The Ikhwān al-Ṣafāʾ (The Brethren of Purity or The Sincere Brethren) were the anonymous adepts of an esoteric fraternity of lettered urbanites that was principally based in the Mesopotamian cities of Basra and Baghdad in the second half of the tenth century CE. This learned brotherhood occupied a prominent station in the history of science and philosophy in Islam due to the wide intellectual reception of their famed tracts the Rasāʾil Ikhwān al-Ṣafāʾ (The epistles of the brethren of purity). The exact dating and authorship of this encyclopedic compendium remain unsettled polemical questions, and it is widely assumed that the provenance of the Ikhwān's ideas is primarily ascribable to Ismāʿīlī sources. Nonetheless, this is controversial, and it is rather more circumspect to attribute...