The arguments for the existence, unity, and incorporeity of God divide the Arabic philosophers into two schools. Maimonides naturally espoused the view permitting the most exalted conception of God, that is, the conception of God free from human attributes. He recognizes none but negative attributes; in other words, he defines God by means of negations only. For instance, asserting that the Supreme Being is omniscient or omnipotent, is not investing Him with a positive attribute, it is simply denying imperfection. The student knows that in the history of the doctrine of attributes, the recognition of negative attributes marks a great advance in philosophic reasoning. Maimonides holds that the conception of the Deity as a pure abstraction is the only one truly philosophic. His evidences for the existence, the immateriality, and the unity of God, are conceived in the same spirit. In offering them he follows Aristotle's reasoning closely, adding only one other proof, the cosmological, which he took from his teacher, the Arab Avicenna. He logically reaches this proof by more explicitly defining the God-idea, and, at the same time, taking into consideration the nature of the world of things and their relation to one another. Acquainted with Ptolemy's "Almagest" and with the investigations of the Arabs, he naturally surpasses his Greek master in astronomical knowledge. In physical science, however, he gives undivided allegiance to the Aristotelian theory of a sublunary and a celestial world of spheres, the former composed of the sublunary elements in constantly shifting, perishable combinations, and the latter, of the stable, unchanging fifth substance (quintessence). But the question, how God moves these spheres, separates Maimonides from his master. His own answer has a Neoplatonic ring. He holds, with Aristotle, that there are as many separate Intelligences as spheres. Each sphere is supposed to aspire to the Intelligence which is the principle of its motion. The Arabic thinkers assumed ten such independent Intelligences, one animating each of the nine permanent spheres, and the tenth, called the "Active Intellect," influencing the sublunary world of matter. The existence of this tenth Intelligence is proved by the transition of our own intellect from possible existence to actuality, and by the varying forms of all transient things, whose matter at one time existed only in a potential state. Whenever the transition from potentiality to actuality occurs, there must be a cause. Inasmuch as the tenth Intelligence (_Sechel Hapoel_, Active Intellect) induces form, it must itself be form, inasmuch as it is the source of intellect, it is itself intellect. This is, of course, obscure to us, but we must remember that Maimonides would not have so charming and individual a personality, were he not part and parcel of his time and the representative of its belief. Maimonides, having for once deviated from the peripatetic