Objection 1. It would seem that use is not an act of the will. For Augustine says (De Doctr. Christ. i, 4) that "to use is to refer that which is the object of use to the obtaining of something else." But "to refer" something to another is an act of the reason to which it belongs to compare and to direct. Therefore use is an act of the reason and not of the will.
Objection 2. Further, Damascene says (De Fide Orth. ii, 22) that man "goes forward to the operation, and this is called impulse; then he makes use (of the powers) and this is called use." But operation belongs to the executive power; and the act of the will does not follow the act of the executive power, on the contrary execution comes last. Therefore use is not an act of the will.
Objection 3. Further, Augustine says (QQ. 83, qu. 30): "All things that were made were made for man's use, because reason with which man is endowed uses all things by its judgment of them." But judgment of things created by God belongs to the speculative reason; which seems to be altogether distinct from the will, which is the principle of human acts. Therefore use is not an act of the will.
On the contrary, Augustine says (De Trin. x, 11): "To use is to apply to something to purpose of the will."
I answer that, The use of a thing implies the application of that thing to an operation: hence the operation to which we apply a thing is called its use; thus the use of a horse is to ride, and the use of a stick is to strike. Now we apply to an operation not only the interior principles of action, viz. the powers of the soul or the members of the body; as the intellect, to understand; and the eye, to see; but also external things, as a stick, to strike. But it is evident that we do not apply external things to an operation save through the interior principles which are either the powers of the soul, or the habits of those powers, or the organs which are parts of the body. Now it has been shown above (Question 9, Article 01) that it is the will which moves the soul's powers to their acts, and this is to apply them to operation. Hence it is evident that first and principally use belongs to the will as first mover; to the reason, as directing; and to the other powers as executing the operation, which powers are compared to the will which applies them to act, as the instruments are compared to the principal agent. Now action is properly ascribed, not to the instrument, but to the principal agent, as building is ascribed to the builder, not to his tools. Hence it is evident that use is, properly speaking, an act of the will.
Reply to Objection 1. Reason does indeed refer one thing to another; but the will tends to that which is referred by the reason to something else. And in this sense to use is to refer one thing to another.
Reply to Objection 2. Damascene is speaking of use in so far as it belongs to the executive powers.
Reply to Objection 3. Even the speculative reason is applied by the will to the act of understanding or judging. Consequently the speculative reason is said to use, in so far as it is moved by the will, in the same way as the other powers.
Source: Summa Theologica (New Advent)