The Ikhwān al-Ṣafāʾ (Brethren of Purity), the anonymous adepts of a tenth-century esoteric fraternity based in Basra and Baghdad, hold an eminent position in the history of science and philosophy in Islam due to the wide reception and assimilation of their monumental encyclopaedia, the Rasāʾil Ikhwān al-Ṣafāʾ (Epistles of the Brethren of Purity). This compendium contains fifty-two epistles offering synoptic accounts of the classical sciences and philosophies of the age; divided into four classificatory parts, it treats themes in mathematics, logic, natural philosophy, psychology, metaphysics, and theology, in addition to didactic fables.

In Epistle 49, the Brethren utilize an array of sources, predominantly Hellenic and Islamic, in probing the entire hierarchy of existence, from the nature of God to the most basic elements. Epistle 50 describes the ‘proper attitudes’ towards body and soul for the attainment of wellness in this world and the hereafter, before addressing religious and philosophical worship. Lastly, in Epistle 51, the Brethren again consider the arrangement of the world as a whole in terms of Pythagorean number theory. With this penultimate epistle the encyclopedia thus comes full-circle, in preparation for the final epistle (on magic), directing the reader back to the topic of numbers with which the corpus begins.