Margaret Cavendish is widely known as a materialist. However, since Cavendishian matter is always in motion, “matter” and “motion” are equally important foundational concepts for her natural philosophy. In Philosophical Letters (1664), she takes to task her materialist rival Thomas Hobbes by assaulting his account of accidents in general and his concept of “rest” in particular. In this article, I argue that Cavendish defends her continuous-motion view in two ways: first, she claims that her account avoids seeing accidents as capable (...) of generation and annihilation, which she argues is inconceivable; and second, she contends that according to Hobbes’s own view “rest” is an absurd conception since it cannot be drawn from experience. Beyond its function as a defense, I claim that Cavendish’s focused criticism of “rest” shows that she is a perceptive reader of Hobbes’s natural philosophy, insofar as her criticisms undercut the two a priori principles of Hobbesian physics. Finally, I show how her views developed in more detail in Philosophical and Physical Opinions (1663) and Observations upon Experimental Philosophy ([1666] 2001) avoid the worries she raises for Hobbesian materialism. (shrink)