cannot be doubted by rational minds, facts that constitute the real world as distinct from the opinions, desires, hopes, and beliefs of human beings. Now, this whole way of trying to understand the world rests upon beliefs that are simply not questioned. Every attempt to understand and cope with experience has to begin with some act of faith. Every such belief is of course open to critical question, but no criticism is possible except by relying on beliefs that in the act of criticism are not criticized.