The Consolation of Philosophy Book IV

For a thing is which maintains its place in nature and acts in accord with its nature. Whatever fails to do this loses the existence which is proper to its nature. But you may argue that evil men are capable of action. I will not deny it, but such capability is the product of weakness, not of strength. For they can do evil acts which they could not have done if they had been able to remain capable of good. And that possibility of doing evil shows clearly that they can do nothing. For, if our earlier conclusion that evil is nothing still stands, it is clear that the wicked can do nothing since they can do only evil.