Here’s a nice challenge: What is the fundamental problem of evil? If there is a fundamental problem of evil P, then there is some organization among the many proposed problems of evil. That would be good news. Let a fundamental problem of evil P be such that (i) a solution to P would solve all other problems of evil and (ii) a solution to other problems of evil would not (necessarily) solve P. Let’s say that a problem of evil P is more fundamental than a problem P’ just in case (i) a solution to P would solve P and (ii) a solution to P’ would not (necessarily) solve P.
The challenge in the logical problem of evil is to show that it is metaphysically possible that God co-exists with evil, E. There are various ways to take the challenge; one natural way is to take it as asserting that it is impossible that God cannot eliminate E without a cost of a greater good G. All possible evil is gratuitous.
The challenge of the evidential argument is to show that God coexists with all actual evil. One natural way to take the challenge is as asserting that there is some actual evil E that God can eliminate without the cost of any greater good, G. The evidential problem has given rise to a host of epistemological replies concerning how we might know that God could eliminate E without the cost of G. These are not solutions to the problem; they are dissolutions: they remove the challenge without showing that, for any actual evil E, there is a greater good G such that G entails E. But if it could be shown that there is a solution to the evidential problem of evil, then there would be a solution to the logical problem of evil.
But the fundamental problem of evil, I’d propose, is the modal problem of evil. The challenge in the modal problem of evil is to show that, in every possible world W, if E exists in W then God co-exists with E. One natural way to take the modal problem is as asserting that there is a possible world W and some evil E in W such that God can eliminate E without a cost of a greater good G.
It is useful to contrast the logical problem of evil with the modal problem of evil. The logical problem of evil claims that every evil in every world in which they occur–in the vastness of metaphysical space–is gratuitous. To show that that’s mistaken we need to show that just one instance of evil in just one world is not gratuitous. FWD addresses this problem. But the modal problem of evil claims that there is some evil, in some possible world–in the vastness of metaphysical space–that is gratuitous. To show that that’s mistaken we need to show that there are no instances of evil in any possible world that are gratuitous. FWD is useless here. Indeed, just the sort of theistic defense that FWD presents can be used as an atheistic defense in the modal problem of evil. For all we know, the atheist might urge, there is some strange and distant possible world in which, as Guleserian says, a single sentient being suffers pointlessly for a few hours. For all we know modally, aren’t there such worlds?
The modal problem of evil seems like too imposing a challenge. But it is a serious and genuine problem for theism. Possible gratuitous evil is no less a challenge for theism than actual gratuitous evil, since God exists in every world and has the divine attributes in every world. A solution to the modal problem of evil would show that there is no possible world in which there is an instance of evil E such that God can eliminate E without the cost of a greater good. A solution to the modal problem of evil would solve all other problems of evil.