In Summa Theologiae I.76.1 Aquinas presents an argument for the hylomorphic union of body and soul that he attributes to Aristotle. Aquinas builds on Aristotle’s original argument, however, offering his own short but powerful line of reasoning in support of one of the main premises. This additional argument involves an appeal to the principle that nothing acts except insofar as it is in act. This principle has roots in the thought of Aristotle, but is not explicitly used by him. It (...) is, however, fundamental for Aquinas and pervasive throughout his work. In this paper I examine the principle and its implications for Aquinas’ version of the argument. Furthermore, I argue that the principle is foundational to Aquinas’ criticisms of Averroes’ account of the intellective soul and that its inclusion renders Aquinas’ version of the argument incompatible with Averroes’ view. (shrink)