Epicurus is traditionally credited with the sharpest early formulation, and the structure has barely changed since. Is God willing to prevent evil but unable? Then he is not omnipotent. Able but unwilling? Then he is not good. Both able and willing? Then where does evil come from? The argument's power is that it uses only premises the believer already accepts.
Philosophers distinguish two versions, and the distinction matters because they have had very different fates.
The logical problem claims outright contradiction: the propositions "God exists" and "evil exists" cannot both be true. This is the stronger claim and it is now generally thought to fail. Alvin Plantinga's free will defence showed that the two are not strictly incompatible — it is at least possible that a world containing free creatures who sometimes choose wrongly is better than a world without freedom, and mere possibility is all that is needed to defeat a claim of contradiction. Note how modest that conclusion is: it does not show the reason is actual, only that no contradiction has been proved.
The evidential problem is where the argument now lives, and it is much harder. It concedes there is no contradiction and argues instead that the sheer quantity and distribution of suffering is powerful evidence against God. The examples that carry the weight are deliberately chosen: a fawn burning to death in a forest fire no one witnesses, a child suffering an illness that produces no discernible good in anyone. These cases are hard to fit to any of the standard justifications, because there is no free choice involved and no soul being made.
The main responsesWhat defenders say.
Free will. A world containing beings who genuinely choose is more valuable than one containing beings who cannot, and genuine choice entails the possibility of choosing badly. The obvious limit is that this addresses moral evil — what people do — and says nothing about earthquakes, parasites, or childhood cancer. Some extend it by attributing natural evil to non-human free agents, which most find unpersuasive.
Soul-making. Associated with John Hick: a world designed for the development of character cannot be a world without difficulty. Courage is impossible without danger, compassion without suffering, perseverance without obstacles. The objection is proportionality — the argument might justify some hardship, but it strains badly against suffering that destroys people rather than developing them, and against the suffering of animals and infants, who develop no character from it.
Sceptical theism. Perhaps the reasons exist but lie beyond our grasp, as a child cannot see why a painful medical procedure is warranted. Our inability to identify a justifying reason is then poor evidence that none exists. Critics reply that this proves too much: if our moral judgement is that unreliable, it undermines our ability to trust any of our moral perceptions, including the ones religious ethics relies on.
Where it standsAn honest summary.
The logical problem is widely regarded as answered. The evidential problem is not, and the disagreement is genuine rather than a matter of one side missing something obvious. Theists generally argue that the problem is real but not decisive, especially when set against whatever positive reasons they hold for believing. Atheists generally argue that the evidential weight is overwhelming and the defences are too speculative to bear it.
It is also worth marking that this is not only an argument. For many people the problem arrives through their own suffering or someone else's, and encountering it as a formal exchange of premises can feel beside the point. That reaction is not philosophically naive — several religious traditions treat the question as one to be lived with rather than solved, and the Book of Job notably ends without giving the explanation it spent the whole book demanding.