And so we must first know that the movers of the heavens] are substances separate from matter, namely Intelligences, which the common people call Angels.’ (Dante, Convivium, 2.2)
Dante’s doctrine of the angels comes from the amalgamation of traditions typified by thirteenth-century Scholastic angelology, whose two essential threads are the Judeo-Christian angels, or messengers, who intercede between God and man in the Bible; and Aristotle’s “intelligences” responsible for the apparent motions of astronomical entities, in turn derived from Plato’s Ideas. In his harmonization of these divergent traditions in the Convivio, Dante notes that the separated were the species or models of sensible things. For Aristotle, they were species of an altogether superior order, which nonetheless acted upon lower bodies, affecting their variety, mutations, and reproduction and bringing about their degradation. Because this plethora of unmoved-movers considered as the final causes of particular celestial movements seemed to contradict Aristotle’s other tenet of a single Prime Mover, it fell to Neoplatonists to derive these multiple intelligences from the first in a series of “emanations,” each the cause of subsequent, inferior beings. Arab philosophers who were heavily influenced by Neoplatonism, like Alfarabi and Avicenna, were the first to identify the Aristotelian intelligences with the angel-messengers of revealed religion. With the notable exception of Albertus Magnus, most Scholastic thinkers assumed that Aristotle’s intelligences were in fact angels.
There is no doubt that Dante emphatically subscribed to the notion of angels as movers of the astronomical spheres (Conv. 2.2.7, 2.4.2). As the intelligences responsible for the apparent motions of the stars and planets, angels explain celestial mechanics. They move the spheres, not by means of physical contact but by pure understanding, solo intendendo—by spiritual contact with their virtue, tatto di vertù (Conv. 2.5.18; cf. Summa contra Gentiles 2.92: per intellectum movent; 2.56 tactus . . . virtutis). In the Convivio, Dante assumes that there must be at least three movers of the Heaven of Venus in order to account for the three distinct proper movements discernible in the behavior of that planet. In that text, angels whose sole operation is to move the spheres seem to belong to a special class devoted to the active life, while the vast majority of these creatures are wholly engaged in pure contemplation (Conv. 2.4.9–12). In an effort to conform better to Aristotle, however, Dante goes on to make the apparently contradictory claim that even the sphere-movers contemplate, but that from their speculation results the circulation of Heaven (Conv. 2.4.13). Dante’s assignment of angels from every hierarchy to the task of moving a planetary sphere is essentially unique. (Aquinas, for example, supposed such movers came only from the hierarchy called Virtues.) Dante may be reflecting a Thomistic distinction between ministers and contemplators when he claims that we receive gifts from the bottom hierarchy of angels because it is closest to us (Conv. 2.5.8).
In the Paradiso, the causal link between planets and angels is even stronger, as Beatrice denies (against Jerome) that the one could have preceded the other in the order of Creation, because the “perfection” of the “movers” is to turn the material heavens (Par. 29.44–45). Aquinas explicitly states that the moving of the spheres may be considered a ministry of angels, but it by no means constitutes the fulfillment of their nature (De potentia Dei 3.19 ad 3). Beatrice further seems to suggest that the whole array of angelic hosts, with the exception of those who sinned, are involved in the “art” of celestial circulation (Par. 29.52–54), although some scholars strenuously insist that this circling refers to their activity of contemplation, not sphere moving.’
The Dante Encyclopedia, ed. R. Lansing, pp. 39b-41a)