Objection 1. It would seem that man has not free-will. For whoever has free-will does what he wills. But man does not what he wills; for it is written (Romans 7:19): "For the good which I will I do not, but the evil which I will not, that I do." Therefore man has not free-will.
Objection 2. Further, whoever has free-will has in his power to will or not to will, to do or not to do. But this is not in man's power: for it is written (Romans 9:16): "It is not of him that willeth"--namely, to will--"nor of him that runneth"--namely, to run. Therefore man has not free-will.
Objection 3. Further, what is "free is cause of itself," as the Philosopher says (Metaph. i, 2). Therefore what is moved by another is not free. But God moves the will, for it is written (Proverbs 21:1): "The heart of the king is in the hand of the Lord; whithersoever He will He shall turn it" and (Philippians 2:13): "It is God Who worketh in you both to will and to accomplish." Therefore man has not free-will.
Objection 4. Further, whoever has free-will is master of his own actions. But man is not master of his own actions: for it is written (Jeremiah 10:23): "The way of a man is not his: neither is it in a man to walk." Therefore man has not free-will.
Objection 5. Further, the Philosopher says (Ethic. iii, 5): "According as each one is, such does the end seem to him." But it is not in our power to be of one quality or another; for this comes to us from nature. Therefore it is natural to us to follow some particular end, and therefore we are not free in so doing.
On the contrary, It is written (Sirach 15:14): "God made man from the beginning, and left him in the hand of his own counsel"; and the gloss adds: "That is of his free-will."
I answer that, Man has free-will: otherwise counsels, exhortations, commands, prohibitions, rewards, and punishments would be in vain. In order to make this evident, we must observe that some things act without judgment; as a stone moves downwards; and in like manner all things which lack knowledge. And some act from judgment, but not a free judgment; as brute animals. For the sheep, seeing the wolf, judges it a thing to be shunned, from a natural and not a free judgment, because it judges, not from reason, but from natural instinct. And the same thing is to be said of any judgment of brute animals. But man acts from judgment, because by his apprehensive power he judges that something should be avoided or sought. But because this judgment, in the case of some particular act, is not from a natural instinct, but from some act of comparison in the reason, therefore he acts from free judgment and retains the power of being inclined to various things. For reason in contingent matters may follow opposite courses, as we see in dialectic syllogisms and rhetorical arguments. Now particular operations are contingent, and therefore in such matters the judgment of reason may follow opposite courses, and is not determinate to one. And forasmuch as man is rational is it necessary that man have a free-will.
Reply to Objection 1. As we have said above (81, 3, ad 2), the sensitive appetite, though it obeys the reason, yet in a given case can resist by desiring what the reason forbids. This is therefore the good which man does not when he wishes--namely, "not to desire against reason," as Augustine says.
Reply to Objection 2. Those words of the Apostle are not to be taken as though man does not wish or does not run of his free-will, but because the free-will is not sufficient thereto unless it be moved and helped by God.
Reply to Objection 3. Free-will is the cause of its own movement, because by his free-will man moves himself to act. But it does not of necessity belong to liberty that what is free should be the first cause of itself, as neither for one thing to be cause of another need it be the first cause. God, therefore, is the first cause, Who moves causes both natural and voluntary. And just as by moving natural causes He does not prevent their acts being natural, so by moving voluntary causes He does not deprive their actions of being voluntary: but rather is He the cause of this very thing in them; for He operates in each thing according to its own nature.
Reply to Objection 4. "Man's way" is said "not to be his" in the execution of his choice, wherein he may be impeded, whether he will or not. The choice itself, however, is in us, but presupposes the help of God.
Reply to Objection 5. Quality in man is of two kinds: natural and adventitious. Now the natural quality may be in the intellectual part, or in the body and its powers. From the very fact, therefore, that man is such by virtue of a natural quality which is in the intellectual part, he naturally desires his last end, which is happiness. Which desire, indeed, is a natural desire, and is not subject to free-will, as is clear from what we have said above (82, 1,2). But on the part of the body and its powers man may be such by virtue of a natural quality, inasmuch as he is of such a temperament or disposition due to any impression whatever produced by corporeal causes, which cannot affect the intellectual part, since it is not the act of a corporeal organ. And such as a man is by virtue of a corporeal quality, such also does his end seem to him, because from such a disposition a man is inclined to choose or reject something. But these inclinations are subject to the judgment of reason, which the lower appetite obeys, as we have said (81, 3). Wherefore this is in no way prejudicial to free-will.
The adventitious qualities are habits and passions, by virtue of which a man is inclined to one thing rather than to another. And yet even these inclinations are subject to the judgment of reason. Such qualities, too, are subject to reason, as it is in our power either to acquire them, whether by causing them or disposing ourselves to them, or to reject them. And so there is nothing in this that is repugnant to free-will.
Source: Summa Theologica (New Advent)