In contrast to what is often thought, the work De spiritu is entirely Aristotelian. It provides an indispensable part of Aristotle’s philosophy of living nature. In this work he is the first Greek to argue that the most fundamental vital principle is not breath but vital heat. This vital heat forms a unity with the soul, as its instrumental body (sôma organikon). The treatise is mainly a debate with Plato's Timaeus. This new book consists of an Introduction, a Translation, and an extensive Commentary on the text of De spiritu. The main value of this book is to show convincingly that Aristotle’s theory of soul and biology have been misconstrued since 200 AD due to the intervention of Alexander of Aphrodisias.