The Soul as a Part in Aquinas

I clarify and critically examine Aquinas’s claim that the human soul is a part of the human substance, arguing that the claim that the soul is a part is the key to understanding his ontology of the human being. I begin by setting out Aquinas’s understanding of parthood and then proceed to look critically at the features Aquinas attributes to the soul that are particularly relevant to its parthood. Such features include the soul’s subsistence, its causal interaction with the body, and its status as source of esse of the human being. I conclude with a synthesis in which I argue that the human soul is a part in two ways: as an Aristotelian substantial form and as an incorporeal organ analogous to the heart or the eye. I further argue that the picture of the soul that emerges from this account is internally coherent and that that fact, in itself, makes the view defensible, given its considerable explanatory power.