Objection 1. It would seem that the act of the will is not commanded. For Augustine says (Confess. viii, 9): "The mind commands the mind to will, and yet it does not." But to will is the act of the will. Therefore the act of the will is not commanded.
Objection 2. Further, to receive a command belongs to one who can understand the command. But the will cannot understand the command; for the will differs from the intellect, to which it belongs to understand. Therefore the act of the will is not commanded.
Objection 3. Further, if one act of the will is commanded, for the same reason all are commanded. But if all the acts of the will are commanded, we must needs proceed to infinity; because the act of the will precedes the act of reason commanding, as stated above (Article 1); for if that act of the will be also commanded, this command will be precedes by another act of the reason, and so on to infinity. But to proceed to infinity is not possible. Therefore the act of the will is not commanded.
On the contrary, Whatever is in our power, is subject to our command. But the acts of the will, most of all, are in our power; since all our acts are said to be in our power, in so far as they are voluntary. Therefore the acts of the will are commanded by us.
I answer that, As stated above (Article 1), command is nothing else than the act of the reason directing, with a certain motion, something to act. Now it is evident that the reason can direct the act of the will: for just as it can judge it to be good to will something, so it can direct by commanding man to will. From this it is evident that an act of the will can be commanded.
Reply to Objection 1. As Augustine says (Confess. viii, 9) when the mind commands itself perfectly to will, then already it wills: but that sometimes it commands and wills not, is due to the fact that it commands imperfectly. Now imperfect command arises from the fact that the reason is moved by opposite motives to command or not to command: wherefore it fluctuates between the two, and fails to command perfectly.
Reply to Objection 2. Just as each of the members of the body works not for itself alone but for the whole body; thus it is for the whole body that the eye sees; so is it with the powers of the soul. For the intellect understands, not for itself alone, but for all the powers; and the will wills not only for itself, but for all the powers too. Wherefore man, in so far as he is endowed with intellect and will, commands the act of the will for himself.
Reply to Objection 3. Since command is an act of reason, that act is commanded which is subject to reason. Now the first act of the will is not due to the direction of the reason but to the instigation of nature, or of a higher cause, as stated above (Question 9, Article 4). Therefore there is no need to proceed to infinity.
Source: Summa Theologica (New Advent)