“From the One, Only One Proceeds”

Abstract The separated intellects play a crucial but notoriously controversial role within the Neoplatonic systems of al-Fārābī and Avicenna. While both thinkers provide an array of proofs to support the existence of such immaterial substances, the most enduring of these is based on a metaphysical rule of Avicenna’s metaphysics known as the “rule of one” ( qāʿidat al-wāḥid ): that from the One, only one proceeds ( lā yaṣdur ʿan l-wāḥid illā l-wāḥid ). The following paper explores the various ways in which Avicenna defended this principle and traces their reception in the post-classical period, thereby showing how vigorously the question of emanation was debated among scholars of the later medieval period.