Aquinas does not build his philosophical system around a theory of knowledge. In fact the reverse is true: he builds his epistemology on the basis provided by other parts of his system, in particular, his metaphysics and psychology. To examine what we can recognize as a distinct and systematic theory of knowledge, then, we need to extract his strictly epistemological claims from the metaphysical and psychological discussions in which they are embedded.

COGNITION

Cognition is Aquinas’s fundamental epistemic category. He endorses the Aristotelian view that the soul is potentially all things, and he holds that cognition involves its actually becoming a given thing or, as he sometimes puts it, its being assimilated to that thing in a certain way. As Aquinas sees it, the development of this notion of cognition as the soul’s assimilation to the objects cognized requires him to deal with two sorts of issues. First, he needs a metaphysical account of the two relata: the human soul and the object of human cognition. Here he draws primarily on his Artistotelian hylomorphism.