Having answered the most serious criticisms of his account, Aristotle concludes his arguments that the earth must be a sphere – and so concludes De Caelo II - with the particulars of its size. De Caelo III, 1, opens with a brief summary of what has been discussed thus far concerning the first heaven and its parts, the stars carried in the heaven, and the composition and nature of these things, including, finally, that they are ungenerated and incorruptible (298a24–27). Having dealt with the first (and highest) element, aether, Aristotle turns to the other elements.

These elements – each of which possesses its own specific nature – are “by nature.” Natural things are either substances or operations and affections of substance. “Substances” refers to the elements – aether, air, fire, water, and earth – and things composed of them, such as the heaven, as a whole and its parts, as well as plants and animals and their parts; “operations and affections” include movements of each of these according to their proper power as well as their alterations and transformations into one another. Obviously, Aristotle concludes, the study of nature is for the most part concerned with bodies because all natural substances are either bodies or come to be after bodies, and all involve magnitude. The investigation must also include generation and destruction, as “operations and affections” of the elements and all things composed of them (298b8–11).