Metaphysically, in the classical and Western tradition of Christianity, evil is defined as the absence or lack of a good quality in a thing or being. When we say a knife is dull, we are saying that it is lacking the good quality of sharpness and the ability to cut. When someone chooses to act morally reprehensibly, we are saying that something good is lacking in the individual. Evil can only be measured against what is good. In his Confessions, St. Augustine defined evil as a privation of goodness. Evil is not a positive substance but is the absence of being and goodness (as darkness is the absence of light). Evil can only be parasitic on the good (that is, the prior conception of good is always needed in order to determine something evil). The world and human souls are seen as created by the highest being, God (who is goodness), “from above,” but at the same time as corruptible by nonbeing (or evil) “from below.” God is, thus, responsible for the isness and goodness in the world, but not the nonbeing and evil. According to the classical Christian understanding of evil, the universe is good and is the creation of a good God for a good purpose. Evil—whether it be moral (rooted in the will resulting in pain and suffering), or natural evil—is not placed there by God but represents the distortion of something inherently good. Evil is always ontologically nothing or a privation of goodness.

Here we are only dealing with the metaphysics of evil and what kind of thing it might be. Of course other issues regarding evil remain.

For further reading consult the works of St. Augustine. His Confessions, The Enchiridion on Faith, Hope, and Love, and On Free Choice of the Will are very helpful.