Thomas Aquinas, Compendium of Theology

Since intellectual being is superior to sentient being, just as intellect is superior to sense, and since lower beings imitate higher beings as best they may, just as bodies subject to generation and corruption imitate in some fashion the circulatory motion of heavenly bodies, it follows that sensible beings resemble, in their own way, intellectual beings. Thus from the resemblance of sense to intellect we can mount to some knowledge of intellectual beings.

In sensible beings a certain factor is found to be the highest; this is act, that is, form. Another factor is found to be the lowest, for it is pure potency; this is matter. Midway between the two is the composite of matter and form.

We expect to find something similar in the intellectual world. The supreme intellectual being, God, is pure act. Other intellectual substances have something of act and of potency, but in a way that befits intellectual being. And the lowest among intellectual substances, that whereby man understands, has, so to speak, intellectual being only in potency. This is borne out by the fact that man is at first found to be only potentially intelligent, and this potency is gradually reduced to act in the course of time. And this is why the faculty whereby man understands is called the possible intellect.