Here is a rambling (I'm tempted to say "incoherent") essay, by Robert Wright, about the relation between science and religion. Some atheists appear to reject theism (belief in a personal deity) on the ground that it's unnecessary. "We don't need that hypothesis," they say. Let's reconstruct this reasoning. It seems to go as follows:
1. It's reasonable to believe something only if it's necessary to believe it.
2. It's not necessary to believe in a personal deity.
Therefore,
3. It's not reasonable to believe in a personal deity.
I've seen lots of discussion of the second premise, but almost no discussion of the first. Many atheists came to accept the second premise after Darwin gave a naturalistic explanation of the diversity of life. Before Darwin, the diversity of life needed a supernaturalistic explanation, i.e., an explanation in terms of the creative activity of a deity.
But why should someone accept the first premise? Why should there be a presumption against belief, rebuttable only by necessity? Why should we take the position that we shouldn't believe something unless it's necessary, given other things we believe? Another possibility is that it's reasonable to believe something as long as it's compatible with other things one believes. Darwinism is compatible with theism (as Darwin himself said three years before his death [see here for a transcription of the letter]). It may not be compatible with every version of theism (fundamentalist Christianity, for example), but it's compatible with the existence of an omnipotent, omnipresent, omniscient, omnibenevolent being. (As Wright points out, and as I have said many times in this blog, God could have set the process of natural selection in motion, with the aim of producing human beings.) My point is that many atheists suppress the first premise of their argument. This premise needs to be articulated and defended, openly and honestly.
Addendum: Just to be clear, Darwin was driven to agnosticism not by the argument reconstructed above (there is no evidence that Darwin accepted the first premise, even if he accepted the second), but by the problem of evil. He found it difficult to believe that God could have set the process of natural selection in motion, knowing (as God would) of the terrible suffering that natural selection involves.
Addendum 2: Here is a web page devoted to answering the question, "What did Darwin believe?" It is part of the Darwin Correspondence Project, which is a wonderful resource.
Keith Burgess-Jackson, J.D., Ph.D. 