Evodius. Now, if possible, explain to me why God gave man free choice of the will, since if he had not received it he would not be able to sin.

Augustine. Are you perfectly sure that God gave to man what you think ought not to have been given?

E. As far as I seem to understand the discussion in the first book, we have freedom of will, and could not sin if we were without it.

A. I, too, remember that this was made clear to us. But I just asked you whether you know that it was God who gave us that which we possess, through which it is clear that we commit sin.

E. No one else. For we are from Him, and whether we sin or whether we do right, we earn reward or punishment from Him.

A. I want to ask, as well: do you know this clearly, or do you believe it willingly without really knowing it, because you are prompted by authority?

E. I admit that at first I trusted authority on this point. But what can be more true than that all good proceeds from God, that everything just is good, and that it is just to punish sinners and to reward those who do right? From this it follows that through God sinners are afflicted with unhappiness, and those who do right endowed with happiness.

A. I do not object, but let me ask another question: how do you know that we are from God? You did not answer that: instead, you explained that we merit punishment and reward from God.

E. The answer to that question, too, is clear, if for no other reason than the fact that, as we have already agreed, God punishes sins. All justice is from God, and it is not the role of justice to punish foreigners, although it is the role of goodness to bestow gifts on them. Thus it is clear that we belong to God, since he is not only most generous in bestowing benefits upon us, but also most just in punishing us. Also, we can understand that man is from God through the fact, which I proposed and you conceded, that every good is from God. For man himself, insofar as he is a man, is a good, because he can live rightly when he so wills.

A. If this is so, the question that you proposed is clearly answered. If man is a good, and cannot act rightly unless he wills to do so, then he must have free will, without which he cannot act rightly. We must not believe that God gave us free will so that we might sin, just because sin is committed through free will. It is sufficient for our question, why free will should have been given to man, to know that without it man cannot live rightly. That it was given for this reason can be understood from the following: if anyone uses free will for sinning, he incurs divine punishment. This would be unjust if free will had been given not only that man might live rightly, but also that he might sin. For how could a man justly incur punishment who used free will to do the thing for which it was given? When God punishes a sinner, does He not seem to say, "Why have you not used free will for the purpose for which I gave it to you, to act rightly?" Then too, if man did not have free choice of will, how could there exist the good according to which it is just to condemn evildoers and reward those who act rightly? What was not done by will would be neither evildoing nor right action. Both punishment and reward would be unjust if man did not have free will. Moreover, there must needs be justice both in punishment and in reward, since justice is one of the goods that are from God. Therefore, God must needs have given free will to man.

--Augustine, On Free Choice of the Will, Book Two, 1-7