This is the first full-scale commentary on Aristotle's de Caelo III to appear in recent decades. de Caelo III can serve as a good introduction to Aristotle's physics and its character. In it he answers some very general questions about the elements of all material things except celestial objects: how many these elements are, why they cannot be infinitely many but must be more than one, whether they are eternal or can be generated and decay, and, if the second, how. His discussion is often framed as a critique of rival theories, and he argues systematically against the geometrical theory of the elements in Plato's Timaeus, which adds greatly to the interest of the work. The commentary adduces many parallel passages from Aristotle's other works to round off the readers understanding, and the introduction offers a brief but comprehensive overview of the Aristotelian theory of the elements, which de Caelo III often takes for granted.