I answer that, Whoever knows anything perfectly, is able to distinguish its acts, powers, and nature, down to the minutest details, whereas he who knows a thing in an imperfect manner can only distinguish it in a general way, and only as regards a few points. Thus, one who knows natural things imperfectly, can distinguish their orders in a general way, placing the heavenly bodies in one order, inanimate inferior bodies in another, plants in another, and animals in another; whilst he who knows natural things perfectly, is able to distinguish different orders in the heavenly bodies themselves, and in each of the other orders.
Now our knowledge of the angels is imperfect, as Dionysius says (Coel. Hier. vi). Hence we can only distinguish the angelic offices and orders in a general way, so as to place many angels in one order. But if we knew the offices and distinctions of the angels perfectly, we should know perfectly that each angel has his own office and his own order among things, and much more so than any star, though this be hidden from us.
Reply Obj. 1: All the angels of one order are in some way equal in a common similitude, whereby they are placed in that order; but absolutely speaking they are not equal. Hence Dionysius says (Coel. Hier. x) that in one and the same order of angels there are those who are first, middle, and last.
Reply Obj. 2: That special distinction of orders and offices wherein each angel has his own office and order, is hidden from us.
Reply Obj. 3: As in a surface which is partly white and partly black, the two parts on the borders of white and black are more akin as regards their position than any other two white parts, but are less akin in quality; so two angels who are on the boundary of two orders are more akin in propinquity of nature than one of them is akin to the others of its own order, but less akin in their fitness for similar offices, which fitness, indeed, extends to a definite limit. _______________________
FOURTH ARTICLE [I, Q. 108, Art. 4]
Whether the Distinction of Hierarchies and Orders Comes from the
Angelic Nature?
Objection 1: It would seem that the distinction of hierarchies and of orders is not from the nature of the angels. For hierarchy is "a sacred principality," and Dionysius places in its definition that it "approaches a resemblance to God, as far as may be" (Coel. Hier. iii). But sanctity and resemblance to God is in the angels by grace, and not by nature. Therefore the distinction of hierarchies and orders in the angels is by grace, and not by nature.
Obj. 2: Further, the Seraphim are called "burning" or "kindling," as Dionysius says (Coel. Hier. vii). This belongs to charity which comes not from nature but from grace; for "it is poured forth in our hearts by the Holy Ghost Who is given to us" (Rom. 5:5): "which is said not only of holy men, but also of the holy angels," as Augustine says (De Civ. Dei xii). Therefore the angelic orders are not from nature, but from grace.
Obj. 3: Further, the ecclesiastical hierarchy is copied from the heavenly. But the orders among men are not from nature, but by the gift of grace; for it is not a natural gift for one to be a bishop, and another a priest, and another a deacon. Therefore neither in the angels are the orders from nature, but from grace only.