In "The City of God" (Bk 12, ch. 7), Augustine famously argued that it is a mistake to look for the active, effective cause of the wills consent to evil. Moral evil occurs, he argued, because of a defective will, not an effective one, and he said that it made no more sense to look for an intelligible cause of sin than to desire to see darkness or to hear silence. But Augustine also set out the same view in at least three other places.
The Apostles comment, "That which I work, I do not know," can seem to those who dont understand to be contrary to his other statement, "Sin, that it may appear sin, by that which is good, wrought death in me" (Rm 7:, 15 and 13). For how can it appear if it is not known? But "I do not know" here means, "I do not approve." Just as darkness is not seen but is felt by comparison with light, so that to sense darkness is not to see, so also sin, because it is not illumined by the light of justice, is discerned by its not being understood, just as darkness is said to be sensed by its not being seen. That is the point of the Psalmist: "Who can understand sins?" (Ps 18:13). (Augustine, Expos. on Romans, 43: PL 35, 2071)"From my secret things cleanse me, O Lord." For "who can understand sins?" When you can see darkness youll be able to understand sins. (Augustine, En. in Ps 18, sermon 2, 13; PL 36, 162)"Who understands sins" (Ps 18:13). What sweetness can there be in sins when there is no understanding of them? How can anyone understand sins when they close the very eye to which truth is sweet, to which Gods commands are desirable and sweet? Just as darkness closes the eyes, so sins close the mind and do not allow it to see the light, nor to see itself. (Augustine, Enar. In Ps 18, sermon 1, 13; PL 36, 156)