At the bottom of all philosophy, of all science and of all thinking, you will find the one all-inclusive question: How is man to tell truth from error? The ignorant man solves this problem in a very simple manner: he holds that whatever he believes, he _knows;_ and that whatever he knows is true. This is the attitude of all amateur and professional theologians, politicians and other numbskulls of that sort. The pious old maid, for example, who believes in the doctrine of the immaculate conception looks upon her faith as proof, and holds that all who disagree with her will suffer torments in hell. Opposed to this childish theory of knowledge is the chronic doubt of the educated man. He sees daily evidence that many things held to be true by nine-tenths of all men are, in reality, false, and he is thereby apt to acquire a doubt of everything, including his own beliefs. At different times in the history of man, various methods of solving or evading the riddle have been proposed. In the age of faith it was held that, by his own efforts alone, man was unable, even partly, to distinguish between truth and error, but that he could always go for enlightenment to an infallible encyclopedia: the word of god, as set forth, through the instrumentality of inspired scribes, in the holy scriptures. If these scriptures said that a certain proposition was true, it _was_ true, and any man who doubted it was either a lunatic or a criminal.[1] This doctrine prevailed in Europe for many years and all who ventured to oppose it were in danger of being killed, but in the course of time the number of doubters grew so large that it was inconvenient or impossible to kill all of them, and so, in the end, they had to be permitted to voice their doubts unharmed. The first man of this new era to inflict any real damage upon the ancient churchly idea of revealed wisdom was Nicolas of Cusa, a cardinal of the Roman Catholic Church, who lived in the early part of the fifteenth century.[2] Despite his office and his time, Nicolas was an independent and intelligent man, and it became apparent to him, after long reflection, that mere belief in a thing was by no means a proof of its truth. Man, he decided was prone to err, but in the worst of his errors, there was always some kernel of truth, else he would revolt against it as inconceivable. Therefore, he decided, the best thing for man to do was to hold all of his beliefs lightly and to reject them whenever they began to appear as errors. The real danger, he said, was not in making mistakes, but in clinging to them after they were known to be mistakes.