As has often been suggested by atheists and theists
alike, the argument from evil is probably the single greatest intellectual threat
to the truth of Christian theism. In its most basic form the argument from evil
derives from the following propositions:
3. Evil exists.
A reductio ad absurdum is said to result when one simply adds a fourth proposition, "God exists." That is, the set becomes logically inconsistent. Epicurus famously said it like this: "Is God willing but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil?" This in a nutshell is the logical argument from evil.
The logical problem of evil has fallen on hard times. Ever since Alvin Plantinga published his "Free Will Defense," the burden of proof has shifted to the atheologian, to show the logical inconsistency of the set of premises constituting the problem of evil. Perhaps surprisingly to some, this has been done to almost no one's satisfaction. Plantinga points out that to derive a formal contradiction (as the logical argument must) would require additional premises, such as "God is not all powerful," or "Evil does not exist." It may seem counterintuitive that evil could coexist with an all-good, all-powerful God – all the more so during times of intense suffering – but it's not illogical, strictly speaking.
So long as it is possible for there to be an overriding justification for God to permit evil that creates a greater good, suggests Plantinga, it is possible for God and evil to coexist. Moral free will is one such possible justification. My fellow blogger here, Joe Hinman, has made a good case for the moral necessity of free will (see his "Soteriological Drama" post) as a valid justification for evil in the world (even if it necessarily entails risk of evil and suffering).
Now let's turn the tables for a moment and consider an argument from evil against naturalism:
1. Nature is all that exists.
2. Nature is amoral (neither good nor evil).
3. Evil exists.
Here we have another reductio. For nature to be all that exists, and nature to be non-evil, evil cannot exist. Alternatively, it may be that naturalism is not all that exists, or that nature is not amoral. Either way, given the reality of evil naturalism as commonly understood is false.
Or at least that's how it appears at a glance. Of course, naturalists can get around this by explaining how evil doesn’t really exist, how nature is not amoral, or how evil is a mere by-product or epiphenomenon of naturalistic evolution. But then that would clearly take the sting out of the atheistic argument from evil. For if moral evil can emerge as a by-product of a thoroughly amoral natural system, there's no reason to think it can't emerge as a by-product of a thoroughly good world, inhabited by free moral agents under the supervision of a thoroughly good God. So at worst evil is no more a threat to theism than it is to naturalism.