 Following Aristotle’s lead in De anima III.5, Aquinas distinguishes between two distinct intellectual powers, the possible intellect and the agent intellect. The first starts out in potentiality to all intelligibles, a tabula rasa, “like a tablet on which nothing has been written” (ST 1a 79.2c), and gradually becomes actualized, taking in universal concepts that are then stored there as dispositions. But because such concepts are not immediately available through sensory experience—one cannot grasp what it is to be a cat just by looking at a cat—there must also be an active intellectual power, “a power on the side of intellect to actualize intelligible things by abstracting the species from material conditions” (ST 1a 79.3c). By Aquinas’s time, generations of Aristotelians had already argued over the correct understanding of these two powers, with some treating the agent intellect as a single higher mind, separate from human beings, and perhaps even to be identified with God.  Ibn Rushd  (Averroes), the Spanish philosopher active a half century before Aquinas, had even proposed treating the possible intellect as a single higher power to which every human being has shared access. Aquinas, in contrast, thinks that each human being has their own intellect, agent and possible. That we all share in a single agent intellect is, he thinks, wrong but not incredible—it has affinities with the venerable Augustinian doctrine of  divine illumination—but  Aquinas regards the Averroistic doctrine of a separate possible intellect as not just contrary to the faith but entirely preposterous, since it would make it impossible to explain how we are each capable of individual acts of thought. As he describes the absurd implication in his treatise On the Unity of the Intellect against the Averroists, “if I think about stone and you do likewise, then it would be necessary that you and I have one and the same intellectual operation” (4.101–3; see Ogden 2022).