 In Cartesian natural philosophy, all natural change is due to the impacts that material bodies make upon one another’s surfaces (this is reflected in Descartes’s first two laws of nature). The concept of a force plays little if any role. Unlike Descartes, Newton placed the concept of a force at the very center of his thinking about motion and its causes within nature. In that regard, his reactions to the shortcomings of Cartesian natural philosophy parallel Leibniz’s, who coined the term “dynamics”, and who obviously regarded force as a fundamental concept in metaphysics as well (Westfall 1971). But Newton’s attitude toward understanding the forces of nature involved an especially intricate method that generated intense scrutiny and debate amongst many philosophers and mathematicians, including Leibniz (Garber 2012). Newton’s canonical notion of a force, which he calls a vis impressa or “impressed force”, is the notion of an “action exerted on a body” that changes its state of motion. This was a confusing notion at the time. Perhaps it is not difficult to see why that should be so. To take one of Newton’s own examples: suppose I hit a tennis ball with my racquet—according to Newton, I have impressed a force on the tennis ball, for I have changed its state of motion (hopefully!). We have a reasonably good idea of what the tennis ball is, of what the racquet is, and even of what I am, and a Cartesian might wish to stop her analysis there. But what exactly is this “force” that I impressed on the tennis ball? The ball, the racquet and I are physical things of one sort or another, but is the force physical? Is it not physical? It does not seem likely that a force is itself a physical thing in the sense of being a substance, to use a philosophical notion popular in Newton’s day (as we saw above in his first optics paper). The reason is that in Definition Four in the Principia, which defines an impressed force for the first time, Newton remarks: “This force consists solely in the action and does not remain in a body after the action has ceased”. So when I hit the tennis ball over the net, the force I impressed on it was the action of hitting the ball, or an action associated with hitting the ball, and not a property of me or of the ball after the action had ceased. This idea confused many of Newton’s readers. By the mid-eighteenth century, the time of Hume’s analysis of causation in the Treatise and the Enquiry, many philosophers started to think that actions and other kinds of event are important items to have in one’s ontology, and they often contended in particular that causal relations hold between events. But in Newton’s day, philosophers typically regarded objects or substances as the causal relata. Indeed, one actually finds an equivocation between thinking of events and thinking of objects as the relevant causal relata even in Hume: in his Enquiry, he first defines a “cause” so that “objects” are the causal relata, but then gives an example in which one of the relata is the vibration of a string (Enquiry, §VII, 51). So actions were difficult to analyze, left out of analyses, or conflated with objects. As a result, Newton’s conception of force proved confusing, even to his most sympathetic interpreters. Moreover, it was unclear to many of Newton’s mechanist readers how his forces fit into their rather austere ontological view that material bodies consist solely of properties such as size, shape, mobility and solidity.