The God of classical theism is defined as omnipotent, omniscient, and all good. But this conflicts with the existence of pain and suffering in the world, which indicates that God is either not omnipotent, not omniscient, or not all good. The formal argument:
(1) God is omniscient, omnipotent, and all good (from the classical definition of theism)
(2) Evil exists in the world
(3) If God is unable to prevent evil, then he is not omnipotent
(4) If God is unaware of evil, then he is not omniscient
(5) If God is unwilling to prevent evil, then he is not all good
(6) Therefore, God is either not omniscient, not omnipotent, or not good
(7) Both 1 and 6 are contradictory
(8) Therefore, the God of classical theism does not exist
Attempts to resolve this problem results in a lesser God than theists would be willing to accept. The problem of evil as a logical problem was primarily defended by J.L. Mackie in the mid-20th Century.
Logical consistency requires that it be possible for several statements to all be true at the same time. If it can be shown that there is a possible way of reconciling them, even if implausible or unlikely, then the statements are not logically contradictory.
Logical consistency and the logical problem of evil
If it is possible that God has a morally sufficient reason to permit evil, then the logical contradiction does not hold. The conclusion would then be that either God does not exist or that he has a morally sufficient reason to permit evil. An example would be of a child receiving a vaccination. So long as it's even possible that God has long-term, morally sufficient reasons for permitting evil, there is no logical contradiction between an all good God and the existence of evil.
Plantinga's Free Will Defense
Alvin Plantinga makes a suggestion for one possible morally sufficient reason that God would have for permitting evil: morally significant free will. This is defined as the ability of someone to choose between good and evil. Just as it would be absurd to praise a robot for emptying the trash when it was programmed to do that and nothing else, so too would it be absurd to praise someone for making a good moral decision if they had no moral choice in the matter. Consider possible worlds:
World 1: God gives people free will, and does not force them to always choose good; there is evil and suffering.
World 2: God does not give people free will, and forces them to always choose good; there is no evil and suffering.
World 3: God gives people free will, and forces them to always choose good; there is no evil and suffering.
World 4: God gives people free will, and does not force them to always choose good; there is no evil and suffering.
Plantinga says that World 3 is logically impossible; even God cannot force someone to freely do something. World 2 contains only automatons, and is hence undesirable on classical theism. This leaves World 1 and 4. World 1 describes the actual world, and World 4 describes a way the world could be: everyone has free will, but they all choose to do only good. The Book of Genesis hints that God intended the world to be like this.
Flew and Mackie maintain that God could and should have created World 3; Plantinga maintains that this is logically impossible.
Divine omnipotence and the Free Will Defense
Some scholars maintain that "omnipotence" should mean the ability to anything, even logically impossible things. But there are many things God cannot do: lie, cheat, be ignorant, etc. This supports the thesis there are things even an omnipotent being cannot do.
Objection: Free Will and Natural Evil
The free will defense does not say anything about natural evil (earthquakes, mosquitoes, etc), and so some think it is incomplete. Plantinga suggests that invisible demons could be responsible for natural evil and hence, are abusing their free will in exactly the same way humans do. This response seems implausible, but to rebut a logical inconsistency all that needs to be shown is a possible way it can be reconciled, even if that is not the real reason for it or even if its plausible at all.
Was Plantinga's victory too easy?
J.L. Mackie, mostly associated with mid-20th Century atheism, concedes that Plantinga did away with the logical problem of evil. But he says that it is still unsatisfactory because Plantinga only deals with the possible and not the plausible; Plantinga does not give any evidence for his claim. However, most philosopher's agree that the blame lies with Mackie in making too strong a claim for the problem of evil and as a result most debate has turned to the evidential problem of evil instead.
Evaluation of the Free Will Defense
If a situation is claimed to be impossible, all that needs to be shown is a way for that situation to be possible. The possible situation does not need to be actual or even plausible, as long as it is conceivable then it is logically possible and hence the strong claim that the situation is impossible is refuted. The free will defense seems to fulfill this need and hence, the logical problem of evil is generally considered to be refuted.
Other Solutions to the Logical Problem of Evil
John Hick suggests that evil and suffering are part of a plan of "soul making," that is, shaping humans into the best that they can be. Evolution was a part of this process. This response is a theodicy instead of a defense: an attempt to provide an actual reason for evil rather than just a possible reason, as Plantinga does.
Eleonore Stump suggests that evil is what brings people closer to God; that without evil, constant joy and happiness would make people in love with worldly things rather than God.
Problems with the Free Will Defense
Heaven is usually conceived of as a place where it is impossible for people to do wrong. But if this is the ultimate desirable reward, then why is free will so valuable here on earth? If people lose their free will in heaven then are they the same people they were on earth? And if they keep their free will but are forced to always do good, then doesn't this mean that World 3 (above) is actually possible after all? If so, then Flew and Mackie's objections above are correct.
Secondly, If God is incapable of doing wrong, then it seems he does not have free will and hence, is not morally praiseworthy; this conflicts with classical theism. But if he does have free will but is incapable of doing wrong, then this seems to make World 3 (above) possible after all. If so, then Flew and Mackie's objections above are correct.