Objection 1. It would seem that free-will is not an appetitive, but a cognitive power. For Damascene (De Fide Orth. ii, 27) says that "free-will straightway accompanies the rational nature." But reason is a cognitive power. Therefore free-will is a cognitive power.
Objection 2. Further, free-will is so called as though it were a free judgment. But to judge is an act of a cognitive power. Therefore free-will is a cognitive power.
Objection 3. Further, the principal function of free-will is to choose. But choice seems to belong to knowledge, because it implies a certain comparison of one thing to another, which belongs to the cognitive power. Therefore free-will is a cognitive power.
On the contrary, The Philosopher says (Ethic. iii, 3) that choice is "the desire of those things which are in us." But desire is an act of the appetitive power: therefore choice is also. But free-will is that by which we choose. Therefore free-will is an appetitive power.
I answer that, The proper act of free-will is choice: for we say that we have a free-will because we can take one thing while refusing another; and this is to choose. Therefore we must consider the nature of free-will, by considering the nature of choice. Now two things concur in choice: one on the part of the cognitive power, the other on the part of the appetitive power. On the part of the cognitive power, counsel is required, by which we judge one thing to be preferred to another: and on the part of the appetitive power, it is required that the appetite should accept the judgment of counsel. Therefore Aristotle (Ethic. vi, 2) leaves it in doubt whether choice belongs principally to the appetitive or the cognitive power: since he says that choice is either "an appetitive intellect or an intellectual appetite." But (Ethic. iii, 3) he inclines to its being an intellectual appetite when he describes choice as "a desire proceeding from counsel." And the reason of this is because the proper object of choice is the means to the end: and this, as such, is in the nature of that good which is called useful: wherefore since good, as such, is the object of the appetite, it follows that choice is principally an act of the appetitive power. And thus free-will is an appetitive power.
Reply to Objection 1. The appetitive powers accompany the apprehensive, and in this sense Damascene says that free-will straightway accompanies the rational power.
Reply to Objection 2. Judgment, as it were, concludes and terminates counsel. Now counsel is terminated, first, by the judgment of reason; secondly, by the acceptation of the appetite: whence the Philosopher (Ethic. iii, 3) says that, "having formed a judgment by counsel, we desire in accordance with that counsel." And in this sense choice itself is a judgment from which free-will takes its name.
Reply to Objection 3. This comparison which is implied in the choice belongs to the preceding counsel, which is an act of reason. For though the appetite does not make comparisons, yet forasmuch as it is moved by the apprehensive power which does compare, it has some likeness of comparison by choosing one in preference to another.
Source: Summa Theologica (New Advent)