The human soul is subsistent, which is to say that it is in some way an independent substance. This does not mean that we are our souls, because human beings have an essential bodily component (§2.1). Our soul's subsistence is a matter of its having a function – thought – that it performs on its own, independently of the body. More hangs on this argument than on any other passage in the Treatise, but it unfortunately seems to be one of the weaker arguments of the Treatise (§2.2). Unlike the rational soul, the souls of other animals are not subsistent, because sensation requires the body. Indeed, for Aquinas, sensation is a wholly corporeal activity. Aquinas is therefore a materialist regarding sensation, whereas he rejects materialism in the case of the mind (§2.3). Yet Aquinas is not a dualist in any meaningful sense, and his account of the soul-body relationship is an entirely consistent application of his broader metaphysical principles (§2.4).

The essential bodily component

Aquinas begins the Treatise with a striking characterization of his subject matter. Human beings, he says, are “composed of a spiritual and corporeal substance” (75pr). It is hard to imagine, at first glance, a clearer statement of the dualist doctrine: the human mind is one thing, the human body another, each its own independent substance. This reading seems to find confirmation in 75.2, where Aquinas argues that the human soul subsists on its own.