Chapter 83 of the Compendium Theologiae explains why part of our intellectual armamentarium is called the agent intellect; but it’s also an outstanding summary of how we learn. Here’s a diagram I made after reading the chapter; you might want to refer to it as we go along.
Here’s what Thomas has to say:
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.
Thomas, like Aristotle, is no Platonic Idealist. Plato said that when we see a dog we remember Dog…but no, the intelligible species Dog doesn’t exist on its own, floating out there in space somewhere. We learn what it is to be a Dog by studying real dogs.
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.
I see this dog, Fido; how do I get from this particular, this dog, this Fido, to Dog, an understanding of Dog in general? There’s no effect without a cause, and especially an efficient cause, an agent. The agent in this case is me, of course; I’m the one who’s learning what a Dog is. But I see with my eyes, I manipulate with my hands; I must have some faculty which performs this translation from particular to universal. What is it?
The possible intellect cannot perform this service, for it is in potency with respect to intelligible objects rather than active in rendering them intelligible.
The possible intellect is like a storehouse for intelligible species. As such, it’s part of my memory, as the term is commonly used.
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.
I suppose that it’s due to Thomas’ analogy, here, that we refer to the “natural light of reason”.
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.
What I understand him to be saying is this, as I diagrammed it above. The intelligible species enters my intellect in pure potency, as perhaps when I have seen dogs but not regarded them, not reflected upon them. And then, it strikes me that I’ve been seeing these odd furry things about, and I realize that they are all more or less similar. I reflect upon the sense impressions, and I—my agent intellect—bring the intelligible species Dog into full act. My possible intellect now has the species in full, indeed, is the species for that moment:
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.
After a moment, I contemplate something else, a passing car, perhaps, and forget about dogs. But I have changed. With regard to dogs, my possible intellect is now my habitual intellect: I have the species Dog in a way I previously didn’t. And I can call it to mind and understand in act whenever I choose. In short, I have learned what a dog is.