Objection 1. It would seem that free-will is not a power. For free-will is nothing but a free judgment. But judgment denominates an act, not a power. Therefore free-will is not a power.
Objection 2. Further, free-will is defined as "the faculty of the will and reason." But faculty denominates a facility of power, which is due to a habit. Therefore free-will is a habit. Moreover Bernard says (De Gratia et Lib. Arb. 1,2) that free-will is "the soul's habit of disposing of itself." Therefore it is not a power.
Objection 3. Further, no natural power is forfeited through sin. But free-will is forfeited through sin; for Augustine says that "man, by abusing free-will, loses both it and himself." Therefore free-will is not a power.
On the contrary, Nothing but a power, seemingly, is the subject of a habit. But free-will is the subject of grace, by the help of which it chooses what is good. Therefore free-will is a power.
I answer that, Although free-will [Liberum arbitrium--i.e. free judgment] in its strict sense denotes an act, in the common manner of speaking we call free-will, that which is the principle of the act by which man judges freely. Now in us the principle of an act is both power and habit; for we say that we know something both by knowledge and by the intellectual power. Therefore free-will must be either a power or a habit, or a power with a habit. That it is neither a habit nor a power together with a habit, can be clearly proved in two ways.
First of all, because, if it is a habit, it must be a natural habit; for it is natural to man to have a free-will. But there is not natural habit in us with respect to those things which come under free-will: for we are naturally inclined to those things of which we have natural habits--for instance, to assent to first principles: while those things which we are naturally inclined are not subject to free-will, as we have said of the desire of happiness (82, 1,2). Wherefore it is against the very notion of free-will that it should be a natural habit. And that it should be a non-natural habit is against its nature. Therefore in no sense is it a habit.
Secondly, this is clear because habits are defined as that "by reason of which we are well or ill disposed with regard to actions and passions" (Ethic. ii, 5); for by temperance we are well-disposed as regards concupiscences, and by intemperance ill-disposed: and by knowledge we are well-disposed to the act of the intellect when we know the truth, and by the contrary ill-disposed. But the free-will is indifferent to good and evil choice: wherefore it is impossible for free-will to be a habit. Therefore it is a power.
Reply to Objection 1. It is not unusual for a power to be named from its act. And so from this act, which is a free judgment, is named the power which is the principle of this act. Otherwise, if free-will denominated an act, it would not always remain in man.
Reply to Objection 2. Faculty sometimes denominates a power ready for operation, and in this sense faculty is used in the definition of free-will. But Bernard takes habit, not as divided against power, but as signifying a certain aptitude by which a man has some sort of relation to an act. And this may be both by a power and by a habit: for by a power man is, as it were, empowered to do the action, and by the habit he is apt to act well or ill.
Reply to Objection 3. Man is said to have lost free-will by falling into sin, not as to natural liberty, which is freedom from coercion, but as regards freedom from fault and unhappiness. Of this we shall treat later in the treatise on Morals in the second part of this work (I-II, 85, seqq.; 109).
Source: Summa Theologica (New Advent)