To Florus 5.51

Unless the capacity had existed necessarily, its possible result would not have existed. That a person can do evil and good is therefore necessary; his doing evil, however, is owed not to his necessity but to his possibility. Where either side is possible, neither is necessary. Thus sin is nothing other than the will to do what justice forbids and from which one is free to abstain. If will is nothing other than a movement of the mind under no compulsion, then just as God possessed the possibility of making the world, while the world itself received existence under necessity, so something similar is understood in God’s image. A person is not compelled to possess the will he chooses; it comes to him from possibility. The evil he has done, however, carries the necessity of guilt. Thus a crime necessarily incurs its abhorrence, although it arose not from the doer’s necessity but from his possibility. The work of possibility therefore testifies to a free living being.