I. “Our reason must be consider'd as a kind of cause, of which truth is the natural effect.”1 In these quaint words, David Hume expresses the Philosophers’ point of view. By means of reason we must be able to see the truth of principles and to see that truth without any possibility of error. This view has been so long and so firmly held that it may be called the philosophical ideal of knowledge. Reason is not truly reason, unless by it we can come to know truths that are absolutely certain; truths of principle that are absolutely universal. It has always been admitted that much that is called “knowledge” in ordinary conversation, much that passes for knowledge in the schools, is neither absolutely certain nor absolutely universal. But it has been held that knowledge has been achieved or might be achieved in certain sciences: in ethics (according to some), or in metaphysics, or in physics or in mathematics.