In the 13th century, Christian theologians worried in varying degrees about the way “the” Commentator, Averroes, tended to separate intellect from the individual human soul (see digression on this aspect of Averroes in What Is “I”?) — enough so that the reception of Aristotle into the Latin world was for a time threatened. Aquinas wrote a famous little treatise in 1270 On the Unity of the Intellect, mustering as many arguments as possible for a reading of Aristotle that avoided this separation, and gave each soul its own individual intellect. Due to the minimalist nature of Aristotle’s own account, the argument has continued to the present day. Both sides of the dispute have some textual basis on their side. Supported by Augustinian orthodoxy and the writings of Avicenna, the theologians generally argued for a strongly unified intellectual soul. Part of their concern seems to have been a clearly nonphilosophical one, having to do with moral justification of the possible eternal damnation of a human. More purely philosophical readers of Aristotle tended to be less worried about these matters.
The French Thomist scholar E. H. Wéber wrote a couple of fascinating books, L’Homme en Discussion a l’Université de Paris en 1270 (1970) and La Personne Humaine au XIIIe Siècle (1991), about the way Aquinas in this context also, rather unexpectedly, drew on the early Christian neoplatonic writings attributed to a fictitious Greek disciple of the apostle Paul called Dionysius the Areopagite.
Both Albert the Great and Aquinas made considerable use of pseudo-Dionysius in their theology. A bit like Augustine in this regard, pseudo-Dionysius had a strong neoplatonic notion of divine illumination in the soul. As with intellect in Averroes, this also comes from outside, but unlike anything in Averroes, it involves a direct relation between God and the soul. Wéber argued that this played a larger role in the thought of Aquinas than has been generally recognized, and it does seem to me that when Aquinas talks about the natural light of reason, it has something of the character of a divine illumination, quite different from the mainly linguistic, social, and ethical view of reason I find in Aristotle. This view of reason as divine illumination in the soul did not require any “separate” intellect, leaving Aquinas free to argue that both the active and the potential intellect were strictly parts of the individual soul.
Wéber recounts that Aquinas (like Albert) was initially only moderately concerned about the views of Averroes on the soul, but later took a stronger position, harshly condemning this aspect of Averroes’ thought. Politically speaking, it seems that Aristotle had to be separated from Averroes on this matter, in order to make Aristotle safe for Christianity at the time. Matters of theological diplomacy were an important practical part of the unity of truth in Aquinas. Whatever we think of this particular development, we should be grateful to Aquinas for his role in historically securing Latin acceptance of Aristotle. (See also Archaeology of the Subject; Intelligence from Outside; Parts of the Soul; God and the Soul; Fortunes of Aristotle; Errors of the Philosophers; Subject; Mind Without Mentalism. )