“For evil has no positive nature; but the loss of good has received the name “evil.””

XI, 9
The City of God (early 400s)
Context: For when God said, “Let there be light, and there was light,” if we are justified in understanding in this light the creation of the angels, then certainly they were created partakers of the eternal light which is the unchangeable Wisdom of God, by which all things were made, and whom we call the only-begotten Son of God; so that they, being illumined by the Light that created them, might themselves become light and be called “Day,” in participation of that unchangeable Light and Day which is the Word of God, by whom both themselves and all else were made. “The true Light, which lighteth every man that cometh into the world,” — this Light lighteth also every pure angel, that he may be light not in himself, but in God; from whom if an angel turn away, he becomes impure, as are all those who are called unclean spirits, and are no longer light in the Lord, but darkness in themselves, being deprived of the participation of Light eternal. For evil has no positive nature; but the loss of good has received the name “evil.”