We live in a world filled with competing truth claims. Every day, we are bombarded with declarations that something is true and that something else is false. We are told what to believe and what not to believe. We are asked to behave one way but not another way. In her monthly column “What I Know for Sure,” Oprah Winfrey tells us how to handle our lives and our relationships. The New York Times editorial page regularly tells us what approach we should take to the big moral, legal, or public-policy issues of our day. Richard Dawkins, the British atheist and evolutionist, tells us how to think of our historical origins and our place in this universe.