With this account of “the where” – place, not void – I turn to the De Caelo and its topics, particularly the elements. As a topic, the elements present a problem: the elements – themselves unquestionably “things that are by nature” – appear in the Physics only within the examination of other topics, but are never themselves examined as a topic. A direct examination of them appears in the De Caelo. But historically the coherence of the De Caelo as a set of logoi, the definition of its topic (s), and the relation of its arguments to those of the Physics have been problematic. A consideration of the Physics and the De Caelo as topical investigations solves both problems, substantive and historical.

The Topic of the De Caelo

In the Physics, Aristotle investigates strictly defined topics. Physics II, 1, identifies things that are by nature: animals, their parts, plants, and the elements earth, air, fire, and water (192b9–11). Nature is a principle of “being moved and being at rest in that to which it belongs in virtue of itself” (192b21–22); consequently, Aristotle asserts at Physics III, 1, we must know motion if we are to understand nature, and he lists the “common and universal things” without which motion seems to be impossible (200b20–23). The investigation of proper things, he says, will come later because universal things, including the continuous, the infinite, place, void, and time should be investigated first (200b23–25).