I answer that, A thing is said to be assumable as being capable of being assumed by a Divine Person, and this capability cannot be taken with reference to the natural passive power, which does not extend to what transcends the natural order, as the personal union of a creature with God transcends it. Hence it follows that a thing is said to be assumable according to some fitness for such a union. Now this fitness in human nature may be taken from two things, viz. according to its dignity, and according to its need. According to its dignity, because human nature, as being rational and intellectual, was made for attaining to the Word to some extent by its operation, viz. by knowing and loving Him. According to its need—because it stood in need of restoration, having fallen under original sin. Now these two things belong to human nature alone. For in the irrational creature the fitness of dignity is wanting, and in the angelic nature the aforesaid fitness of need is wanting. Hence it follows that only human nature was assumable.
Reply Obj. 1: Creatures are said to be "such" with reference to their proper causes, not with reference to what belongs to them from their first and universal causes; thus we call a disease incurable, not that it cannot be cured by God, but that it cannot be cured by the proper principles of the subject. Therefore a creature is said to be not assumable, not as if we withdrew anything from the power of God, but in order to show the condition of the creature, which has no capability for this.
Reply Obj. 2: The likeness of image is found in human nature, forasmuch as it is capable of God, viz. by attaining to Him through its own operation of knowledge and love. But the likeness of trace regards only a representation by Divine impression, existing in the creature, and does not imply that the irrational creature, in which such a likeness is, can attain to God by its own operation alone. For what does not come up to the less, has no fitness for the greater; as a body which is not fitted to be perfected by a sensitive soul is much less fitted for an intellectual soul. Now much greater and more perfect is the union with God in personal being than the union by operation. And hence the irrational creature which falls short of the union with God by operation has no fitness to be united with Him in personal being.
Reply Obj. 3: Some say that angels are not assumable, since they are perfect in their personality from the beginning of their creation, inasmuch as they are not subject to generation and corruption; hence they cannot be assumed to the unity of a Divine Person, unless their personality be destroyed, and this does not befit the incorruptibility of their nature nor the goodness of the one assuming, to Whom it does not belong to corrupt any perfection in the creature assumed. But this would not seem totally to disprove the fitness of the angelic nature for being assumed. For God by producing a new angelic nature could join it to Himself in unity of Person, and in this way nothing pre-existing would be corrupted in it. But as was said above, there is wanting the fitness of need, because, although the angelic nature in some is the subject of sin, their sin is irremediable, as stated above (I, Q. 64, A. 2).
Reply Obj. 4: The perfection of the universe is not the perfection of one person or suppositum, but of something which is one by position or order, whereof very many parts are not capable of assumption, as was said above. Hence it follows that only human nature is capable of being assumed. _______________________
SECOND ARTICLE [III, Q. 4, Art. 2]
Whether the Son of God Assumed a Person?
Objection 1: It would seem that the Son of God assumed a person. For Damascene says (De Fide Orth. iii, 11) that the Son of God "assumed human nature in atomo," i.e. in an individual. But an individual in rational nature is a person, as is plain from Boethius (De Duab. Nat.). Therefore the Son of God assumed a person.
Obj. 2: Further, Damascene says (De Fide Orth. iii, 6) that the Son of God "assumed what He had sown in our nature." But He sowed our personality there. Therefore the Son of God assumed a person.
Obj. 3: Further, nothing is absorbed unless it exist. But Innocent III [*Paschas. Diac., De Spiritu Sanct. ii] says in a Decretal that "the Person of God absorbed the person of man." Therefore it would seem that the person of man existed previous to its being assumed.