Objection 1. It would seem that free-will is a power distinct from the will. For Damascene says (De Fide Orth. ii, 22) that thelesis is one thing and boulesis another. But thelesis is the will, while boulesis seems to be the free-will, because boulesis, according to him, is will as concerning an object by way of comparison between two things. Therefore it seems that free-will is a distinct power from the will.
Objection 2. Further, powers are known by their acts. But choice, which is the act of free-will, is distinct from the act of willing, because "the act of the will regards the end, whereas choice regards the means to the end" (Ethic. iii, 2). Therefore free-will is a distinct power from the will.
Objection 3. Further, the will is the intellectual appetite. But in the intellect there are two powers--the active and the passive. Therefore, also on the part of the intellectual appetite, there must be another power besides the will. And this, seemingly, can only be free-will. Therefore free-will is a distinct power from the will.
On the contrary, Damascene says (De Fide Orth. iii, 14) free-will is nothing else than the will.
I answer that, The appetitive powers must be proportionate to the apprehensive powers, as we have said above (Question 64, Article 2). Now, as on the part of the intellectual apprehension we have intellect and reason, so on the part of the intellectual appetite we have will, and free-will which is nothing else but the power of choice. And this is clear from their relations to their respective objects and acts. For the act of "understanding" implies the simple acceptation of something; whence we say that we understand first principles, which are known of themselves without any comparison. But to "reason," properly speaking, is to come from one thing to the knowledge of another: wherefore, properly speaking, we reason about conclusions, which are known from the principles. In like manner on the part of the appetite to "will" implies the simple appetite for something: wherefore the will is said to regard the end, which is desired for itself. But to "choose" is to desire something for the sake of obtaining something else: wherefore, properly speaking, it regards the means to the end. Now, in matters of knowledge, the principles are related to the conclusion to which we assent on account of the principles: just as, in appetitive matters, the end is related to the means, which is desired on account of the end. Wherefore it is evident that as the intellect is to reason, so is the will to the power of choice, which is free-will. But it has been shown above (Question 79, Article 8) that it belongs to the same power both to understand and to reason, even as it belongs to the same power to be at rest and to be in movement. Wherefore it belongs also to the same power to will and to choose: and on this account the will and the free-will are not two powers, but one.
Reply to Objection 1. Boulesis is distinct from thelesis on account of a distinction, not of powers, but of acts.
Reply to Objection 2. Choice and will--that is, the act of willing --are different acts: yet they belong to the same power, as also to understand and to reason, as we have said.
Reply to Objection 3. The intellect is compared to the will as moving the will. And therefore there is no need to distinguish in the will an active and a passive will.
Source: Summa Theologica (New Advent)