Objection 1. It would seem that the will does not move the intellect. For what moves excels and precedes what is moved, because what moves is an agent, and "the agent is nobler than the patient," as Augustine says (Gen. ad lit. xii, 16), and the Philosopher (De Anima iii, 5). But the intellect excels and precedes the will, as we have said above (Article 3). Therefore the will does not move the intellect.
Objection 2. Further, what moves is not moved by what is moved, except perhaps accidentally. But the intellect moves the will, because the good apprehended by the intellect moves without being moved; whereas the appetite moves and is moved. Therefore the intellect is not moved by the will.
Objection 3. Further, we can will nothing but what we understand. If, therefore, in order to understand, the will moves by willing to understand, that act of the will must be preceded by another act of the intellect, and this act of the intellect by another act of the will, and so on indefinitely, which is impossible. Therefore the will does not move the intellect.
On the contrary, Damascene says (De Fide Orth. ii, 26): "It is in our power to learn an art or not, as we list." But a thing is in our power by the will, and we learn art by the intellect. Therefore the will moves the intellect.
I answer that, A thing is said to move in two ways:
First, as an end; for instance, when we say that the end moves the agent. In this way the intellect moves the will, because the good understood is the object of the will, and moves it as an end.
Secondly, a thing is said to move as an agent, as what alters moves what is altered, and what impels moves what is impelled. In this way the will moves the intellect and all the powers of the soul, as Anselm says (Eadmer, De Similitudinibus). The reason is, because wherever we have order among a number of active powers, that power which regards the universal end moves the powers which regard particular ends. And we may observe this both in nature and in things politic. For the heaven, which aims at the universal preservation of things subject to generation and corruption, moves all inferior bodies, each of which aims at the preservation of its own species or of the individual. The king also, who aims at the common good of the whole kingdom, by his rule moves all the governors of cities, each of whom rules over his own particular city. Now the object of the will is good and the end in general, and each power is directed to some suitable good proper to it, as sight is directed to the perception of color, and the intellect to the knowledge of truth. Therefore the will as agent moves all the powers of the soul to their respective acts, except the natural powers of the vegetative part, which are not subject to our will.
Reply to Objection 1. The intellect may be considered in two ways: as apprehensive of universal being and truth, and as a thing and a particular power having a determinate act. In like manner also the will may be considered in two ways: according to the common nature of its object--that is to say, as appetitive of universal good--and as a determinate power of the soul having a determinate act. If, therefore, the intellect and the will be compared with one another according to the universality of their respective objects, then, as we have said above (Article 3), the intellect is simply higher and nobler than the will. If, however, we take the intellect as regards the common nature of its object and the will as a determinate power, then again the intellect is higher and nobler than the will, because under the notion of being and truth is contained both the will itself, and its act, and its object. Wherefore the intellect understands the will, and its act, and its object, just as it understands other species of things, as stone or wood, which are contained in the common notion of being and truth. But if we consider the will as regards the common nature of its object, which is good, and the intellect as a thing and a special power; then the intellect itself, and its act, and its object, which is truth, each of which is some species of good, are contained under the common notion of good. And in this way the will is higher than the intellect, and can move it. From this we can easily understand why these powers include one another in their acts, because the intellect understands that the will wills, and the will wills the intellect to understand. In the same way good is contained in truth, inasmuch as it is an understood truth, and truth in good, inasmuch as it is a desired good.
Reply to Objection 2. The intellect moves the will in one sense, and the will moves the intellect in another, as we have said above.
Reply to Objection 3. There is no need to go on indefinitely, but we must stop at the intellect as preceding all the rest. For every movement of the will must be preceded by apprehension, whereas every apprehension is not preceded by an act of the will; but the principle of counselling and understanding is an intellectual principle higher than our intellect --namely, God--as also Aristotle says (Eth. Eudemic. vii, 14), and in this way he explains that there is no need to proceed indefinitely.
Source: Summa Theologica (New Advent)