The Ikhwan al-Safa' (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 Rasa 'il Ikhwan al-Safa' (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 by the Ikhwan in light of theological issues, affording an occasion to refute eternalism. Epistle 39 explains movement and rest, the kinds of physical movement, 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 various types of causes and effects, including 'divine gifts', God's origination and organization of the world, emanation, and the frequently invoked analogy of numbers. Drawing heavily on al-Kindi's risala of the same name, the main section of Epistle 41: 'On the Definitions and Descriptions' conforms to the title, defining variously categorized phenomena; then follows a diverse range of chapters detailing colours, numbers, ratios, and geometry. The detailed survey of the world presented in these three Epistles concludes the penultimate section of the encyclopaedia, on soul and intellect.