The published work of the brotherhood appears in a series of 51 epistles, the _Rasa´il ikhwani s-Safa_, which form an encyclopædia of philosophy and science as known to the Arabic-speaking world in the 4th cent. A.H. They do not propose any new theories but simply furnish a manual of current material. The whole text of these epistles has been printed at Calcutta, whilst portions of the voluminous whole have been edited by Prof. Dieterici between 1858 and 1872, and these were followed in 1876 and 1879 by two volumes called _Makrokosmos_ and _Mikrokosmos_, in which an epitome is presented of the whole work. It appears that the leading spirit in the preparation of this encyclopædia was Zayd b. Rifa´a, and with him were associated Abu Sulayman Muhammad al-Busti, Abu l-Hasan `Ali az-Zanjani, Abu Ahmad al-Mahrajani, and al-Awfi, but it does not follow that these were the founders of the brotherhood, as some have supposed. A great part of the _Epistles of the Brotherhood_ deals with logic and the natural sciences, but when the writers turn to metaphysics, psychology, or theology, we find very clear traces of the neo-Platonic doctrines as contained in Alexander of Aphrodisias and matured by Plotinus. God, we read, is above all knowledge and above all the categories of human thought. From God proceeds the _`aql_ or intelligence, a complete spiritual emanation which contains in itself the forms of all things, and from the _`aql_ proceeds the Universal Soul, and from that Soul comes primal matter: when this primal matter becomes capable of receiving dimensions it becomes secondary matter, and from that the universe proceeds. The Universal Soul permeates all matter and is itself sustained by the perpetual emanation of itself from the _`aql_. This Universal Soul permeating all things yet remains one; but each individual thing has a part-soul, which is the source of its force and energy, this part-soul having a varying degree of intellectual capacity. The union of soul and matter is temporary; by wisdom and faith the soul tends to be set free from its material fetters, and so to approach nearer to the present spirit or _`aql_. The right aim of life is the emancipation of the soul from matter, so that it may be absorbed in the parent spirit and thus approach nearer to the Deity. All this is but a repetition of the teaching of al-Farabi and the neo-Platonists, slightly coloured, perhaps, by Sufism, and expressed less logically and lucidly than in the teaching of the philosophers. In general character it shows a tendency towards pantheism, akin to the tendency we have already observed in certain of the Mu`tazilites. God, properly so called, is outside, or rather on such a plane that man does not know, and never can know, anything about Him. Even the _`aql_ is on a plane other than that on which the human soul lives. But the Universal Soul which permeates all things is an emanation from this Spirit, and the Spirit emanates from the unknowable God.