The conclusion that the elements are generated from one another raises the question of how this generation occurs. This question – indeed, the issue of the generation of the elements generally – presupposes a prior problem: what differentiates the elements? This problem is serious for two reasons. First, whatever differentiates each element must be generated when the element is generated. Second, whatever generates each element renders it unique and so makes the element be what it is according to its definition. Hence an account of what differentiates each element is central to the nexus of topics concerning the generation of the elements, their natures and motions. This “prior problem” is solved in the remainder of De Caelo III and IV, and the final account of the generation of the elements appears in the De Generatione et Corruptione.

Aristotle first criticizes his predecessors (De Caelo III, 7 and 8). The followers of Empedocles and Democritus explain the generation of the elements as an excretion of what is already there; this view reduces generation to an illusion – as if it requires a vessel rather than matter (De Caelo III, 7, 305b–5). And Aristotle quickly shows that it entails that an infinite body is contained in a finite body – which is impossible (305b20–25).

On other accounts, the elements change into one another, by means of shape or by resolution into planes (305b26).