Thomas Aquinas, Compendium of Theology

This discussion brings out the truth that knowledge of things in our intellect is not caused by any participation or influence of forms that are intelligible in act and that subsist by themselves, as was taught by the Platonists and certain other philosophers who followed them in this doctrine. No, the intellect acquires such knowledge from sensible objects, through the intermediacy of the senses. However, since the forms of objects in the sense faculties are particular, as we just said, they are intelligible not in act, but only in potency. For the intellect understands nothing but universals. But what is in potency is not reduced to act except by some agent. Hence there must be some agent that causes the species existing in the sense faculties to be intelligible in act. The possible intellect cannot perform this service, for it is in potency with respect to intelligible objects rather than active in rendering them intelligible. Therefore we must assume some other intellect, which will cause species that are intelligible in potency to become intelligible in act, just as light causes colors that are potentially visible to be actually visible. This faculty we call the agent intellect, which we would not have to postulate if the forms of things were intelligible in act, as the Platonists held.

To understand, therefore, we have need, first, of the possible intellect which receives intelligible species, and secondly, of the agent intellect which renders things intelligible in act. Once the possible intellect has been perfected by the intelligible species, it is called the habitual intellect (intellectus in habitu), for then it possesses intelligible species in such a way that it can use them at will; in other words, it possesses them in a fashion that is midway between pure potency and complete act. But when it has these species in full actuality, it is called the intellect in act. That is, the intellect actually understands a thing when the species of the thing is made the form of the possible intellect. This is why we say that the intellect in act is the object actually understood.