The Jews did not get all this in its purity for various reasons. In the first place it was only gradually that the Jews became acquainted with the wealth of Aristotelian material. We are sure that Abraham Ibn Daud, the forerunner of Maimonides, had a thorough familiarity with the ideas of Aristotle; and those who came after him, for example Maimonides, Gersonides, Hasdai Crescas, show clearly that they were deep students of the ideas represented in the writings of the Stagirite. But there is not the same evidence in the earlier writings of Isaac Israeli, Saadia, Joseph Ibn Zaddik, Gabirol, Bahya Ibn Pakuda, Judah Halevi. They had picked up Aristotelian ideas and principles, but they had also absorbed ideas and concepts from other schools, Greek as well as Arabian, and unconsciously combined the two. Another explanation for the rarity of the complete and unadulterated Aristotle among the Jewish thinkers of the middle ages is that people in those days were very uncritical in the matter of historical facts and relations. Historical and literary criticism was altogether unknown, and a number of works were ascribed to Aristotle which did not belong to him, and which were foreign in spirit to his mode of thinking. They emanated from a different school of thought with different presuppositions. I am referring to the treatise called the "Theology of Aristotle,"[6] and that known as the "Liber de Causis."[7] Both were attributed to Aristotle in the middle ages by Jews and Arabs alike, but it has been shown recently[8] that the former represents extracts from the works of Plotinus, the head of the Neo-Platonic school of philosophy, while the latter is derived from a treatise of Proclus, a Neo-Platonist of later date.