Abstract  Aristotle accounted for the fundamental dynamics of the cosmos in terms of the tendencies of the various elements to distinct types of natural motions, and (in the case of the sublunary elements) to rest in their natural places. In so doing, he introduced a fifth element, the ether, with a natural and unceasing tendency to revolve, as the matter for the heavenly bodies. This paper deals with some of the objections raised to this model, and to its conceptual underpinnings, raised by Xenarchus of Seleuceia, an unorthodox Peripatetic of the 1st century BC, and of the attempts of later philosophers to rebut them. In so doing it casts light on a little-known, but historically important and interesting, episode in the development of physical dynamics.