Alexander of Aphrodisias’s commentary (about AD 200) is the earliest extant commentary on Aristotle’s Metaphysics, and it is the most valuable indirect witness to the Metaphysics text and its transmission. Mirjam Kotwick’s study is a systematic investigation into the version of the Metaphysics that Alexander used when writing his commentary, and into the various ways his text, his commentary, and the texts transmitted through our manuscripts relate to one another. Through a careful analysis of lemmata, quotations, and Alexander’s discussion of Aristotle’s argument Kotwick shows how to uncover and partly reconstruct a Metaphysics version from the second century AD. Kotwick then uses this version for improving the text that came down to us by the direct manuscript tradition and for finding solutions to some of the puzzles in this tradition. Through a side-by-side examination of Alexander’s text, his interpretation of Aristotle’s thought, and the directly transmitted versions of the Metaphysics, Kotwick reveals how Alexander’s commentary may have influenced the text of our manuscripts at different stages of the transmission process. This study is the first book-length examination of a commentary as a witness to an ancient philosophical text. This blend of textual criticism and philosophical analysis both expands on existing methodologies in classical scholarship and develops new ones.