We have a tendency to think of understanding or knowing as involving a single binary relation to something, but there are others of thinking, In a well-known passage in De Potentia 8, Aquinas argues that in understanding a person is involved in four distinct relations:
(1) We are related to the thing understood (res intellecta).
(2) We are related to the intelligible species.
(3) We are related to our own intellectual action.
(4) We are related to our intellectual conception.
All of these have to be distinct. The res intellecta cannot, as such, be the other three, because unlike them it is sometimes (interdum) outside the intellect. The intelligible species, which is how things experienced and imagined and remembered appear to the intellect, activates the intellect, and therefore is the beginning of the intellectual action, distinguishing it from the conception, which is the end-result. The intelligible species can't be the intellectual action itself because it has to pre-exist it; we receive the intelligible species simply by experiencing things as intellectual beings, and therefore our intellectual actions presuppose it. Since the conception is the result of the intellectual action, and formed by it for intellectual ends, the action and the conception are not the same.
In reality, of course, this is simplified, because, at least for human beings, between the thing understood and the intelligible species we find of sensation and sensory processing, and in being related to the intelligible species, intellectual action, and intellectual conception, the intellect has to be related to the phantasm and the thing sensed or imagined. Moreover, if we consider what happens beyond the intellectual conception, we find that it does not stop -- we link, distinguish, compare, contrast, modify intellectual conceptions in light of other intellectual conceptions, and thus judge and reason, both of which we do in part so that we might understand more.
The temptation in epistemology is to treat 'knowledge' or 'cognition' as a single monolithic thing. But one of the most obvious facts of our experience as sensers and imaginers and knowers and understanders is the coordination of distinct relations into a unified harmony. We coordinate sight and hearing, we coordinate sensing and imagining, we coordinate imagining and understanding, and in each of these we are relating ourselves to more than one thing. We cannot have a good account thought of any kind without considering the variety of relations we are integrating into a whole whenever we do any kind of thinking at all.
