When Aristotle began to be studied in his larger treatises and the details of the psychology and the metaphysics became known especially through Averroes, we find among the Jews also an interest in the finer points of the problem of knowledge. The motives of Plato's idealism and Aristotle's conceptualism (if this inexact description may be allowed for want of a more precise term) are discussed with fulness and detail by Levi ben Gerson. He realizes the difficulty involved in the problem. Knowledge must be of the real and the permanent. But the particular is not permanent, and the universal, which is permanent, is not real. Hence either there is no knowledge or there is a reality corresponding to the universal concept. This latter was the view adopted by Plato. Gersonides finds the reality in the thoughts of the Active Intellect, agreeing in this with the views of Philo and Augustine, substituting only the Active Intellect for their Logos. Maimonides does not discuss the question, but it is clear from a casual statement that like Aristotle he does not believe in the independent reality of the universal (Guide III, 18). In theoretical physics the Arabian Mutakallimun, we have seen (p. xxii), laid great stress on the theory of atom and accident as opposed to the concepts of matter and form by which Aristotle was led to believe in the eternity of the world. Accordingly every Mutakallim laid down his physical theory and based on it his proof of creation. This method was followed also by the early Jewish thinkers. The Karaites before Maimonides adopted the atomic theory without question. And Aaron ben Elijah, who had Maimonides's "Guide" before him, was nevertheless sufficiently loyal to his Karaite predecessors to discuss their views side by side with those of the Aristotelians and to defend them against the strictures of Maimonides. Saadia, the first Rabbanite philosopher, discusses no less than thirteen erroneous views concerning the origin and nature of the world, but he does not lay down any principles of theoretical physics explicitly. He does not seem to favor the atomic theory, but he devotes no special treatment to the subject, and in his arguments for creation as opposed to eternity he makes use of the Kalamistic concepts of substance and accident and composition and division. The same is true of Bahya Ibn Pakuda. Joseph Ibn Zaddik is the first who finds it necessary to give an independent treatment of the sciences before proceeding to construct his religious philosophy, and in so doing he expounds the concepts of matter and form, substance and accident, genesis and destruction, the four elements and their natures and so on--all these Aristotelian concepts. Ibn Daud follows in the path of Ibn Zaddik and discusses the relevant concepts of potentiality and actuality and the nature of motion and infinity, upon which his proof is based of the existence of God. Maimonides clears the ground first by a