The characteristic of all that is spiritual is its knowledge of its own essential nature. Will is nothing else, in Behmenyar’s opinion, than the knowledge of that which is the necessary outcome of that nature. Farther, the life and the joy of rational souls consist in self-knowledge. 11. Ibn Sina achieved a far-reaching influence. His Canon of Medicine was highly esteemed even in the West, from the 13th century to the 16th, and it is still the authority for medical treatment among the Persians of the present day. On Christian Scholasticism his influence was important. Dante placed him between Hippocrates and Galen; and Scaliger maintained that he was Galen’s equal in Medicine, and much his superior in Philosophy. For the East he stood and yet stands as the Prince of Philosophy. In that region Neo-Platonic Aristotelianism continues to be known under the form which was given it by Ibn Sina. Manuscripts of his works abound,—an evidence of his popularity,—while commentaries on his writings, and epitomes of them, are countless. He was studied by physicians and statesmen, and even by theologians: It was only a few who went farther back and consulted his sources. From the very first, of course, he had many enemies, and they were more noisy in their demonstrations than his friends. Poets cursed him: theologians either chimed in with him, or tried to refute him. And in Bagdad in the year 1150, the Caliph Mustandjid consigned to the flames Ibn Sina’s writings, as part of a certain judge’s philosophical library. 5. Ibn al-Haitham. 1. After the days of Ibn Sina and his school, little more attention was paid to the cultivation of Speculative Philosophy in the Eastern regions of the Muslim empire. In these lands Arabic was forced more and more to yield to Persian, both in life and in literature. That the Persian tongue is not so well adapted for abstract logical and metaphysical discussion—might be only of quite secondary importance, in connection with this decline in speculation; but the conditions of civilization, and with them the subjects which interested men, were sadly changed. Ethics and Politics came more to the front, although without assuming an actually new form. But in the later Persian literature the predominant place was unmistakeably held by Poetry, partly of a free-thinking tendency, partly, and indeed preponderatingly, of a mystic kind, which satisfied the need for wisdom, experienced by people of culture. From about the middle of the 10th century, the scientific movement which originated at Bagdad had in part turned westward. We have already found Farabi in Syria, and Masudi in Egypt: In the latter country Cairo was becoming a second Bagdad.