On the contrary, Whatever the Son can do, so can the Father and the Holy Ghost, otherwise the power of the three Persons would not be one. But the Son was able to become incarnate. Therefore the Father and the Holy Ghost were able to become incarnate.
I answer that, As was said above (AA[1],2,4), assumption implies two things, viz. the act of the one assuming and the term of the assumption. Now the principle of the act is the Divine power, and the term is a Person. But the Divine power is indifferently and commonly in all the Persons. Moreover, the nature of Personality is common to all the Persons, although the personal properties are different. Now whenever a power regards several things indifferently, it can terminate its action in any of them indifferently, as is plain in rational powers, which regard opposites, and can do either of them. Therefore the Divine power could have united human nature to the Person of the Father or of the Holy Ghost, as It united it to the Person of the Son. And hence we must say that the Father or the Holy Ghost could have assumed flesh even as the Son.
Reply to Objection 1: The temporal sonship, whereby Christ is said to be the Son of Man, does not constitute His Person, as does the eternal Sonship; but is something following upon the temporal nativity. Hence, if the name of son were transferred to the Father or the Holy Ghost in this manner, there would be no confusion of the Divine Persons.
Reply to Objection 2: Adoptive sonship is a certain participation of natural sonship; but it takes place in us, by appropriation, by the Father, Who is the principle of natural sonship, and by the gift of the Holy Ghost, Who is the love of the Father and Son, according to Gal. 4:6 : "God hath sent the Spirit of His Son into your hearts crying, Abba, Father." And therefore, even as by the Incarnation of the Son we receive adoptive sonship in the likeness of His natural sonship, so likewise, had the Father become incarnate, we should have received adoptive sonship from Him, as from the principle of the natural sonship, and from the Holy Ghost as from the common bond of Father and Son.
Reply to Objection 3: It belongs to the Father to be innascible as to eternal birth, and the temporal birth would not destroy this. But the Son of God is said to be sent in regard to the Incarnation, inasmuch as He is from another, without which the Incarnation would not suffice for the nature of mission.
Whether several Divine Persons can assume one and the same individual nature?
Objection 1: It would seem that two Divine Persons cannot assume one and the same individual nature. For, this being granted, there would either be several men or one. But not several, for just as one Divine Nature in several Persons does not make several gods, so one human nature in several persons does not make several men. Nor would there be only one man, for one man is "this man," which signifies one person; and hence the distinction of three Divine Persons would be destroyed, which cannot be allowed. Therefore neither two nor three Persons can take one human nature.
Objection 2: Further, the assumption is terminated in the unity of Person, as has been said above (A[2]). But the Father, Son, and Holy Ghost are not one Person. Therefore the three Persons cannot assume one human nature.
Objection 3: Further, Damascene says (De Fide Orth. iii, 3,4), and Augustine (De Trin. i, 11,12,13), that from the Incarnation of God the Son it follows that whatever is said of the Son of God is said of the Son of Man, and conversely. Hence, if three Persons were to assume one human nature, it would follow that whatever is said of each of the three Persons would be said of the man; and conversely, what was said of the man could be said of each of the three Persons. Therefore what is proper to the Father, viz. to beget the Son, would be said of the man, and consequently would be said of the Son of God; and this could not be. Therefore it is impossible that the three Persons should assume one human nature.
On the contrary, The Incarnate Person subsists in two natures. But the three Persons can subsist in one Divine Nature. Therefore they can also subsist in one human nature in such a way that the human nature be assumed by the three Persons.
I answer that, As was said above (Q[2], A[5], ad 1), by the union of the soul and body in Christ neither a new person is made nor a new hypostasis, but one human nature is assumed to the Divine Person or hypostasis, which, indeed, does not take place by the power of the human nature, but by the power of the Divine Person. Now such is the characteristic of the Divine Persons that one does not exclude another from communicating in the same nature, but only in the same Person. Hence, since in the mystery of the Incarnation "the whole reason of the deed is the power of the doer," as Augustine says (Ep. ad Volusianum cxxxvii), we must judge of it in regard to the quality of the Divine Person assuming, and not according to the quality of the human nature assumed. Therefore it is not impossible that two or three Divine Persons should assume one human nature, but it would be impossible for them to assume one human hypostasis or person; thus Anselm says in the book De Concep. Virg. (Cur Deus Homo ii, 9), that "several Persons cannot assume one and the same man to unity of Person."
Reply to Objection 1: In the hypothesis that three Persons assume one human nature, it would be true to say that the three Persons were one man, because of the one human nature. For just as it is now true to say the three Persons are one God on account of the one Divine Nature, so it would be true to say they are one man on account of the one human nature. Nor would "one" imply unity of person, but unity in human nature; for it could not be argued that because the three Persons were one man they were one simply. For nothing hinders our saying that men, who are many simply, are in some respect one, e.g. one people, and as Augustine says (De Trin. vi, 3): "The Spirit of God and the spirit of man are by nature different, but by inherence one spirit results," according to 1 Cor. 6:17 : "He who is joined to the Lord is one spirit."
Reply to Objection 2: In this supposition the human nature would be assumed to the unity, not indeed of one Person, but to the unity of each Person, so that even as the Divine Nature has a natural unity with each Person, so also the human nature would have a unity with each Person by assumption.
Reply to Objection 3: In the mystery of the Incarnation, there results a communication of the properties belonging to the nature, because whatever belongs to the nature can be predicated of the Person subsisting in that nature, no matter to which of the natures it may apply. Hence in this hypothesis, of the Person of the Father may be predicated what belongs to the human nature and what belongs to the Divine; and likewise of the Person of the Son and of the Holy Ghost. But what belongs to the Person of the Father by reason of His own Person could not be attributed to the Person of the Son or Holy Ghost on account of the distinction of Persons which would still remain. Therefore it might be said that as the Father was unbegotten, so the man was unbegotten, inasmuch as "man" stood for the Person of the Father. But if one were to go on to say, "The man is unbegotten; the Son is man; therefore the Son is unbegotten," it would be the fallacy of figure of speech or of accident; even as we now say God is unbegotten, because the Father is unbegotten, yet we cannot conclude that the Son is unbegotten, although He is God.
Whether one Divine Person can assume two human natures?
Objection 1: It would seem that one Divine Person cannot assume two human natures. For the nature assumed in the mystery of the Incarnation has no other suppositum than the suppositum of the Divine Person, as is plain from what has been stated above (Q[2], AA[3],6). Therefore, if we suppose one Person to assume two human natures, there would be one suppositum of two natures of the same species; which would seem to imply a contradiction, for the nature of one species is only multiplied by distinct supposita.
Objection 2: Further, in this hypothesis it could not be said that the Divine Person incarnate was one man, seeing that He would not have one human nature; neither could it be said that there were several, for several men have distinct supposita, whereas in this case there would be only one suppositum. Therefore the aforesaid hypothesis is impossible.
Objection 3: Further, in the mystery of the Incarnation the whole Divine Nature is united to the whole nature assumed, i.e. to every part of it, for Christ is "perfect God and perfect man, complete God and complete man," as Damascene says (De Fide Orth. iii, 7). But two human natures cannot be wholly united together, inasmuch as the soul of one would be united to the body of the other; and, again, two bodies would be together, which would give rise to confusion of natures. Therefore it is not possibly for one Divine Person to assume two human natures.
On the contrary, Whatever the Father can do, that also can the Son do. But after the Incarnation the Father can still assume a human nature distinct from that which the Son has assumed; for in nothing is the power of the Father or the Son lessened by the Incarnation of the Son. Therefore it seems that after the Incarnation the Son can assume another human nature distinct from the one He has assumed.
I answer that, What has power for one thing, and no more, has a power limited to one. Now the power of a Divine Person is infinite, nor can it be limited by any created thing. Hence it may not be said that a Divine Person so assumed one human nature as to be unable to assume another. For it would seem to follow from this that the Personality of the Divine Nature was so comprehended by one human nature as to be unable to assume another to its Personality; and this is impossible, for the Uncreated cannot be comprehended by any creature. Hence it is plain that, whether we consider the Divine Person in regard to His power, which is the principle of the union, or in regard to His Personality, which is the term of the union, it has to be said that the Divine Person, over and beyond the human nature which He has assumed, can assume another distinct human nature.
Reply to Objection 1: A created nature is completed in its essentials by its form, which is multiplied according to the division of matter. And hence, if the composition of matter and form constitutes a new suppositum, the consequence is that the nature is multiplied by the multiplication of supposita. But in the mystery of the Incarnation the union of form and matter, i.e. of soul and body, does not constitute a new suppositum, as was said above (A[6]). Hence there can be a numerical multitude on the part of the nature, on account of the division of matter, without distinction of supposita.
Reply to Objection 2: It might seem possible to reply that in such a hypothesis it would follow that there were two men by reason of the two natures, just as, on the contrary, the three Persons would be called one man, on account of the one nature assumed, as was said above (A[6], ad 1). But this does not seem to be true; because we must use words according to the purpose of their signification, which is in relation to our surroundings. Consequently, in order to judge of a word's signification or co-signification, we must consider the things which are around us, in which a word derived from some form is never used in the plural unless there are several supposita. For a man who has on two garments is not said to be "two persons clothed," but "one clothed with two garments"; and whoever has two qualities is designated in the singular as "such by reason of the two qualities." Now the assumed nature is, as it were, a garment, although this similitude does not fit at all points, as has been said above (Q[2], A[6], ad 1). And hence, if the Divine Person were to assume two human natures, He would be called, on account of the unity of suppositum, one man having two human natures. Now many men are said to be one people, inasmuch as they have some one thing in common, and not on account of the unity of suppositum. So likewise, if two Divine Persons were to assume one singular human nature, they would be said to be one man, as stated (A[6], ad 1), not from the unity of suppositum, but because they have some one thing in common.
Reply to Objection 3: The Divine and human natures do not bear the same relation to the one Divine Person, but the Divine Nature is related first of all thereto, inasmuch as It is one with It from eternity; and afterwards the human nature is related to the Divine Person, inasmuch as it is assumed by the Divine Person in time, not indeed that the nature is the Person, but that the Person of God subsists in human nature. For the Son of God is His Godhead, but is not His manhood. And hence, in order that the human nature may be assumed by the Divine Person, the Divine Nature must be united by a personal union with the whole nature assumed, i.e. in all its parts. Now in the two natures assumed there would be a uniform relation to the Divine Person, nor would one assume the other. Hence it would not be necessary for one of them to be altogether united to the other, i.e. all the parts of one with all the parts of the other.
Whether it was more fitting that the Person of the Son rather than any other Divine Person should assume human nature?
Objection 1: It would seem that it was not more fitting that the Son of God should become incarnate than the Father or the Holy Ghost. For by the mystery of the Incarnation men are led to the true knowledge of God, according to Jn. 18:37 : "For this was I born, and for this came I into the world, to give testimony to the truth." But by the Person of the Son of God becoming incarnate many have been kept back from the true knowledge of God, since they referred to the very Person of the Son what was said of the Son in His human nature, as Arius, who held an inequality of Persons, according to what is said ( Jn. 14:28 ): "The Father is greater than I." Now this error would not have arisen if the Person of the Father had become incarnate, for no one would have taken the Father to be less than the Son. Hence it seems fitting that the Person of the Father, rather than the Person of the Son, should have become incarnate.