(3) We ask concerning their number.
(4) Of their difference from each other.
(5) Of their immortality or incorruptibility. _______________________
FIRST ARTICLE [I, Q. 50, Art. 1]
Whether an Angel Is Altogether Incorporeal?
Objection 1: It would seem that an angel is not entirely incorporeal. For what is incorporeal only as regards ourselves, and not in relation to God, is not absolutely incorporeal. But Damascene says (De Fide Orth. ii) that "an angel is said to be incorporeal and immaterial as regards us; but compared to God it is corporeal and material. Therefore he is not simply incorporeal."
Obj. 2: Further, nothing is moved except a body, as the Philosopher says (Phys. vi, text 32). But Damascene says (De Fide Orth. ii) that "an angel is an ever movable intellectual substance." Therefore an angel is a corporeal substance.
Obj. 3: Further, Ambrose says (De Spir. Sanct. i, 7): "Every creature is limited within its own nature." But to be limited belongs to bodies. Therefore, every creature is corporeal. Now angels are God's creatures, as appears from Ps. 148:2: "Praise ye" the Lord, "all His angels"; and, farther on (verse 4), "For He spoke, and they were made; He commanded, and they were created." Therefore angels are corporeal.
On the contrary, It is said (Ps. 103:4): "Who makes His angels spirits."
I answer that, There must be some incorporeal creatures. For what is principally intended by God in creatures is good, and this consists in assimilation to God Himself. And the perfect assimilation of an effect to a cause is accomplished when the effect imitates the cause according to that whereby the cause produces the effect; as heat makes heat. Now, God produces the creature by His intellect and will (Q. 14, A. 8; Q. 19, A. 4). Hence the perfection of the universe requires that there should be intellectual creatures. Now intelligence cannot be the action of a body, nor of any corporeal faculty; for every body is limited to "here" and "now." Hence the perfection of the universe requires the existence of an incorporeal creature.