Du Châtelet as Philosopher of Physics

Most commentators have assumed that while Émilie Du Châtelet’s Foundations of Physics (1740) is an important and original work that demonstrates her commitment to Leibnizian metaphysics and the Principle of Sufficient Reason (PSR), Du Châtelet herself does not have an original argument for either a commitment to the PSR or its truth. I argue against this widespread assumption by showing that implicit in the Foundations is an argument for a commitment to the PSR from the possibility of scientific reasoning. This argument takes as its starting point our commitment to scientific reasoning, and in particular to abductive reasoning in science. It then shows that the PSR is a presupposition of such reasoning. Thus, insofar as we are committed to abductive—and more generally, scientific—reasoning, we are also committed to the PSR. I show that this argument in Du Châtelet is both original and distinct from any argument for the PSR presented by Leibniz. I further argue that the argument provides a significant insight into the way in which Du Châtelet’s views about substance differ from Leibniz’s.

In this paper, we argue that Du Châtelet’s account of motion is an important contribution to the history of the absolute versus relative motion debate. The arguments we lay out have two main strands. First, we clarify Du Châtelet’s threefold taxonomy of motion, using Musschenbroek as a useful Newtonian foil and showing that the terminological affinity between the two is only apparent. Then, we assess Du Châtelet’s account in light of the conceptual, epistemological, and ontological challenges posed by Newton to any relational theory of motion. What we find is that, although Du Châtelet does not meet all the challenges to their full extent, her account of motion is adequate for the goal of the Principia: determining the true motions in our planetary system.

Newton’s startling conclusion in Book III of the Principia that all bodies gravitate defied easy interpretation. Whereas the editor of the Principia’s second edition (1713), Roger Cotes, claimed that gravity is a primary quality, Newton himself was more cautious. He claimed only that all bodies gravitate without saying that gravity is a property of bodies, adding the caveat that he was not ipso facto contending that gravity is essential to matter. In this same period, when Samuel Clarke defended Newton’s view against Leibniz’s famous criticisms, he presented a third, deflationary approach by treating gravity instrumentally. But an instrumentalist approach to gravity seemed to conflict with Newton’s own proclamation in the Principia that “it is enough that gravity really exists.” The tensions amongst these disparate ideas were never resolved. To address these tensions, Émilie Du Châtelet published a work entitled Foundations of Physics (Paris, 1740–42). She argued that neither Cotes’s approach, nor Newton’s, were apt methods of expressing the conclusion of the new theory of gravity. The theory indicates empirically that gravity is a universal force, she argued, but did not support Cotes’s interpretation because there was insufficient evidence to show that gravity is a property of all bodies. Among other things, it was not yet clear whether gravity depends in some way upon a subtle medium like an ether, in which case it might not be a property of bodies. (Similarly, Locke’s famous claim that God may have superadded gravity to bodies is suspect because it presupposes that the theory indicates that gravity is a property.) However, Newton’s alternative approach, which avoids contending that gravity is a property of all bodies or essential to matter, even while proclaiming that it is a universal force, was equally unsatisfactory. For starters, Du Châtelet argues that one must first clarify what one means by the essence of matter, which Newton avoided, and then one must also track the various conceptions of essences in play at the time. The overarching goal was to show that Newton’s abstemious approach toward classic metaphysical issues, often trumpeted as a feature of his physics, conflicted with attempts to interpret his theory of gravity. This episode reflects Du Châtelet’s overarching methodological approach to the “foundations” of physics. Although we do not require metaphysical foundations for physics in the way (e.g.) that Descartes presented in his Principles, whereby the first two laws of nature are purportedly deduced from God’s property of immutability, we also cannot eschew metaphysical questions as Newton attempted to do. Instead, the profound conclusions of the new physics require a foray into classic metaphysical topics like the essence of matter if those conclusions are to be widely understood. Du Châtelet’s approach to these problems is distinctive and deserves scholarly attention today.

In her second edition of the Foundations of Physics, Du Châtelet advocates a three-fold distinction of explanation: the metaphysical, the mechanical, and the physical. While her use of metaphysical explanation (i.e., explaining via the Principle of Sufficient Reason) has received some attention in the literature, little has been written about the distinction she draws between mechanical and physical explanations, including their demand, scope, and use in physical theorizing. This paper aims to fill this void, arguing that making this distinction is a crucial piece of Du Châtelet’s scientific method. According to Du Châtelet, a mechanical explanation is one that ‘explains a phenomenon by the shape, size, situation, and so on, of parts’, whereas a physical explanation is one that ‘uses physical qualities to explain (such as elasticity) … without searching whether the mechanical cause of these qualities is known or not’ (Du Châtelet 1742, 181). I will analyze Du Châtelet’s views regarding (1) What counts as a good physical explanation, (2) Why a mechanical explanation is not necessary for answering most research questions in physics, and (3) Why a good physical explanation, instead, is sufficient for answering those questions. In so doing, I argue that Du Châtelet is proposing an independent criterion of what counts as a good explanation in physics: on the one hand, it frees physicists from the methodological constraint imposed by mechanical philosophy, which was still an influential school of thought at her time; on the other, it replaces this constraint with the requirements of attention to empirical evidence, for that alone determines which physical qualities are apt to serve as good explanans.