Objection 1. It would seem that the will is not moved by the intellect. For Augustine says on Psalm 118:20: "My soul hath coveted to long for Thy justifications: The intellect flies ahead, the desire follows sluggishly or not at all: we know what is good, but deeds delight us not." But it would not be so, if the will were moved by the intellect: because movement of the movable results from motion of the mover. Therefore the intellect does not move the will.
Objection 2. Further, the intellect in presenting the appetible object to the will, stands in relation to the will, as the imagination in representing the appetible will to the sensitive appetite. But the imagination, does not remove the sensitive appetite: indeed sometimes our imagination affects us no more than what is set before us in a picture, and moves us not at all (De Anima ii, 3). Therefore neither does the intellect move the will.
Objection 3. Further, the same is not mover and moved in respect of the same thing. But the will moves the intellect; for we exercise the intellect when we will. Therefore the intellect does not move the will.
On the contrary, The Philosopher says (De Anima iii, 10) that "the appetible object is a mover not moved, whereas the will is a mover moved."
I answer that, A thing requires to be moved by something in so far as it is in potentiality to several things; for that which is in potentiality needs to be reduced to act by something actual; and to do this is to move. Now a power of the soul is seen to be in potentiality to different things in two ways: first, with regard to acting and not acting; secondly, with regard to this or that action. Thus the sight sometimes sees actually, and sometimes sees not: and sometimes it sees white, and sometimes black. It needs therefore a mover in two respects, viz. as to the exercise or use of the act, and as to the determination of the act. The first of these is on the part of the subject, which is sometimes acting, sometimes not acting: while the other is on the part of the object, by reason of which the act is specified.
The motion of the subject itself is due to some agent. And since every agent acts for an end, as was shown above (Question 1, Article 2), the principle of this motion lies in the end. And hence it is that the art which is concerned with the end, by its command moves the art which is concerned with the means; just as the "art of sailing commands the art of shipbuilding" (Phys. ii, 2). Now good in general, which has the nature of an end, is the object of the will. Consequently, in this respect, the will moves the other powers of the soul to their acts, for we make use of the other powers when we will. For the end and perfection of every other power, is included under the object of the will as some particular good: and always the art or power to which the universal end belongs, moves to their acts the arts or powers to which belong the particular ends included in the universal end. Thus the leader of an army, who intends the common good--i.e. the order of the whole army--by his command moves one of the captains, who intends the order of one company.
On the other hand, the object moves, by determining the act, after the manner of a formal principle, whereby in natural things actions are specified, as heating by heat. Now the first formal principle is universal "being" and "truth," which is the object of the intellect. And therefore by this kind of motion the intellect moves the will, as presenting its object to it.
Reply to Objection 1. The passage quoted proves, not that the intellect does not move, but that it does not move of necessity.
Reply to Objection 2. Just as the imagination of a form without estimation of fitness or harmfulness, does not move the sensitive appetite; so neither does the apprehension of the true without the aspect of goodness and desirability. Hence it is not the speculative intellect that moves, but the practical intellect (De Anima iii, 9).
Reply to Objection 3. The will moves the intellect as to the exercise of its act; since even the true itself which is the perfection of the intellect, is included in the universal good, as a particular good. But as to the determination of the act, which the act derives from the object, the intellect moves the will; since the good itself is apprehended under a special aspect as contained in the universal true. It is therefore evident that the same is not mover and moved in the same respect.
Source: Summa Theologica (New Advent)