The most excellent philosophers of our time, such as the authors of The Art of Thinking of The Search for Truth and of the Essay concerning Human Understanding have been very far from indicating to us the true means fitted to assist the faculty whose business it is to make us weigh the probabilities of the true and the false: not to mention the art of discovery, in which success is still more difficult of attainment, and whereof we have nothing beyond very imperfect samples in mathematics.