Descartes’ influential account of motion constitutes one of the central pillars of his natural philosophy and has provided the foundation for many later methods of understanding this most basic of phenomena, both among the Cartesians in the seventeenth century and in modern times. As a major advocate of the mechanical school of natural philosophy, Descartes rejected the complex scheme of substantial forms that the Scholastics had employed to explain nature; rather, bodies and their various phenomena “can be explained without having to assume anything else … in their matter but motion, size, shape, and the arrangement of their parts” (AT XI 26, CSMK 89). Moreover, unlike the Scholastics, whose intricate conception of motion embraced a wide variety of qualitative changes, such as generation and corruption, Descartes followed many of the other mechanical philosophers of the period by limiting motion (motus) to simply change of place. Descartes insists that all movement is “local movement, because I can conceive no other kind” (AT VIIIA 53, MM 50); and, in The World, he comments that the Aristotelian definition of motion (“as the actualization of a potential in so far as it is a potential”) is obscure (AT XI 39). From a metaphysical perspective, however, Descartes regards motion as a “mode” of extension, that is, as a way that extension manifests itself, or as a property of extension (Principles I.53; e.g., shape is mentioned as an additional mode of extension).