A few weeks ago, I heard Michael Shermer being interviewed on the radio. He has recently written a book called (if I remember right) The Believing Brain. One of the findings in this book, he claims, is that people generally don't arrive at their beliefs through reason, as much as we'd like to believe otherwise. Rather, we form beliefs and retroactively justify them with reason. We seek out and almost always find some line of reasoning which will provide our beliefs with rational warrant, in our own minds at least.