The longest article of the Treatise, 76.1, is devoted to accounting for the unity of soul and body. Much of the article works to establish the unworkability of accounts on which soul and body are two things that must somehow be tied together (§3.1). Aquinas's own solution is a form of Aristotelian hylomorphism that is meant to guarantee soul-body unity. To understand this special sort of unity, one has to understand Aquinas's general theory of substantial forms, which imposes strict constraints on what counts as a genuine substance (§3.2). The soul is a substantial form, on this account, because all the parts of the body have life and existence if and only if they are actualized by the soul (§3.3). But this unified account of soul and body by no means pushes Aquinas toward reductive materialism; instead, a proper understanding of form shows precisely why reductive materialism is false (§3.4).

The failure of nonreductive theories

After considering the soul in its own right (Q75), Aquinas turns to the relationship of soul and body. More specifically, he turns to “the soul's union with the body” (76pr). To say that soul and body are united (unitur) is simply to say that they make one (unum) thing. The first article of Q76 focuses on how this union is accomplished, taking for granted that soul and body do in fact make one thing.