Objection 1. It would seem that God does not immediately move the created intellect. For the action of the intellect is governed by its own subject; since it does not pass into external matter; as stated in Metaph. ix, Did. viii, 8. But the action of what is moved by another does not proceed from that wherein it is; but from the mover. Therefore the intellect is not moved by another; and so apparently God cannot move the created intellect.
Objection 2. Further, anything which in itself is a sufficient principle of movement, is not moved by another. But the movement of the intellect is its act of understanding; in the sense in which we say that to understand or to feel is a kind of movement, as the Philosopher says (De Anima iii, 7). But the intellectual light which is natural to the soul, is a sufficient principle of understanding. Therefore it is not moved by another.
Objection 3. Further, as the senses are moved by the sensible, so the intellect is moved by the intelligible. But God is not intelligible to us, and exceeds the capacity of our intellect. Therefore God cannot move our intellect.
On the contrary, The teacher moves the intellect of the one taught. But it is written (Psalm 93:10) that God "teaches man knowledge." Therefore God moves the human intellect.
I answer that, As in corporeal movement that is called the mover which gives the form that is the principle of movement, so that is said to move the intellect, which is the cause of the form that is the principle of the intellectual operation, called the movement of the intellect. Now there is a twofold principle of intellectual operation in the intelligent being; one which is the intellectual power itself, which principle exists in the one who understands in potentiality; while the other is the principle of actual understanding, namely, the likeness of the thing understood in the one who understands. So a thing is said to move the intellect, whether it gives to him who understands the power of understanding; or impresses on him the likeness of the thing understood.
Now God moves the created intellect in both ways. For He is the First immaterial Being; and as intellectuality is a result of immateriality, it follows that He is the First intelligent Being. Therefore since in each order the first is the cause of all that follows, we must conclude that from Him proceeds all intellectual power. In like manner, since He is the First Being, and all other beings pre-exist in Him as in their First Cause, it follows that they exist intelligibly in Him, after the mode of His own Nature. For as the intelligible types of everything exist first of all in God, and are derived from Him by other intellects in order that these may actually understand; so also are they derived by creatures that they may subsist. Therefore God so moves the created intellect, inasmuch as He gives it the intellectual power, whether natural, or superadded; and impresses on the created intellect the intelligible species, and maintains and preserves both power and species in existence.
Reply to Objection 1. The intellectual operation is performed by the intellect in which it exists, as by a secondary cause; but it proceeds from God as from its first cause. For by Him the power to understand is given to the one who understands.
Reply to Objection 2. The intellectual light together with the likeness of the thing understood is a sufficient principle of understanding; but it is a secondary principle, and depends upon the First Principle.
Reply to Objection 3. The intelligible object moves our human intellect, so far as, in a way, it impresses on it its own likeness, by means of which the intellect is able to understand it. But the likenesses which God impresses on the created intellect are not sufficient to enable the created intellect to understand Him through His Essence, as we have seen above (12, 2; 56, 3). Hence He moves the created intellect, and yet He cannot be intelligible to it, as we have explained (12, 4).
Source: Summa Theologica (New Advent)