The structure of the organization and the identities of its members have never been clear.[4][5] Their esoteric teachings and philosophy are expounded in an epistolary style in the Encyclopedia of the Brethren of Purity (Rasā'il Ikhwān al-Ṣafā'), 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.
^They are generally considered a secret society because of their closed and private meetings every 12 days, as mentioned in the Rasa'il.
^Al-Fārūq?, I. R. (1960), "On the Ethics of the Brethren of Purity", The Muslim World, 50: 109–121.
^"Having been hidden within the cloak of secrecy from its very inception, the Rasa'il have provided many points of contention and have been a constant source of dispute among both Muslim and Western scholars. The identification of the authors, or possibly one author, the place and time of writing and propagation of their works, the nature of the secret brotherhood, the outer manifestation of which comprises the Rasa'il – these and many secondary questions have remained without answer." pg 25, Nasr (1964)
^William Bayne Fisher, Richard Nelson Frye, John Andrew Boyle, The Cambridge History of Iran, Published by Cambridge University Press, 1975, ISBN0-521-20093-8, p. 428