We call whatever is known an object of thought, and everything that is an object of thought is known. It follows, then, that the contrary of an object, as such, is unknowable. But subject is the contrary of object. So the subject, as such, is unknowable.
Subject, as is clear from both its etymology and its denotation, is a sort of potency, and every potency is a certain subject receiving act. This is why all things are knowable inasmuch as they are in act. The act of the knowable is, moreover, one with the act of the knower. To the extent that something is potential, then, it is not a knower, and to the extent that something is a non-knower, it is potential in some way. It follows from this that the being that is pure act is a knower in an absolutely perfect sense.
Just Thomism 