"'Works on astrology are the product of fools, who mistook vanity for wisdom. Men are inclined to believe whatever is written in a book, especially if the book be ancient; and in olden times disaster befell Israel because men devoted themselves to such idolatry instead of practising the arts of martial defence and government.' He says, that he had himself studied every extant astrological treatise, and had convinced himself that none deserved to be called scientific. Maimonides then proceeds to distinguish between astrology and astronomy, in the latter of which lies true and necessary wisdom. He ridicules the supposition that the fate of man could be dependent on the constellations, and urges that such a theory robs life of purpose, and makes man a slave of destiny. 'It is true,' he concludes, 'that you may find strange utterances in the Rabbinical literature which imply a belief in the potency of the stars at a man's nativity, but no one is justified in surrendering his own rational opinions because this or that sage erred, or because an allegorical remark is expressed literally. A man must never cast his own judgment behind him; the eyes are set in front, not in the back.'" While Maimonides could be so positive in his opinions with regard to a subject on which he felt competent to say something, he was extremely modest with regard to many of the great problems of medicine. He often uses the expression in his writings, "I do not see how to explain this matter." He quotes with approval from a Rabbi of old who had counselled his students, "teach thy tongue to say, I do not know." In this, of course, he has given the best possible evidence of his largeness of mind and his capacity for making advance in knowledge. It is when men are ready to say, "I do not know," that progress becomes possible. It is very easy to rest in a conscious or unconscious pretence of knowledge that obscures the real question at issue. A great thinker, who lived in the century in which Maimonides died, Roger Bacon, set down as one of the four principal obstacles to advance in knowledge indeed, as _the_ one of the four that hampered intellectual progress the most, the fact that men feared to say, "I do not know."