It seems probable that the intellectual efforts of the Fatimids should be connected with the _Ikhwanu s-Safa_, “the brotherhood of purity” and with the Assassins. The former began as a kind of masonic society at Basra soon after the capture of Baghdad by the Buwayhids in 334. Undoubtedly it had some connection with the sect established by ʿAbdullah the son of Maymun, but it is not possible to specify accurately what that connection was. It may have been a more cultured off-shoot, just as the Qarmatians were a cruder branch; but the more probable explanation is that it was a descendant of the movement which produced ʿAbdullah, but free from the Shiʿite elements which he inherited from the sect founded by his father Maymun. To a large extent it seems that the “Brotherhood” displayed the true principles adopted by the Ismaʿilians free from the Shiʿite ideas and free from the political opportunism which marked the development of the Fatimids in Africa and Egypt. To the Assassins we shall have occasion to return at a later stage. The “Brethren of purity” were disposed in four grades, the highest of which was composed of those who desired the union of their souls with the world-spirit, so that their final doctrine was a species of pantheism. They were a body of religious and ethical reformers, a purified and gentle society, at the opposite pole to the fierce Qarmatians. On the literary side they are best known as the producers of the fifty-one “Epistles of the Brethren of Purity,” an encyclopaedia of philosophy and science as known in the Arabic speaking world of the fourth century. These “Epistles” were edited and translated by Prof. Dieterici between 1858 and 1879, and show a general scheme of education in grammar, theology, philosophy, and physics, the latter including mineralogy, chemistry, botany, and zoology. It is in no sense an original work, but simply an encyclopaedic compilation of all the material then available. The whole Fatimid movement took place in an atmosphere saturated with Hellenistic thought, and the revived study of the Greek material was the direct inspiration both of the Ismaʿilian sect as organised by ʿAbdullah and of the “Brethren.” But the influence of these latter was checked by the strong tendency towards reaction in Muslim theology and thought generally which was gathering even in the fourth century in Asiatic Islam. The future of the philosophers lay in the far west: Ibn Sina (d. 428) was the last of the Muslim philosophers in the east, and he was associated with Shiʿite circles, whilst al-Farabi had lived under the shelter of the Shiʿite Hamdanids, and the “Brethren” flourished under the Buwayhids who also were Shiʿites. For the most part the study of Greek philosophy, therefore, progressed under Shiʿite influences.