Thomas Aquinas, Compendium of Theology

A thing may be in potency in two ways: either naturally, that is, with respect to perfections that can be reduced to act by a natural agent; or else with respect to perfections that cannot be reduced to act by a natural agent but require some other agent. This is seen to take place even in corporeal beings. The boy grows up to be a man; the spermatozoon develops into an animal. This is within the power of nature. But that lumber becomes a bench or that a blind man receives sight, is not within the power of nature.

The same is the case with our minds. Our intellect has a natural potency with regard to certain intelligible objects, namely, those that can be reduced to act by the agent intellect. We possess this faculty as an innate principle that enables us to understand in actuality. However, we cannot attain our ultimate end by the actuation of our intellect through the instrumentality of the agent intellect. For the function of the agent intellect consists in rendering actually intelligible the phantasms that of themselves are only potentially intelligible. This was explained above. These phantasms are derived from the senses. Hence the efficacy of the agent intellect in reducing our intellect to act is restricted to intelligible objects of which we can gain knowledge by way of sense perception. Man's last end cannot consist in such cognition.

The reason is that, once the ultimate end has been reached, natural desire ceases. But no matter how much we may advance in this kind of understanding, whereby we derive knowledge from the senses, there still remains a natural desire to know other objects. For many things are quite beyond the reach of the senses. We can have but a slight knowledge of such things through information based on sense experience. We may get to know that they exist, but we cannot know what they are, for the natures of immaterial substances belong to a different genus from the natures of sensible things and excel them, we may say, beyond all proportion.

Moreover, as regards objects that fall under sense experience, there are many whose nature we cannot know with any certainty. Some of them, indeed, elude our knowledge altogether; others we can know but vaguely. Hence our natural desire for more perfect knowledge ever remains. But a natural desire cannot be in vain.

Accordingly we reach our last end when our intellect is actualized by some higher agent than an agent connatural to us, that is, by an agent capable of gratifying our natural, inborn craving for knowledge. So great is the desire for knowledge within us that, once we apprehend an effect, we wish to know its cause. Moreover, after we have gained some knowledge of the circumstances investing a thing, our desire is not satisfied until we penetrate to its essence. Therefore our natural desire for knowledge cannot come to rest within us until we know the first cause, and that not in any way, but in its very essence. This first cause is God. Consequently the ultimate end of an intellectual creature is the vision of God in His essence.