Objection 1. It would seem that the angels exercise functions of life in assumed bodies. For pretence is unbecoming in angels of truth. But it would be pretence if the body assumed by them, which seems to live and to exercise vital functions, did not possess these functions. Therefore the angels exercise functions of life in the assumed body.
Objection 2. Further, in the works of the angels there is nothing without a purpose. But eyes, nostrils, and the other instruments of the senses, would be fashioned without a purpose in the body assumed by the angel, if he perceived nothing by their means. Consequently, the angel perceives by the assumed body; and this is the most special function of life.
Objection 3. Further, to move hither and thither is one of the functions of life, as the Philosopher says (De Anima ii). But the angels are manifestly seen to move in their assumed bodies. For it was said (Genesis 18:16) that "Abraham walked with" the angels, who had appeared to him, "bringing them on the way"; and when Tobias said to the angel (Tobit 5:7-8): "Knowest thou the way that leadeth to the city of Medes?" he answered: "I know it; and I have often walked through all the ways thereof." Therefore the angels often exercise functions of life in assumed bodies.
Objection 4. Further, speech is the function of a living subject, for it is produced by the voice, while the voice itself is a sound conveyed from the mouth. But it is evident from many passages of Sacred Scripture that angels spoke in assumed bodies. Therefore in their assumed bodies they exercise functions of life.
Objection 5. Further, eating is a purely animal function. Hence the Lord after His Resurrection ate with His disciples in proof of having resumed life (Luke 24). Now when angels appeared in their assumed bodies they ate, and Abraham offered them food, after having previously adored them as God (Genesis 18). Therefore the angels exercise functions of life in assumed bodies.
Objection 6. Further, to beget offspring is a vital act. But this has befallen the angels in their assumed bodies; for it is related: "After the sons of God went in to the daughters of men, and they brought forth children, these are the mighty men of old, men of renown" (Genesis 6:4). Consequently the angels exercised vital functions in their assumed bodies.
On the contrary, The bodies assumed by angels have no life, as was stated in the previous article (ad 3). Therefore they cannot exercise functions of life through assumed bodies.
I answer that, Some functions of living subjects have something in common with other operations; just as speech, which is the function of a living creature, agrees with other sounds of inanimate things, in so far as it is sound; and walking agrees with other movements, in so far as it is movement. Consequently vital functions can be performed in assumed bodies by the angels, as to that which is common in such operations; but not as to that which is special to living subjects; because, according to the Philosopher (De Somn. et Vig. i), "that which has the faculty has the action." Hence nothing can have a function of life except what has life, which is the potential principle of such action.
Reply to Objection 1. As it is in no wise contrary to truth for intelligible things to be set forth in Scripture under sensible figures, since it is not said for the purpose of maintaining that intelligible things are sensible, but in order that properties of intelligible things may be understood according to similitude through sensible figures; so it is not contrary to the truth of the holy angels that through their assumed bodies they appear to be living men, although they are really not. For the bodies are assumed merely for this purpose, that the spiritual properties and works of the angels may be manifested by the properties of man and of his works. This could not so fittingly be done if they were to assume true men; because the properties of such men would lead us to men, and not to angels.
Reply to Objection 2. Sensation is entirely a vital function. Consequently it can in no way be said that the angels perceive through the organs of their assumed bodies. Yet such bodies are not fashioned in vain; for they are not fashioned for the purpose of sensation through them, but to this end, that by such bodily organs the spiritual powers of the angels may be made manifest; just as by the eye the power of the angel's knowledge is pointed out, and other powers by the other members, as Dionysius teaches (Coel. Hier.).
Reply to Objection 3. Movement coming from a united mover is a proper function of life; but the bodies assumed by the angels are not thus moved, since the angels are not their forms. Yet the angels are moved accidentally, when such bodies are moved, since they are in them as movers are in the moved; and they are here in such a way as not to be elsewhere which cannot be said of God. Accordingly, although God is not moved when the things are moved in which He exists, since He is everywhere; yet the angels are moved accidentally according to the movement of the bodies assumed. But they are not moved according to the movement of the heavenly bodies, even though they be in them as the movers in the thing moved, because the heavenly bodies do not change place in their entirety; nor for the spirit which moves the world is there any fixed locality according to any restricted part of the world's substance, which now is in the east, and now in the west, but according to a fixed quarter; because "the moving energy is always in the east," as stated in Phys. viii, text 84.
Reply to Objection 4. Properly speaking, the angels do not talk through their assumed bodies; yet there is a semblance of speech, in so far as they fashion sounds in the air like to human voices.
Reply to Objection 5. Properly speaking, the angels cannot be said to eat, because eating involves the taking of food convertible into the substance of the eater.
Although after the Resurrection food was not converted into the substance of Christ's body, but resolved into pre-existing matter; nevertheless Christ had a body of such a true nature that food could be changed into it; hence it was a true eating. But the food taken by angels was neither changed into the assumed body, nor was the body of such a nature that food could be changed into it; consequently, it was not a true eating, but figurative of spiritual eating. This is what the angel said to Tobias: "When I was with you, I seemed indeed to eat and to drink; but I use an invisible meat and drink" (Tobit 12:19).
Abraham offered them food, deeming them to be men, in whom, nevertheless, he worshipped God, as God is wont to be in the prophets, as Augustine says (De Civ. Dei xvi).
Reply to Objection 6. As Augustine says (De Civ. Dei xv): "Many persons affirm that they have had the experience, or have heard from such as have experienced it, that the Satyrs and Fauns, whom the common folk call incubi, have often presented themselves before women, and have sought and procured intercourse with them. Hence it is folly to deny it. But God's holy angels could not fall in such fashion before the deluge. Hence by the sons of God are to be understood the sons of Seth, who were good; while by the daughters of men the Scripture designates those who sprang from the race of Cain. Nor is it to be wondered at that giants should be born of them; for they were not all giants, albeit there were many more before than after the deluge." Still if some are occasionally begotten from demons, it is not from the seed of such demons, nor from their assumed bodies, but from the seed of men taken for the purpose; as when the demon assumes first the form of a woman, and afterwards of a man; just as they take the seed of other things for other generating purposes, as Augustine says (De Trin. ii.), so that the person born is not the child of a demon, but of a man.
Source: Summa Theologica (New Advent)