It was during Almamun's reign (813-833) that the Caliphate of the East flourished most luxuriantly. As he was imbued with tolerance, it was possible for the sciences and a certain form of philosophy to develop. Bagdad, Kairuan in northern Africa, and Merv in Khorasan, became the centers of science, such as Europe did not possess until many centuries later. The genius of the Greeks celebrated its resurrection in Arabic garb. Statesmen competed with men of leisure for the palm of erudition. The Jews did not remain unaffected by this enthusiasm for science. Investigation and subtle inquiry are indeed part of their innermost nature. They took earnest interest in these intellectual activities, and many of their achievements gained the approbation of the Arabs. The history of Arab civilization has several Jewish names recorded in its annals. Sahal, surnamed Rabban (the Rabbanite, the authority on the Talmud), of Taberistan on the Caspian Sea (about the year 800), was celebrated as a physician and a mathematician. He translated into Arabic the Almagest of the Greek astronomer Ptolemy, the text-book of astronomy during the Middle Ages, and was the first to note the refraction of light. His son Abu-Sahal Ali (835-853) is placed among those that advanced the study of medicine, and was the teacher of two Arabic medical authorities, Razi and Anzarbi. With even more ardor than that with which they had applied themselves to medicine, mathematics and astronomy, the Mussulmans prosecuted the study of the science of religion, a sort of philosophy of religion (Kalâm). It was invested with as much importance as the affairs of state, and exercised a certain influence on politics. The expounders of the Koran, in trying to explain away the grossly sensual references to God, and to reconcile the contradictions contained in that work, developed ideas which projected far beyond the restricted horizon of Islam. Many commentators, by reason of their rationalistic explanations, came into conflict with the champions of the text, and were branded by them as heretics. The Mutazilists (heretics) laid great stress upon the unity of God, and desired that no definite attributes should be ascribed to him; for thereby the essence of God appeared to them to be divided into parts, and several beings to be included in the idea of God, whose unity was thus negatived. They further asserted the freedom of the human will, because the unconditional predetermination by God, which the Oriental mind believes, and the Koran confirms, was incompatible with divine justice, which rewards the good and punishes the bad. They believed, however, that they still stood on the same ground as the Koran, although, of course, going far beyond it, and in order to bring their doctrine into harmony with the blunt sayings of their religious book, they employed the same method as the Alexandrian-Jewish philosophers of religion had used to reconcile the