[12][13] The subject of the work is vast and ranges from mathematics, music, astronomy, and natural sciences, to ethics, politics, religion, and magic—all compiled for one, basic purpose, that learning is training for the soul and a preparation for its eventual life once freed from the body.

All these people are according to Henry Corbin, Ismailis[22] Other scholars, such as Susanne Diwald and Abdul Latif Tibawi have asserted a Sunni-Sufi nature of the work.

[12][13] Further perplexities abound; the use of pronouns for the authorial "sender" of the rasa'il is not consistent, with the writer occasionally slipping from third person to first-person (for example, in Epistle 44, "The Doctrine of the Sincere Brethren").

[23] This has led some to suggest that the rasa'il were not in fact written co-operatively by a group or consolidated notes from lectures and discussions, but were actually the work of a single person.

[24] Of course, if one accepts the longer time spans proposed for the composition of the Encyclopedia, or the simpler possibility that each risala was written by a separate person, sole authorship would be impossible.

[9][25] Its philosophical outlook was Neoplatonic and it tried to integrate Greek philosophy (and especially the dialectical reasoning and logic of Aristotelianism) with various astrological, Hermetic, Gnostic and Islamic schools of thought.

[10] Others, however, hold the Brethren to be "free-thinkers" who transcended sectarian divisions and were not bound by the doctrines of any specific creed.

[9] Their unabashed eclecticism[27] is fairly unusual in this period of Arabic thought, characterised by fierce theological disputes; they refused to condemn rival schools of thought or religions, instead insisting that they be examined fairly and open-mindedly for what truth they may contain: ...to shun no science, scorn any book, or to cling fanatically to no single creed.

This creed is the consideration of all existing things, both sensible and intelligible, from beginning to end, whether hidden or overt, manifest or obscure .

- (from the Ikhwan al-Safa, or Encyclopedia of the Brethren of Purity; Rasa'il IV, pg 52) [15]In total, they cover most of the areas an educated person was expected to understand in that era.

The Brethren divided mathematics itself into four sections: arithmetic was Pythagoras and Nicomachus' domain; Ptolemy ruled over astronomy with his Almagest; geometry was associated with Euclid, naturally; and the fourth and last division was that of music.

Seyyed Hossein Nasr suggests that the origin of the preference for 17 stemmed from the alchemist Jābir ibn Hayyān's numerological symbolism.

The King was convinced by this argument, and granted his judgement to them, but strongly cautioned them that the same Qur'an that supported them also promised them hellfire should they mistreat their animals.

More metaphysical were the four ranks (or "spiritual principles"), which apparently were an elaboration of Plotinus' triad of Thought, Soul, and the One, known to the Brethren through The Theology of Aristotle (a version of Plotinus' Enneads in Arabic, modified with changes and paraphrases, and attributed to Aristotle);[31] first, the Creator (al-Bārī) emanated down to Universal Intellect (al-'Aql al-Kullī), then to Universal Soul (al-Nafs), and through Prime Matter (al-Hayūlā al-Ūlā), which emanated still further down through (and creating) the mundane hierarchy.

Since they refused to accept transmigration, then the Platonic idea that all learning is "remembrance" and that man can never attain to complete knowledge whilst shackled in his body must be false; the Brethren's stance was rather that a person could potentially learn everything worth knowing and avoid the snares and delusion of this sinful world, eventually attaining to Paradise, Allah, and salvation, but unless they studied wise men and wise books - like their encyclopedia, whose sole purpose was to entice men to learn its knowledge and possibly be saved - that possibility would never become an actuality.

As Netton writes, "The magpie eclecticism with which they surveyed and utilized elements from the philosophies of Pythagoras, Plato, Aristotle and Plotinus, and religions such as Nestorian Christianity, Judaism and Hinduism,[23] was not an early attempt at ecumenism or interfaith dialogue.

To use their own image, they perceived their Brotherhood, to which they invited others, as a "Ship of Salvation" that would float free from the sea of matter; the Ikhwan, with their doctrines of mutual cooperation, asceticism, and righteous living, would reach the gates of Paradise in its care.

"[11]The text in the "Encyclopedia of the Brethren of Purity" describes biological diversity in a manner similar to the modern day theory of evolution.

The reason for the conservation of forms, genus and species in matter is fixity of their celestial cause because their efficient cause is the Universal Soul of the spheres instead of the change and continuous flux of individuals which is due to the variability of their cause”.

Muhammad Hamidullah describes the ideas on evolution found in the Encyclopedia of the Brethren of Purity (The Epistles of Ikhwan al-Safa) as follows: "[These books] state that God first created matter and invested it with energy for development.

It is an anecdote of a "prince who strays from his palace during his wedding feast and, drunk, spends the night in a cemetery, confusing a corpse with his bride.

[48] This series began in 2008 with an introductory volume of studies edited by Nader El-Bizri, and continued with the publication of:[49] Both the editors' approach to the project and the quality of its English translations have been criticized.