This is the first great commentary in the Western European tradition of expounding Aristotle's On the Soul. Dated about 1235, this work by Richard Rufus of Cornwall is a major contribution to the history of Western philosophy and the study of Aristotle. Indeed, no future account of thirteenth century philosophical psychology will be able to ignore the contribution of Richard Rufus. Following Aristotle, Rufus addresses questions as diverse as 'how do we reproduce and grow', 'how do we see and hear', 'how do we understand ourselves', and 'how is our immortal soul united with our body?'Its exposition and its questions date from about 35 years before Thomas Aquinas wrote his commentary on On the Soul, so its publication will prompt a re-evaluation of Aquinas's theory of the soul. As the copious notes to this edition indicate, not only is this the earliest surviving commentary on Aristotle's On the Soul taught at a Western University, but it was read by most of Rufus's early successors. Part of this commentary was published in 1952 but this is the first complete edition of a work that disappeared from the historical record 700 years ago. In addition to the text itself, this edition features an extensive introduction which presents the reader with the subsequent tradition, both published and unpublished.