This paper examines Aquinas's theory of dualist mental causation, that is, his theory of how human beings can efficiently cause changes in their bodies in virtue of two non-physical mental states of theirs, specifically an act of the intellect and an act of the will. It is first shown that Aquinas's hylomorphism does not lie at the heart of this theory. Rather, a relation that he calls “contact of power” (tactus virtutis) does. The remainder of the paper then investigates the (...) nature of this relation. Since Aquinas discusses key marks of it by contrasting it with physical contact, the paper focuses on this contrast. It is argued that, for Aquinas, the human soul stands in a relation of contact of power to the body or, more precisely, a certain part of the body when it causes a change in it and is holenmerically co-located with it. (shrink)