Our version of the argument proceeds as follows:
(1) If God exists, God is a perfectly benevolent omnipotent being. (By definition)
(2) A perfectly benevolent being would prevent any unnecessary natural evil if he could. (Premise)
(3) An omnipotent being could prevent all unnecessary natural evil. (Premise)
(4) Therefore, if there were a God, there would be no unnecessary natural evil. (From 1, 2, and 3)
(5) But there is unnecessary natural evil. (Premise derived from experience)
(6) Therefore, there is no God. (From 4 and 5)
The argument is valid. The only real question is whether we have reason to accept the premises.
The first premise is a simple consequence of the definition of "God" that we have agreed to accept; so there is no room to quibble at this step.
Premise (3) seems rather hard to deny. Omnipotence is (at least) the power to bring about anything that is logically possible. But it's hard to think of a case of unnecessary human suffering the prevention of which would imply a contradiction or some other sort of logical impossibility. The paradigm cases will be episodes of human suffering in the wake of floods, earthquakes, plagues and other natural disasters. And it would seem to be a straightforward consequence of our understanding of divine omnipotence that an omnipotent deity could easily prevent this sort of suffering if he chose to do so.
This leaves the theist with two plausible lines of response: He can deny premise (2) and maintain that divine benevolence is compatible with the existence of unnecessary natural evil, or he can deny the existence of such evil altogether. Let's consider these options in reverse order:http://www.princeton.edu/~grosen/puc/phi203/evil.html
