Aristotle's Book 4 provides an account of the formation of minerals, metals and other homogeneous stuffs. Eric Lewis argues that, in doing so, it offers fresh insight into Aristotle's concept of matter. The four elements (earth, air, fire and water) do have matter, and their matter is the contraries - hot and cold, moist and dry. Lewis further argues that in the text translated here, the only extant ancient commentary on the , Alexander of Aphrodisias supports this interpretation of Aristotle. Such a conception of matter complements the account given at an earlier point in the corpus of Aristotle's work in and is confirmed by the account at later points in the biological works, although it adds further detail. 4 emerges as an important book. Alexander's commentary is here translated into English for the first time.