Kendall A. Fisher

In his discussion of the rational soul as form of the body in Summa Theologiae I.76.1, St. Thomas Aquinas largely devotes his response to the consideration and elimination of a few competing accounts of the relationship between the human body and soul. After rejecting these alternatives, he concludes that his account, on which the soul is the substantial form of the body, must be correct. I argue that Aquinas’s argument, which seems to involve a hasty move to affirm his own view, in fact reveals a specific and plausible set of desiderata based on Aristotle’s definition of the human being as a rational animal in the category of substance. Aquinas’s argument, therefore, provides the constraints on any viable account of human ontology. Moreover, given his metaphysical framework, only his hylomorphic account can satisfy the desiderata. These considerations allow us to understand and appreciate Aquinas’s argumentative strategy more fully.

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.