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.

Epistles 39 and 40 return to the Aristotelian philosophy of certain earlier Epistles. Yet the topics taken up here are considered in light of theological issues, affording the occasion to refute eternalism. Epistle 39 explains movement and rest, the kinds of motion, and the species of moved beings, before introducing the divine Mover and the idea that when He ceases to move the world, it will end. The highly composite Epistle 40 addresses themes beyond the different types of causes and effects, including ‘divine gifts’, God’s origination and organization of the world, emanation, and the frequently invoked analogy of numbers. Epistle 41: ‘On Definitions and Descriptions’ draws heavily on al-Kindī’s risāla of the same name, mainly conforming to the title. Folllowing the definitions of variously categorized phenomena comes a diverse range of chapters detailing colours, numbers, ratios, and geometry. The survey of the world and existent beings presented in this volume concludes the penultimate section of the Rasāʾil, on soul and intellect.