The “One in Number” from al-Fārābī to Avicenna: Identity and Unity

Abstract The “one in number” that the falāsifa inherit from Aristotle undergoes philosophically interesting transformations in their works. By tracing these transformations in the metaphysical work of al-Fārābī, Yaḥyā Ibn ʿAdī, and Avicenna, it is possible to see how central the “one in number” is for Yaḥyā Ibn ʿAdī’s and Avicenna’s ontology of essence. This will also allow to shed new light on the relation between essence and unity in Avicenna. He creates the basis for his systematic separation of essence and unity by hypostatizing the unity of individuals in external reality through the “one in number.”