Levi ben Gerson, or Leon de Bagnols, was more renowned and more talented than any of these. He was also called Leo the Hebrew, but more usually by his literary name Gersonides (born 1288, died about 1345). He belonged to a family of scholars, and among his ancestors he reckoned that Levi of Villefranche who had indirectly caused the prohibition of scientific study. In spite of the interdict of Ben Adret forbidding the instruction of youths in science, Gersonides was initiated into it at a very early age, and before he had reached his thirtieth year he was at work at a comprehensive and profound work upon philosophy. Gersonides was gifted with a versatile and profound intellect, and averse to all superficiality and incompleteness. In astronomy he corrected his predecessors, and made such accurate observations that specialists based their calculations upon them. He invented an instrument by means of which observations of the heavens could be made more certain. This discovery filled him with such ecstasy that he composed a Hebrew poem, a kind of riddle, upon it, though he was an unpoetical man, and had his head filled with dry calculations and logical conclusions. He also wrote works upon the science of medicine, and discovered new remedies. At the same time he was held in very high repute by his contemporaries as a profound Talmudist, and inspired by his love for systematic arrangement, wrote a methodology of the Mishna.
Maestro Leon de Bagnols, as he was called as a physician, fortunately did not belong to the Jews of France proper: he successively lived in Orange, Perpignan, and in Avignon, at this time the home of popedom. Therefore, he had not been a sufferer in the expulsion of his co-religionists from this land; but his heart bled at the sight of the sufferings which the exiles were made to undergo. He moreover escaped from the effects of the rising of the Shepherds, and the subsequent bitter calamities. At about the same time, his fertile powers of production began to put forth fruit, and he began the series of writings which continued for more than twenty years (1321-1343). None of his writings created such a sensation as his work on the philosophy of religion (Milchamoth Adonaï). In this he set forth the boldest metaphysical thoughts with philosophical calmness and independence, as if paying no heed to the fact that by his departure from the hitherto received notions upon these questions, he was laying himself open to the charges of heresy and heterodoxy. "If my observations are correct," he remarked, "then all blame leveled against me, I regard as praise." Leon de Bagnols belonged to a class of thinkers seldom met with, who, with majestic brow, seek truth for its own intrinsic value, without reference to other ends and results which might cause conflict. Levi ben Gerson thus expressed his opinion upon this subject: Truth must be brought out and placed beneath the glare of open daylight, even if it should contradict the Torah in the strongest possible manner. The Torah is no tyrannical law, which desires to force one to accept untruth as truth, on the contrary, it seeks to lead man to a true understanding of things. If the truth arrived at by investigation is in harmony with the utterances of the Bible, then so much the better. In his independence of thought, the only parallel to Gersonides among Jewish inquirers is Spinoza. Unlike many of his predecessors, he would not look upon science as a body of occult doctrines designed for an inner circle of the initiated. He moreover refused to follow slavishly the authorities in philosophy regarded as infallible. He propounded independent views in opposition not only to Maimuni and Averroes, but also to Aristotle. Leon de Bagnols did not establish a perfect and thoroughly organized system of the philosophy of religion, but treated of the difficulties which interested the thinkers of the age more incisively than any of his predecessors.