Descartes' account of the material world relies heavily on time. Most importantly, time is a component of speed, which figures in his fundamental conservation principle and laws. However, in his most systematic discussion of the concept, time is treated as some-how reducible both to thought and to motion. Such reductionistic views, while common among Descartes' late scholastic contemporaries, are very ill-suited to Cartesian physics. I show that, in spite of the apparent identifications with thought and motion, Cartesian time retains—in the form of what I will call 'successive duration'—precisely the intrinsic structure necessary to serve as an independent parameter of quantitative physics. As is often the case with Descartes, he gives the impression of embracing traditional doctrines while in fact radically transforming the underlying concepts to serve his scientific agenda. His theory of time, though formulated in Aristotelian terms, anticipates Newton in important respects.