 In many ways, Leibniz and Newton grew up in the same philosophical environment. Each came of age during the heyday of Cartesianism, and each argued in particular that Cartesian views in natural philosophy failed to include a sufficiently robust conception of the forces of bodies in nature. Force would lie at the center of Newton’s mature physics (Westfall 1971), and would become even more central to Leibniz’s thinking, playing an essential role in his metaphysics as well (Garber 2012). Indeed, in Leibniz’s criticisms of Cartesian physics involving what was later called the vis viva controversy (Smith 2006) and in Newton’s mature physics, each argued that Descartes and his followers had failed to understand the importance of the notion of force. Leibniz and Newton knew one another as mathematicians already in the 1670s, and as we have seen, Leibniz discussed Newton’s first optical work with Huygens. But after the publication of the Principia in 1687, their philosophical relationship, which was marked originally by respectful disagreement, began to develop in earnest. Just two years after the Principia appeared, Leibniz published his Essay on the Causes of Celestial Motions (or Tentamen), and then in 1693, the two corresponded with one another on mathematical and philosophical issues (Newton 2004: 106–9). Leibniz initiated their discussion in March of 1693: after highlighting Newton’s “astonishing discovery” that the elliptical planetary orbits found by Kepler can be the result of gravitational attraction within the solar system, Leibniz contends that these motions must be caused by “the motion of a fluid medium” (Newton 2004: 106). He had described such a fluid medium, or vortex, in detail in his own Essay. The background to Leibniz’s comment is his unwavering commitment—one shared by Huygens, whose theory of gravity’s cause Leibniz mentions in the same letter—to the mechanist requirement that all changes in motion must be the result of material impact. Thus for Leibniz, one can (e.g.) speak of the sun as attracting the earth as a façon de parler, but in fact the cause of the earth’s acceleration, of its true motion around the sun, is its interaction with a fluid medium. (Descartes was also a famous proponent of a vortex theory of planetary motion—Aiton 1972: 30–64; Gaukroger 2002: 150–3.) When he replied in October of 1693, however, Newton did not accept Leibniz’s philosophical olive branch. Leibniz had insisted that the vortex theory of planetary motion “would not at all detract from the value and truth of your discovery” that Kepler’s ellipses result simply from the conception of attraction or gravitation (Newton 2004: 107). This olive branch was important because earlier vortex theorists, especially Descartes, had failed to account for Kepler’s laws, so Leibniz was crediting Newton with advancing astronomy in this respect (and indeed, he attempted to reflect Kepler’s results in his Tentamen). But Newton brushed aside the olive branch, noting that vortices would disturb the motions of planets and comets through the solar system. He writes that some “very fine” matter fills the heavens, adding: