Miscellanies 1.17.83

And let them say, then, that not Hector burned the ships of the Greeks, but Achilles, because, being able to prevent Hector, he did not prevent him. But he, out of wrath (and it was in his power to be wrathful or not), did not keep off the fire, and is perhaps a joint cause; whereas the devil, having free will, was able both to repent and to steal, and he himself is the cause of the theft, not the Lord who did not prevent it. Nor was the gift harmful, so that prevention was needed. If we must be exact with them, let them know that what does not prevent, which we say happened in the case of the theft, is no cause at all; but what prevents is liable to the charge of being a cause. For the one who holds a shield before another is the cause of his not being wounded, preventing him from being wounded; and Socrates' divine sign was a cause, not by not preventing, but by turning him away, even if it did not urge him on. Neither praises nor blames, neither honors nor punishments would be just if the soul did not have the power of impulse and aversion.