 The difference between Locke and Hume might be that the former, unlike the latter, nonetheless insisted that mechanism continued to provide philosophers with a canon for the intelligibility of causation within nature (Hume’s critique of earlier conceptions of causation, including Locke’s, is certainly well known). Hume also expended considerable effort in interpreting some of the philosophical aspects or consequences of Newton’s theory of gravity and his corresponding understanding of the nature of matter. In particular, he wished to provide what we would now call an empiricist interpretation of Newton’s three laws of motion, his concept of mass (quantity of matter), and his understanding of gravity; in the process, he does not shy away from interpreting Newton’s sometimes confusing notion of the vis inertiae or force of inertia. When students learn the canonical idea of a Newtonian force, they learn about what Newton called a vis impressa, an impressed force. They also learn that the mass of a body is not a force at all. But in all three editions of the Principia, Newton himself said that although the mass of a body is not an impressed force, it could be called by the “very significant name of force of inertia” because bodies with mass with resist acceleration (Newton 1999, 404). And to resist acceleration seemed to Newton like an action that a body performs, say when some other body collides with it. Now for Hume, this issue was particularly difficult because he had already contended that strictly speaking, we have no idea (no representation) of force or power—these are merely words employed by natural philosophers without any ideas corresponding to them. Thus he must find an interpretation of Newton that is not predicated on our ability to form an idea of forces or powers. In a famous footnote in his Enquiry Concerning Human Understanding, he writes (Hume 1777 [1993: §vii.i note 2, 48–49]):