The most prominent Jewish exponent of philosophy in this age of strenuous interest in metaphysical speculations and contests was Levi ben Gerson (Leon di Bannolas), theologian, scientist, physician, and astronomer. One of his ancestors, Gerson ben Solomon, had written a work typical of the state of the natural sciences in his day. Levi ben Gerson's chief work became famous not among Jews alone. It was referred to in words of praise by Pico della Mirandola, Reuchlin, Kepler, and other Christian thinkers. He was the inventor of an astronomical instrument, a description of which was translated into Latin at the express command of Pope Clement VI., and carefully studied by Kepler. Besides, Levi ben Gerson was the author of an arithmetical work. In those days, in fact up to the seventeenth century, there was but a faint dividing line between astronomy and mathematics, as between medicine and natural history. John of Seville was a notable mathematician, the compiler of a practical arithmetic, the first to make mention of decimal fractions, which possibly may have been his invention, and in the Zohar, the text-book of mediæval Jewish mysticism, which appeared centuries before Copernicus's time, the cause of the succession of day and night is stated to be the earth's revolution on its axis. In this great translation period scarcely a single branch of human science escaped the mental avidity of Jews. They found worthy of translation such essays as "Rules for the Shoeing and Care of Horses in Royal Stables" and "The Art of Carving and Serving at Princely Boards." Translations of works on scholasticism now took rank beside those from Greek and Arabic philosophers, and to translations from the Arabic into Hebrew were added translations from and into Latin, or even into the vernacular idiom wherever literary forms had developed. The bold assertion can be made good that not a single prominent work of ancient science was left untranslated. On the other hand it is hard to speculate what would have been the fate of these treasures of antiquity without Jewish intermediation. Doubtless an important factor in the work was the encouragement given Jewish scholars by enlightened rulers, such as Emperor Frederick II., Charles and Robert of Anjou, Jayme I. of Aragon, and Alfonso X. of Castile, and by popes, and private patrons of learning. Mention has been made of Jewish contributions to the work of the medical schools of Montpellier and Salerno. Under Jayme I. Christian and Jewish savants of Barcelona worked together harmoniously to promote the cause of civilization and culture in their native land. The first to use the Catalan dialect for literary purposes was the Jew Yehuda ben Astruc, and under Alfonso (X.) the Wise, Jews again attained to prominence in the king's favorite science of astronomy. The Alfonsine Tables were chiefly the work of Isaac ibn Sid, a Toledo _chazan_ (precentor). In general, the results reached by Jewish scholarship at