If evil is just a "lack" of good, then evil isn't real and the problem is dodged

Where the idea comes from

Augustine, a fourth-century thinker who wrote about his own spiritual search in Confessions, had once believed (with the Manichees) that evil was a rival power, equal to good. He came to reject that. If God created everything and everything he created is good, then evil can't be a created substance. Instead, it's a corruption or twisting of good things. Thomas Aquinas later developed the same view. C.S. Lewis put it simply in Mere Christianity: evil is a parasite, not an original thing.

Privation doesn't mean unreal

The objection assumes that if evil is an absence, it isn't real. But absences can be devastatingly real. Blindness is a lack of sight; it is not an illusion. A hole in a ship's hull is a lack of wood, and it can sink the ship. Starvation is a lack of food, and it kills. Saying evil is a privation is a claim about what kind of thing evil is, not about whether it's real or serious.

Why it matters

The privation view has some important consequences. First, it means evil is always parasitic on good. Cruelty uses real goods like intelligence, strength and will, and bends them toward harm. A torturer needs a working mind and body. Evil can't create; it can only spoil. Second, it means good is more fundamental than evil. That is why the world is not a battle between equal forces, and why Christians can hope that evil will one day be ended without anything good being lost.

Doesn't this let God off the hook?

No, and serious defenders never claimed it did. The privation view explains what evil is. It doesn't explain why God allows it. That is a separate question, addressed by responses like the free will defence and soul-making (see The problem of evil and Why does God allow suffering?). John Hick, in Evil and the God of Love, traced both the Augustinian and the Irenaean approaches and argued we need both: a view of what evil is and a view of why it is permitted.

What about pain?

Some point out that pain feels positive, not like an absence. That is a fair challenge. Many defenders answer that pain itself is a good warning system, and that what makes suffering evil is the disorder of the situation: bodies breaking, relationships ruined, wills turned against others.

A fair concession

The language of "privation" can sound abstract or even dismissive to someone in pain. It's a philosophical tool, not a pastoral one, and it should be used carefully.

How to explain this to a friend

When explaining this to a friend, use concrete examples rather than philosophical terms. Rot is a lack of health in fruit. A lie is a twisting of speech, which is good in itself. Cruelty is strength and intelligence aimed the wrong way. Once people see that evil always spoils something good rather than standing on its own, the idea stops sounding like a word game and starts to make sense.

The bottom line

Calling evil a privation doesn't make it unreal. Blindness, holes and starvation are all absences that do real harm. The point is that evil is a corruption of something good, which means good is deeper and evil can be defeated. For more on what evil is, see What is evil?