Augustine writes about the problem of evil and describes evil as the absence of good.
IV. The Problem of Evil
A. Refutation of Dualism (C.S. Lewis)
1. How do you know which god is the good god?
2. There must be a higher standard by which we judge.
3. The higher standard must be good, because all evil is a corruption of some higher good.
4. You cannot have two gods; God must be the only standard.
B. Evil is the absence of good.
C. "The Free Will Defense"