edition of his text might appear, he writes:

For two planets separated from each other by a long empty [vacui] distance do not attract each other by any force of gravity or act on each other in any way except by the mediation of some active principle [movente principio] interceding between them by which the force is transmitted from one to the other.[1]

Unlike the Bentley correspondence, this is a direct expression of Newton’s own understanding of gravity, one that is entirely independent of the questions about Newton’s views of the essence of matter and his attempts to distance himself (perhaps) from what were then called “Epicurean” conceptions of atoms in the void (cf. Henry, 2011). There cannot be a clearer statement of his views: two planets do not act on each other in any way except through some mediating item.--Andrew Janiak (2013) "Three concepts of causation in Newton." Studies in History and Philosophy of Science Part A 44.3 (2013): 403

As regular readers may recall, I am one of Janiak's targets in context.* And I have to admit that when I first read Janiak's paper I thought his evidence was so strong that I had to concede that after the publication of the Principia, Newton did not return to the position that I attributed to him (here; here) based on my reading of the posthumously published Treatise of the System of the World, an early version of what became book III of the Principia.

But before I was going to throw in the towel, I wanted to check out the wider context of the passage quoted by Janiak. It reads as follows:

The Epicureans making a distinction of the whole of nature into body and void, denied the existence of God, but very absurdly. For two planets separated from each other by a great expanse of void do not mutually attract each other by any force of gravity or act on each other in any way except by the mediation of some active principle that stands between them by means of which force is propagated from one to the other. [According to the opinion of the ancients, this medium was not corporeal since they held that all bodies by their very natures were heavy and that atoms themselves fall through empty space toward the earth by the eternal force of their nature without being pushed by other bodies.] Therefore the ancients who grasped the mystical philosophy as Thales and the Stoics more correctly taught that a certain infinite spirit pervades all space, and contains and vivifies the entire world; and this supreme spirit was their numen; according to the poet cited by the Apostle: In him we live and move and have our being. Hence the omnipresent God is recognized, and by the Jews is called 'place'. To the mystical philosophers, however, Pan was that supreme numen...By this symbol, the philosophers taught that matter is moved in that infinite spirit and by it is driven, not at random, but harmonically, or according to the harmonic proportions as I have just explained.[2]

As McGuire notes (1968: 169), the manuscript probably dates from the mid-1690s when Newton became eager to situate his own work in light of ancient epicureanism. (I have tried to explain why this would be so here.) Here the mediating active principle just is the Stoic world soul, which appears to be an electric spirit of some sort. So, at first glance the wider context seems to support Janiak’s position.

And before I address the position, I have to note a complication. A very natural reading of the General Scholium, which was drafted a decade or two later, of the Principia is that he denies that God is a world soul (Newton 1999: 940; recall last week's discussion.) Rudolf De Smet & Karin Verelst have convincingly argued that here (and in a few other places of the General Scholium) Newton is echoing Lipsius's attack, inspired by Philo, on the old stoic notion  of anima mundi. So, we are by no means required to accept this manuscript as authoritative expression of Newton’s all-things-considered views. But because I have expressed some doubts that Newton really does reject the world soul altogether, this move is not entirely open to me.

However, and more controversially, I now think Janiak was wrong to think that the manuscript offers "a direct expression of Newton’s own understanding of gravity, one that is entirely independent of the questions about Newton’s views of the essence of matter and his attempts to distance himself (perhaps) from what were then called “Epicurean” conceptions of atoms in the void." In fact, now I deny that we are supposed to treat the key sentence – “For two planets separated from each other by a great expanse of void do not mutually attract each other by any force of gravity” – as really expressing Newton’s own position. For, he could be expressing the Epicurean position here. In fact, I think it is a natural reading of Epicurus' physics as expressed in the "Letter to Herodotus" reproduced in Book X of Diogenes Laertius Lives of Eminent Philosophers.  

Epicurean gravity is directed down to a privileged place (Epicurus, Letter to Herodotus, section 61). And this is so in infinite worlds (section 45). In a void atoms, which constitute bodies, do not interfere with each other unless they accidently deflect each other from their path (Epicurus, Letter to Herodutus, section 44). So, in Epicurean cosmology, the atoms on a planet move down, barring deflection and the swerve, and planets do not attract each other. So, a natural way to read the Epicureans is that two planets separated from each other by a great expanse of void do not “mutually attract each other by any force of gravity.” The atoms that constitute such planets have a tendency to press toward the center of each planet. But there is no reason to think they attract each other across the void. 

So, if one is antecedently convinced that Newton rejects the possibility of action at a distance, then, indeed taken in isolation --- "for two planets separated from each other by a great expanse of void do not mutually attract each other by any force of gravity or act on each other in any way except by the mediation of some active principle that stands between them by means of which force is propagated from one to the other" -- seems like a smoking gun. But the sentence before it and the sentence after it discuss ancient theories. In fact, the following sentence ("bodies by their very natures were heavy and that atoms themselves fall through empty space toward the earth by the eternal force of their nature without being pushed by other bodies") just conveys the Epicurean position. 

So, a natural reading of the first three sentences of the manuscript is as a description of a debate between Epicurean atheists and ancient (mystical-theist) critics. And I am willing to believe that Newton agreed with the critics that the Epicurean position was absurd. Not just because the Epicureans could be taken to deny God's existence, but also because Newton himself has established that Jupiter and Saturn did influence each other (Principia, Book III, Proposition 18, Theorem 18; Newton 1999: 818)

 

 

[1] In the accompanying note Janiak cites it as follows: “unpublished manuscript, University Library Cambridge, Add. MS 3965.6, f.269; quoted in Casini (1984, 38)”

[2] I have consulted Hylarie Kochiras ""Force, matter, and metaphysics in Newton's natural philosophy," 2008: 109 and McGuire (1968: 169).