Lysis continues, in the name of Pythagoras, to trace for the philosopher the course that he must follow in the first part of his doctrine, which is the Purification. After having commended to him moderation and prudence in all things, having exhorted him to be as slow to censure as to approve, he seeks to put him on guard against prejudices and the routine of example, which are, in reality, the greatest obstacles that are encountered by science and truth. This is what Bacon, the regenerator of philosophy in modern Europe, so keenly felt, as I have already cited with praise at the opening of this work. This excellent observer, to whom we owe our freedom from scholastic leading-strings whose ignorance had stifled for us the name of Aristotle, having formed the difficult enterprise of disencumbering and, as it were, clearing the air belonging to the human understanding, in order to put it in a condition to receive an edifice less barbarous, remarked, that one would never attain to establishing there the foundation of true science, if one did not first labour to set aside prejudices.[415] He displayed all his forces against these formidable enemies of human perfectibility, and if he did not overthrow them all, at least he indicated them in such a manner as to make it easier to recognize and destroy them. The prejudices which obsess the human understanding and which he calls idols, are, according to him, of four kinds: these are the idols of the tribe; the idols of the den; the idols of society; and the idols of theories. The first are inherent in human nature; the second are those of each individual; the third result from the equivocal definitions attached to words; the fourth and the most numerous are those that man receives from his teachers and from the doctrines which are current.[416] The last are the most tenacious and the most difficult to conquer. It seems even impossible wholly to resist them. The man who aspires to the perilous glory of improving the human mind, finds himself placed between two formidable dangers, which, like those of Sylla and Charybdis, threaten alternately to break his frail bark: upon one is irresistible routine, upon the other proud innovation. There is danger alike from both sides. He can save himself only by favour of the golden mean, so commended by all the sages and so rarely followed even by them.