This important commentary by Alexander of Aphrodisias on Aristotle's work on biochemistry was previously lost. However, four chapters of it have been re-identified in an Arabic translation by Emma Gannage and are here translated for the first time. The chapters were preserved in the writings of an eighth-century alchemist, Jabir ibn Hayyan. In addition to preserving an interesting example of very early cross-cultural scientific activity in the Muslim world, the newly discovered material is of philosophical importance: We learn how Alexander attempted to provide a unified theory that would unite Aristotle's chemistry with his elemental physics. As well as an English translation of the text, this volume includes a detailed introduction demonstrating the authenticity of the work and discussing its contribution to our understanding of ancient science.