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.