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.
The ancients, however, not properly realizing the force of intelligence, and failing to make a proper distinction between sense and intellect, thought that nothing existed in the world but what could be apprehended by sense and imagination. And because bodies alone fall under imagination, they supposed that no being existed except bodies, as the Philosopher observes (Phys. iv, text 52,57). Thence came the error of the Sadducees, who said there was no spirit (Acts 23:8).
But the very fact that intellect is above sense is a reasonable proof that there are some incorporeal things comprehensible by the intellect alone.
Reply Obj. 1: Incorporeal substances rank between God and corporeal creatures. Now the medium compared to one extreme appears to be the other extreme, as what is tepid compared to heat seems to be cold; and thus it is said that angels, compared to God, are material and corporeal, not, however, as if anything corporeal existed in them.
Reply Obj. 2: Movement is there taken in the sense in which it is applied to intelligence and will. Therefore an angel is called an ever mobile substance, because he is ever actually intelligent, and not as if he were sometimes actually and sometimes potentially, as we are. Hence it is clear that the objection rests on an equivocation.
Reply Obj. 3: To be circumscribed by local limits belongs to bodies only; whereas to be circumscribed by essential limits belongs to all creatures, both corporeal and spiritual. Hence Ambrose says (De Spir. Sanct. i, 7) that "although some things are not contained in corporeal place, still they are none the less circumscribed by their substance."