 In other respects, in France the intellectual situation was rather different than in England. Cartesian ideas held sway for many decades after Newton first emerged on the scene in the 1670s with his optics papers and in the late 1680s with the Principia; they were especially popular at the Académie Royale des Sciences, unlike the situation in the Royal Society (Henry 2013: 120). Voltaire became famous for challenging the Cartesian French orthodoxy, traveling to England for a celebrated visit and bringing home “enlightened” ideas about philosophy, politics, and much else besides in an attempt to foment the emergence and development of a French Newtonian program. His most important publication in this regard is probably the Elements of the philosophy of Newton (Voltaire 1738/1992), which attempts to signal the “errors” of Descartes and to highlight the advantages of the Newtonian program in “Physique”. Of course, Cartesian ideas and methods retained numerous adherents and defenders in France, and so the debate between Cartesian and Newtonian ideas flourished in that environment (cf. Aiton 1972: ch. 8). But even in France, Cartesianism eventually gave way to a newer version of metaphysics associated with Leibniz and his German follower and expositor, Wolff. In 1740, Émilie du Châtelet, published a key text, Institutions de physique, or Foundations of physics (see especially the analysis in Brading 2019). In her text—which was republished in a second edition in 1742 and swiftly translated into German and Italian—she makes precisely the kind of argument that presses Hume a few years later to articulate a conception of the vis inertiae that is consistent with the experimental philosophy. She argues that the Cartesian view that extension is the essence of body is mistaken; we must conceive of material objects as exhibiting a “force d’inertie” if we are to understand their physical interactions properly (Du Châtelet 1742: ch. 1, §2). Perhaps not surprisingly, D’Alembert cites her text approvingly in his Treatise on Dynamics. From Du Châtelet’s point of view, this Newtonian conception of body is not inherently problematic. Problems arise, however, when some Newtonians interpret the theory of universal gravity as proving that bodies have a property of attraction or attractive power endowed by God (Du Châtelet 1742: ch. 16, §385). She accepts the idea that gravity is proportional to the masses of bodies and inversely proportional to the square of their spatial separation, arguing that this Newtonian idea of attraction “marvelously” explains the fall of bodies on earth, the tides, and various astronomical phenomena, something that the vortex theory of the Cartesians fails to do. But she denies that this acceptance entails that she must also accept the conclusion that bodies have an “essential property” called gravity or attractive force, contending that such a conclusion transcends the knowledge available to philosophers (Du Châtelet 1742: ch. 16, §388). The reason is that no one had yet demonstrated that there is no medium for gravity, such as a vortex or an aether: since gravity may depend on some medium, it may not be essential to matter per se. In this specific way, Châtelet’s work is predicated on the familiar notion that although Cartesianism had been overthrown by Newtonian ideas and methods, philosophers must nonetheless develop a proper interpretation of the Newtonian theory of gravity and of its implications. Yet unlike Voltaire, who not only rejected Cartesianism, but became famous for his ridicule of Leibnizian philosophy in Candide, Châtelet argued that philosophers should continue to work on developing the proper methodology in part by taking seriously the ideas and methods of Leibniz and Wolff. In particular, Châtelet argued that the principle of sufficient reason should be a guiding force in metaphysical theorizing, and as Leibniz had argued, it shows the fundamental error in Newton’s concept of absolute space; she added that Clarke’s method of replying to Leibniz’s argument, viz., that one can develop a non-Leibnizian or voluntarist conception of the PSR to evade the problem with absolute space, is underwhelming (Du Châtelet 1742: ch. 5, §74). She also argued that if we use the PSR to guide our thinking about science, we will be skeptical of Locke’s solution to the problem of understanding the philosophical implications of Newton’s theory of universal gravity. The idea that Newton’s theory supports the conclusion that God must have “superadded” gravity to matter avoids potential problems with asserting that gravity is “essential” to matter, something she also regards skeptically, but simply encounters other problems. First, since no one yet understood whether there was any medium underlying gravitational interactions, such as an aether, it was premature to conclude that gravity is a property of matter, even one “superadded” to it by divine fiat. After all, if there is an aether, then matter may gravitate because of its interactions with the aether, and not because of any superadded property. Second, the PSR demands that we seek rational explanations of natural phenomena in our science, but the superaddition thesis is predicated on the idea that we simply cannot understand how matter gravitates toward other matter. Newton’s science is simply taken to indicate that it does so. She was unwilling to leave the most important conclusion of the Scientific Revolution a mere mystery.