This important commentary by Alexander of Aphrodisias on Aristotle's work on biochemistry was previously lost. However, four chapters of it have been reidentified in an Arabic translation by Emma Gannge 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. In addition to a 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.