The influences of Plato, and of the wide variety of ancient, Arabic, and medieval Platonisms, on Aquinas must be distinguished from what he knows about them. The first are pervasive, persistent, and ever increasing. The second change markedly as he reads more of the commentaries and treatises of the Hellenic Neoplatonists and Peripatetics during the last decade of his work (Wayne John Hankey , “Aquinas and the Platonists,” in Stephen Gersh and Martin Hoenen , eds., The Platonic Tradition in the Middle Ages : A Doxographic Approach (Berlin and New York: de Gruyter, 2002), 279–324). Exemplary of these is William of Moerbeke’s translation of Proclus’s Elements of Theology finished in 1268. The Corpus , with its quasi-Apostolic origin for Aquinas, was his most authoritative and influential source of Neoplatonism—a character intensified when conveyed in Paris interlarded with unattributed glosses from Eriugena (Wayne John Hankey , “ Participatio divini luminis , Aquinas’ Doctrine of the Agent Intellect: Our Capacity for Contemplation,” Dionysius 22 (2004): 165 and Larry Michael Harrington , ed., A Thirteenth-Century Textbook of Mystical Theology at the University of Paris). The Elements confirmed what Aquinas discerned to be Dionysius’s Platonic style and way of thinking, when he had explicated The Divine Names (1265–68); his earliest view had been that the Areopagite “mostly followed Aristotle” (Super Sent I.2, d.14, q.1, a.2). In the Arab Peripatetic tradition, where Aquinas’s understanding of Aristotle was formed, the philosopher had absorbed Plotinus and Proclus (Alain de Libera , La Querelle des universaux: De Platon à la fi n du Moyen Age (Paris: Des travaux, 1996), 117).

Besides the Dionysian Corpus and the Liber , his most influential early sources of Platonism were Aristotle and Augustine; he probably read almost nothing by Plato except what was quoted by others, for example, fragments of the Timaeus in the commentary by the Platonist Calcidius. For Aquinas, Augustine, as well as Dionysius, “followed Plato as far as Catholic faith allowed” (QDSC 10, ad 8). Augustine, who shares with Avicenna—as seen by Aquinas (see Dag Nikolaus Hasse , Avicenna’s De Anima in the Latin West: The Formation of a Peripatetic Philosophy of the Soul, 1160–1300)—a doctrine of human knowledge through illumination by intelligible forms, and Dionysius, for whom humans know by turning to the sensible—this is probably why Aquinas initially thought that he was an Aristotelian—are Neoplatonists in markedly different ways. Aquinas greatly prized the Neoplatonic, as well as the Peripatetic, commentaries and paraphrases he gradually acquired, because they enabled getting to the Hellenic sources