Objection 1. It would seem that command is not an act of the reason but of the will. For command is a kind of motion; because Avicenna says that there are four ways of moving, "by perfecting, by disposing, by commanding, and by counselling." But it belongs to the will to move all the other powers of the soul, as stated above (Question 9, Article 1). Therefore command is an act of the will.
Objection 2. Further, just as to be commanded belongs to that which is subject, so, seemingly, to command belongs to that which is most free. But the root of liberty is especially in the will. Therefore to command belongs to the will.
Objection 3. Further, command is followed at once by act. But the act of the reason is not followed at once by act: for he who judges that a thing should be done, does not do it at once. Therefore command is not an act of the reason, but of the will.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xvi.] and the Philosopher (Ethic. i, 13) say that "the appetite obeys reason." Therefore command is an act of the reason.
I answer that, Command is an act of the reason presupposing, however, an act of the will. In proof of this, we must take note that, since the acts of the reason and of the will can be brought to bear on one another, in so far as the reason reasons about willing, and the will wills to reason, the result is that the act of the reason precedes the act of the will, and conversely. And since the power of the preceding act continues in the act that follows, it happens sometimes that there is an act of the will in so far as it retains in itself something of an act of the reason, as we have stated in reference to use and choice; and conversely, that there is an act of the reason in so far as it retains in itself something of an act of the will.
Now, command is essentially indeed an act of the reason: for the commander orders the one commanded to do something, by way of intimation or declaration; and to order thus by intimating or declaring is an act of the reason. Now the reason can intimate or declare something in two ways. First, absolutely: and this intimation is expressed by a verb in the indicative mood, as when one person says to another: "This is what you should do." Sometimes, however, the reason intimates something to a man by moving him thereto; and this intimation is expressed by a verb in the imperative mood; as when it is said to someone: "Do this." Now the first mover, among the powers of the soul, to the doing of an act is the will, as stated above (Question 9, Article 1). Since therefore the second mover does not move, save in virtue of the first mover, it follows that the very fact that the reason moves by commanding, is due to the power of the will. Consequently it follows that command is an act of the reason, presupposing an act of the will, in virtue of which the reason, by its command, moves (the power) to the execution of the act.
Reply to Objection 1. To command is to move, not anyhow, but by intimating and declaring to another; and this is an act of the reason.
Reply to Objection 2. The root of liberty is the will as the subject thereof; but it is the reason as its cause. For the will can tend freely towards various objects, precisely because the reason can have various perceptions of good. Hence philosophers define the free-will as being "a free judgment arising from reason," implying that reason is the root of liberty.
Reply to Objection 3. This argument proves that command is an act of reason not absolutely, but with a kind of motion as stated above.
Source: Summa Theologica (New Advent)