The Brethren of Purity (Arabic transliteration: Ikhwan al-Safa) were a mysterious organization, whose exact identity has never been clear. They were Persian and Arab Muslim philosophers in Basra, Iraq – which was then the seat of the Abbasid Caliphate – sometime during the 10th century CE.
Their esoteric teachings and philosophy are expounded in an epistolary style in the Encyclopedia of the Brethren of Purity (Arabic: Rasa'il Ikhwan al-safa'), a giant compendium of 52 epistles that would greatly influence later encyclopedias. A good deal of Muslim and Western scholarship has been spent on just pinning down the identities of the Brethren and the century in which they were active. This book is a translation of their epistles on natural science.
Abu Hayyan al-Tawhidi (d. 1023) in his Kitab al-Imta' wa'l-Mu'anasa (written between 983 and 985), a collection of 37 seances at the court of Ibn Sa'dan, vizier of the Buyid ruler Samsam ad-Dawla, mentions about identities of founders of this heretic muslim group. Apparently, al-Tawhidi was close to a certain Zaid b. Rifa'a, praising his intellect, ability and deep knowledge – indeed, he had dedicated his Kitab as-Sadiq was-Sadaqa to Zaid – but he was disappointed that Zaid was not orthodox or consistent in his beliefs, and that he was, as Stern puts it:
"...frequenting the society of the heretical authors of the Rasa'il Ikhwan as-Safa, whose names are also recorded as follows: Abu Sulaiman Muhammed b. Ma'shar al-Bisti al-Maqdisi, Abu'l-Hasan 'Ali b. Harun az-Zanjani and Abu Ahmad al-Mihrajani, and al-'Aufi. At-Tauhidi also reports in this connection the opinion expressed by Abu Sulaiman al-Mantiqi, his master, on the Rasa'il and an argument between a certain al-Hariri, another pupil of al-Mantiqi, and Abu Sulaiman al-Maqdisi about the respective roles of Revelation and Philosophy."
Rasa’il Ikhwan al-Safa’ (Epistles of the Brethren of Purity) consist of fifty-two treatises in mathematics, natural sciences, psychology (psychical sciences) and theology. The first part, which is on mathematics, groups fourteen epistles that include treatises in arithmetic, geometry, astronomy, geography, and music, along with tracts in elementary logic, inclusive of: the Isagogue, the Categories, De Interpretatione, the Prior Analytics and the Posterior Analytics. The second part, which is on natural sciences, gathers seventeen epistles on matter and form, generation and corruption, metallurgy, meteorology, a study of the essence of nature, the classes of plants and animals, including a fable. The third part, which is on psychology, comprises ten epistles on the psychical and intellective sciences, dealing with the nature of the intellect and the intelligible, the symbolism of temporal cycles, the mystical essence of love, resurrection, causes and effects, definitions and descriptions. The fourth part deals with theology in eleven epistles, investigating the varieties of religious sects, the virtue of the companionship of the Brethren of Purity, the properties of genuine belief, the nature of the Divine Law, the species of politics, and the essence of magic.
They define a perfect man in their Rasa'il as "of East Persian derivation, of Arabic faith, of Iraqi, that is Babylonian, in education, Hebrew in astuteness, a disciple of Christ in conduct, as pious as a Syrian monk, a Greek in natural sciences, an Indian in the interpretation of mysteries and, above all a Sufi or a mystic in his whole spiritual outlook". There are debates on using this description and other materials of Rasa'il that could help with determination of the identity, affiliation (with Ismaili, Sufism, ...), and other characteristics of Ikhwan al-Safa.