Published online by Cambridge University Press: 01 January 2022
From late 1684 through mid-1685, Isaac Newton turned to developing and refining the conceptual foundations presupposed by his emerging physics. Analysis of his manuscripts from this period reveals that Newton’s understanding of the relativity of acceleration led him to seek a spatiotemporally invariant quantity of matter. He found two such quantities and then designed an experiment to discover their relationship. Interpreting the experiment, however, required distinguishing a new notion of force. Others have recognized the conceptual distinction between inertial and gravitational mass. I show here that Newton clearly saw this distinction and that he provisionally established their equivalence.
I am deeply indebted to George Smith for countless discussions related to this project and for his support and encouragement. I am also grateful to Robert DiSalle and Chris Smeenk, with whom I have had many fruitful discussions. I also thank Howard Stein for his insightful commentary on my talk given during the PSA symposium, Newtonian Relativity. I also thank Stathis Psillos, William Demopoulos, William Harper, Ryan Samaroo, and the anonymous referees for helpful comments on an earlier draft. This research was supported by the Rotman Institute of Philosophy. This article is dedicated to the memory of my father, William Fox.