The Unity of the Intellect

Aristotle's account of the intellect in De Anima was notoriously obscure. He distinguished between a passive intellect (which receives intelligible forms) and an active intellect (which makes things actually intelligible, as light makes colours actually visible). But the relationship between these intellects, and whether either survives the death of the individual, Aristotle left unclear. Earlier Islamic interpreters — al-Farabi, Avicenna — had identified the active intellect with a separate cosmic substance (the Angel of Revelation in Islamic terms). Averroes went further: in his final commentary on De Anima, he identified both the material (possible) intellect and the active intellect as single substances separate from individual human bodies.

Averroes's argument is that the material intellect — the receptive capacity for thought — cannot be a faculty of an individual soul, because an individual soul is tied to a particular body and thus to a particular material substrate. But thought grasps universal truths that are the same for everyone: the truth of a mathematical proof is not my truth or your truth but simply the truth. The universality and necessity of knowledge requires a knowing subject that is itself universal — not my intellect or your intellect but the intellect, the one faculty of universal thought that individual human beings participate in when they think well.

When Averroes's commentaries were translated into Latin in the thirteenth century, his theory of the unity of the intellect became the most controversial philosophical doctrine in the universities. The Latin Averroists — associated with Siger of Brabant and Boethius of Dacia at Paris — taught Averroes's position as philosophical truth while acknowledging its incompatibility with Christian doctrine. Thomas Aquinas attacked the position in his treatise On the Unity of the Intellect Against the Averroists. The Bishop of Paris condemned 219 propositions in 1277, many of them Averroist. The controversy shaped the entire development of late medieval philosophy and the early modern distinction between philosophy and theology.

Averroes's theory of the unity of the intellect is developed in his Long Commentary on Aristotle's De Anima, translated into Latin by Michael Scot around 1230. It is his most philosophically original contribution and the source of the Latin Averroist controversy that dominated thirteenth-century philosophy at Paris.