To Florus 1.47

It is, however, a movement of the mind of someone who is old enough to use rational judgment. When punishment and glory, or on the other side advantage and pleasure, are set before that person, help and, as it were, an opportunity are offered; neither alternative is imposed by necessity. This will, capable of turning either way, received the source of its capacity in free choice. But it brings the actual deed into existence from itself. There is no act of will at all before a person wills, nor can a person will before being capable also of refusing. In the case of sin, a person does not possess both capacities, willing and refusing, before acquiring the use of reason. Taken together, these points show that your definition was entirely true: “Sin is the will to retain or commit something justice forbids and from which one is free to abstain” (Augustine, On Two Souls 15).

This sin, which has been shown to be nothing but will, plainly receives its genus, that is, its very origin, from one's own desire. Its species is then found in each individual, those who are called indivisibles. Its difference lies in the variety of faults and the circumstances of times. Its measure is its very lack of measure. For if the proper measure is to serve the one you ought to serve, whoever neglects this sins by crossing the true limit. Yet one may make the subtle point that sin has a measure because no one sins beyond what he can do. For even if someone sins beyond his powers with an ineffectual will, he was still capable of that very act of willing alone. To vice we must assign a quality through which it shows the bitterness it brings, whether through disgrace or pain. Sin therefore exists; if it did not, you would not pursue your errors. It is nothing other than a will leaving the path on which it ought to remain and from which it is free not to turn aside. It arises from desire for forbidden things, and it exists nowhere except in a person who both had an evil will and could have refrained from having one.