Aristotle was committed to the view that the motion of the celestial bodies cannot be explained without recourse to the postulation of a material principle that is different from, and not reducible to, earth, water, air and fire. This view was very controversial in antiquity and did not gain the acceptance one might expect in the light of the reputation that the same view enjoyed in the late Middle Ages and up until around 1650. Proclus, for example, informs us that some Platonists even reeled back in horror from this view because they felt there was something barbaric in it (In Tim. II 42. 9-12). This is clearly an exaggeration. But, as with all exaggerations, it contains a grain of truth. The truth is that very few people in antiquity were prepared to share with Aristotle the view that celestial bodies are made of a celestial simple body, even if many, if not most, of them accepted his view that the celestial world is a special and somehow distinct region of the natural world. Even within the school of Aristotle, and from the very beginning, the thesis of the existence of a celestial simple body was resisted. The very unsatisfactory state of the information at our disposal does not allow us to establish whether Theophrastus rejected or retained this thesis. But Strato of Lampsacus, the head of the Lyceum after Theophrastus, rejected it and turned to the Platonic view that celestial bodies are made for the most part of fire.
This monograph is a study of Aristotle's argument for the existence of a fifth element and the fortune of this doctrine from the Hellenistic time to late antiquity. Special attention is given to Xenarchus of Seleucia and his critique of the Aristotle. I offer a reconstruction of the objections that Xenarchus moved against Aristotle's arguments for the thesis of the existence of a fifth element. I also show that Xenarchus did not limit himself to raising objections against the doctrine of the fifth substance, but produced a positive doctrine of natural motion. I argue that this doctrine was designed to fit the conception of the sensible world offered in the Timaeus.
Anita Ljubic, The Classical Review, 53 (2003): 305-307