We have still to consider how this union is possible. Some difficulty may seem to arise in this matter. The possible intellect is in potency with respect to all that is intelligible, whereas the agent intellect causes what is intelligible in potency to be intelligible in act, and so must be related to what is intelligible as act to potency. But the same thing, seemingly, cannot be both in potency and in act with respect to the same object. Thus it would appear that the possible intellect and the agent intellect cannot be united in the same substance of the soul.
This doubt is easily resolved if we examine how the possible intellect is in potency with respect to intelligible objects, and how the agent intellect renders them actually intelligible. The possible intellect is in potency with regard to intelligible objects in the sense that it does not contain within its nature any determinate form of sensible things. In the same way the pupil of the eye is in potency with regard to all colors. To the extent, then, that phantasms abstracted from sensible things are likenesses of definite sensible things, they are related to the possible intellect as act to potency. Nevertheless the phantasms are in potency with regard to something that the intellectual soul possesses in act, namely, being as abstracted from material conditions. And in this respect the intellectual soul is related to the phantasms as act to potency. No contradiction is involved if a thing is in act and potency with regard to the same object according to different points of view. Thus natural bodies act upon each other and are acted upon by each other, for each is in potency with respect to the other. The same intellectual soul, therefore, can be in potency with regard to al I intelligible objects and nevertheless, without any contradiction, can be related to them as act, if both a possible intellect and an agent intellect are acknowledged in the soul.
This will be seen more clearly from the way the intellect renders objects actually intelligible. The agent intellect does not render objects actually intelligible in the sense that the latter flow from it into the possible intellect. If this were the case, we would have no need of phantasms and sense in order to understand. No, the agent intellect renders things actually intelligible by abstracting them from phantasms; just as light, in a certain sense, renders colors actual, not as though it contained the colors within itself, but so far as it confers visibility on them. In the same way we are to judge that there is a single intellectual soul that lacks the natures of sensible things but can receive them in an intelligible manner, and that renders phantasms actually intelligible by abstracting intelligible species from them. The power whereby the soul is able to receive intelligible species is called the possible intellect, and the power whereby it abstracts intelligible species from phantasms is called the agent intellect. The latter is a sort of intelligible light communicated to the intellectual soul, in imitation of what takes place among the higher intellectual substances.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Way These Two Faculties Are United in the Same Essence of the Soul, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).