Abstract
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.
