Thomas Aquinas’s commentary on Aristotle’s On the Soul is chronologically the first of his commentaries on Aristotle’s works, written while Aquinas was still in Rome, between 1267–1268. It seems probable then that it was written at the same time as his composition of qq. 75–89 of the Prima Pars of the Summa theologiae, for Aquinas later adopted this same technique while writing the Secunda Pars, commentating simultaneously on Aristotle’s Nicomachean Ethics. It is hard to over-emphasize the importance of this text in revealing Aquinas’s deep, developed understanding of the human person. Though already familiar with the text, Aquinas here applies his intellect to grappling with a new translation of On the Soul (together with Themistius’s paraphrase), which had just been completed by William of Moerbeke. This first of Aquinas’s commentaries on Aristotle was followed shortly thereafter by his commentaries on Aristotle’s On Sense and What is Sensed and On Memory and Recollection, also included in this volume.