The Correspondence between Huygens, Leibniz and Fatio de Duillier

Karin Verelst

Vrije Universiteit Brussel

The correspondence between the two main mechanist natural philosophers of the end of the seventeenth century and a close friend of Newton’s has hitherto received surpisingly little attention. In this contribution I present this correspondence, of which I am preparing a one-volume, commented edition, in its nature and scope. It took place over the crucial period between 1687 and 1695 (death of Huygens), in which Newton envisaged a new edition of his Principia, to be edited by Fatio de Duillier, and later on by David Gregory, who got connected to this exchange as well. It is my aim to show that the Huygens-Leibniz-Fatio correspondence is as important as the Leibniz-Clarke correspondence more than a decade later, and, even more, that the latter is not truly comprehensible without the former. Fatio De Duillier was a Swiss mathematician and protestant radical who worked with the Italian astronomer Cassini and later with Huygens, who recommended him to Leibniz in 1686. Since 1687 he was in London where he regularly visited the Royal Society. He was the intermediary between the three great men, who were in a discrete but thorough exchange of ideas concerning the dynamics of heavenly bodies, the concept of space, and the nature of force, fostered by the publication of Newton’s Principia (1687), Leibniz’s Tentamen de motuum coelestium causis (1689), and Huygens’s Discours de la cause de la pesanteur (1690). The correspondence shows that the debate took place in a much more favourable, open-minded atmosphere than its sequel in 1715. Moreover, Fatio played an intriguing rôle in attempts to bridge the gap bewteen the different positions by proposing an intermediate position in his own De la Cause de la Pesanteur (1691), which was considered for a while by both sides: a mechanist theory of gravity based on the impact from all directions of subtle particles on solid bodies. It reconciles mechanism with the Newtonian vacuum, at the expense of the aequivalence of matter and space. It moreover shows that Fatio had access to some of Newton’s private papers hitherto believed to be seen only by David Gregory. Finally, it allows us to shed light on a fundamental change in Huygens’s own ideas on motus verus toward the end of his life, testified in his letter to Leibniz of May 1694, where he says that he will not change his mind, despite of the raissonement et experiences de Newton dans ses Principes de Phiosophie, que je scay estre dans l’erreur, et j’ay envie de voir s’il ne se retractera pas dans la nouvelle edition de ce livre, que doit procurer David Gregorius, a statement which the editors of the Oeuvres Complètes qualify appropriately enough as an “assertion remarquable”.