The Latin followers of the medieval Islamic philosopher Ibn Rushd or Averroes (1126 - 1198), such as Siger of Brabant, famously taught the doctrine of the unity of the human intellect. The basic idea is this: The intellect, Averroists (like other Aristotelians) argue, is immaterial. But in that case, they conclude (as not all Aristotelians would), it cannot be regarded as the form of a material body. It is instead a substance entirely separated from matter. But matter, the Aristotelian holds, is the principle by which one instance of the form of some species is distinguished from another. Hence there is no way in which one human intellect could be distinguished from another, so that there must be only a single intellect shared by all human beings.
This is groupthink with a vengeance. But though the view is at first blush very odd to modern ears, analogies of the sort beloved of contemporary philosophers of mind can help make it at least partially intelligible. Gyula Klima, in the general introduction to his excellent anthology on medieval philosophy, suggests the image of the Averroist intellect as a mainframe computer, to which all individual humans are connected as terminals. It is really the mainframe that is doing the processing. Another analogy would be cloud computing. For the Averroist, each human being is somewhat like a Kindle Fire tablet whose content is stored entirely in the Amazon Cloud. When you think, and when I think, we are each, as it were, “streaming” different content from the same universal intellect.
For Aquinas, by contrast, we are more like personal computers with their own processing power, and whose content is all stored locally. (This is just an analogy, mind you; Aquinas would certainly not say that we really are computers full stop, after the fashion of computer functionalism.) Nor in Aquinas’s view could two or more human beings intelligibly be said really to be distinct thinkers unless the Averroist position were mistaken -- unless we are in the relevant sense more like personal computers than like terminals or Kindles. (In addition to the passages from the Summa Theologiae just linked to, Aquinas wrote an entire work on the theme: On the Unity of the Intellect against the Averroists.)
That the soul is the form of the body is a crucial component of understanding what is wrong with the Averroist position. For like the Averroists, Aquinas agrees that matter is the principle of individuation between instances of a species. That is why there cannot, in his view, be more than one angel in a species. Each angel -- a disembodied intellect, and a form without matter -- is the unique member of its own species. For since angels have no matter, there is nothing that could distinguish one member of the same angelic species from another. The Averroists’ claim is that since human beings are of the same species, the human intellect -- being, as they claim, entirely separate from matter -- must be the same for all, since there is (given the crucial Averroist claim in question), nothing to individuate two or more distinct human intellects.
Now this raises an interesting question for those attracted to Cartesian forms of dualism -- especially if they are otherwise attracted to Aristotelian-Thomistic hylemorphism as a general philosophy of nature, but even if they are not. Descartes’ conception of the soul famously makes of it a complete substance in its own right (rather than an incomplete substance, as it is for the Thomist). It has no essential connection to the body (unlike, again, the soul as Aquinas understands it). It is like an angel -- an intellectual substance that is nothing but an intellectual substance (i.e. without any bodily operations, which the soul has on the Thomist view).
But in that case, how exactly can one Cartesian soul of the same species be differentiated from another? Cartesian souls of different species could be individuated, as angels are. But Cartesian human souls are, surely, supposed to be souls of the same species. So, again, how can different souls of this same one species be individuated -- as our souls are -- if the Cartesian is correct in claiming that the soul is not related to the body as form to matter?
How, in short, can the Cartesian avoid falling into Averroism? Discuss.