I answer that, The Person or hypostasis of Christ may be viewed in two ways. First as it is in itself, and thus it is altogether simple, even as the Nature of the Word. Secondly, in the aspect of person or hypostasis to which it belongs to subsist in a nature; and thus the Person of Christ subsists in two natures. Hence though there is one subsisting being in Him, yet there are different aspects of subsistence, and hence He is said to be a composite person, insomuch as one being subsists in two.
And thereby the solution to the first is clear.
Reply to Objection 2: This composition of a person from natures is not so called on account of parts, but by reason of number, even as that in which two things concur may be said to be composed of them.
Reply to Objection 3: It is not verified in every composition, that the thing composed is homogeneous with its component parts, but only in the parts of a continuous thing; for the continuous is composed solely of continuous [parts]. But an animal is composed of soul and body, and neither of these is an animal.
Whether in Christ there is any union of soul and body?
Objection 1: It would seem that in Christ there was no union of soul and body. For from the union of soul and body in us a person or a human hypostasis is caused. Hence if the soul and body were united in Christ, it follows that a hypostasis resulted from their union. But this was not the hypostasis of God the Word, for It is eternal. Therefore in Christ there would be a person or hypostasis besides the hypostasis of the Word, which is contrary to AA[2],3.
Objection 2: Further, from the union of soul and body results the nature of the human species. But Damascene says (De Fide Orth. iii, 3), that "we must not conceive a common species in the Lord Jesus Christ." Therefore there was no union of soul and body in Him.
Objection 3: Further, the soul is united to the body for the sole purpose of quickening it. But the body of Christ could be quickened by the Word of God Himself, seeing He is the fount and principle of life. Therefore in Christ there was no union of soul and body.
On the contrary, The body is not said to be animated save from its union with the soul. Now the body of Christ is said to be animated, as the Church chants: "Taking an animate body, He deigned to be born of a Virgin" [*Feast of the Circumcision, Ant. ii, Lauds]. Therefore in Christ there was a union of soul and body.
I answer that, Christ is called a man univocally with other men, as being of the same species, according to the Apostle ( Phil. 2:7 ), "being made in the likeness of a man." Now it belongs essentially to the human species that the soul be united to the body, for the form does not constitute the species, except inasmuch as it becomes the act of matter, and this is the terminus of generation through which nature intends the species. Hence it must be said that in Christ the soul was united to the body; and the contrary is heretical, since it destroys the truth of Christ's humanity.
Reply to Objection 1: This would seem to be the reason which was of weight with such as denied the union of the soul and body in Christ, viz. lest they should thereby be forced to admit a second person or hypostasis in Christ, since they saw that the union of soul and body in mere men resulted in a person. But this happens in mere men because the soul and body are so united in them as to exist by themselves. But in Christ they are united together, so as to be united to something higher, which subsists in the nature composed of them. And hence from the union of the soul and body in Christ a new hypostasis or person does not result, but what is composed of them is united to the already existing hypostasis or Person. Nor does it therefore follow that the union of the soul and body in Christ is of less effect than in us, for its union with something nobler does not lessen but increases its virtue and worth; just as the sensitive soul in animals constitutes the species, as being considered the ultimate form, yet it does not do so in man, although it is of greater effect and dignity, and this because of its union with a further and nobler perfection, viz. the rational soul, as has been said above (A[2], ad 2).
Reply to Objection 2: This saying of Damascene may be taken in two ways: First, as referring to human nature, which, as it is in one individual alone, has not the nature of a common species, but only inasmuch as either it is abstracted from every individual, and considered in itself by the mind, or according as it is in all individuals. Now the Son of God did not assume human nature as it exists in the pure thought of the intellect, since in this way He would not have assumed human nature in reality, unless it be said that human nature is a separate idea, just as the Platonists conceived of man without matter. But in this way the Son of God would not have assumed flesh, contrary to what is written ( Lk. 24:39 ), "A spirit hath not flesh and bones as you see Me to have." Neither can it be said that the Son of God assumed human nature as it is in all the individuals of the same species, otherwise He would have assumed all men. Therefore it remains, as Damascene says further on (De Fide Orth. iii, 11) that He assumed human nature "in atomo," i.e. in an individual; not, indeed, in another individual which is a suppositum or a person of that nature, but in the Person of the Son of God.
Secondly, this saying of Damascene may be taken not as referring to human nature, as if from the union of soul and body one common nature (viz. human) did not result, but as referring to the union of the two natures Divine and human: which do not combine so as to form a third something that becomes a common nature, for in this way it would become predicable of many, and this is what he is aiming at, since he adds: "For there was not generated, neither will there ever be generated, another Christ, Who from the Godhead and manhood, and in the Godhead and manhood, is perfect God and perfect man."
Reply to Objection 3: There are two principles of corporeal life: one the effective principle, and in this way the Word of God is the principle of all life; the other, the formal principle of life, for since "in living things to be is to live," as the Philosopher says (De Anima ii, 37), just as everything is formally by its form, so likewise the body lives by the soul: in this way a body could not live by the Word, Which cannot be the form of a body.
Whether the human nature was united to the Word of God accidentally?
Objection 1: It would seem that the human nature was united to the Word of God accidentally. For the Apostle says ( Phil. 2:7 ) of the Son of God, that He was "in habit found as a man." But habit is accidentally associated with that to which it pertains, whether habit be taken for one of the ten predicaments or as a species of quality. Therefore human nature is accidentally united to the Son of God.
Objection 2: Further, whatever comes to a thing that is complete in being comes to it accidentally, for an accident is said to be what can come or go without the subject being corrupted. But human nature came to Christ in time, Who had perfect being from eternity. Therefore it came to Him accidentally.
Objection 3: Further, whatever does not pertain to the nature or the essence of a thing is its accident, for whatever is, is either a substance or an accident. But human nature does not pertain to the Divine Essence or Nature of the Son of God, for the union did not take place in the nature, as was said above (A[1]). Hence the human nature must have accrued accidentally to the Son of God.
Objection 4: Further, an instrument accrues accidentally. But the human nature was the instrument of the Godhead in Christ, for Damascene says (De Fide Orth. iii, 15), that "the flesh of Christ is the instrument of the Godhead." Therefore it seems that the human nature was united to the Son of God accidentally.
On the contrary, Whatever is predicated accidentally, predicates, not substance, but quantity, or quality, or some other mode of being. If therefore the human nature accrues accidentally, when we say Christ is man, we do not predicate substance, but quality or quantity, or some other mode of being, which is contrary to the Decretal of Pope Alexander III, who says (Conc. Later. iii): "Since Christ is perfect God and perfect man, what foolhardiness have some to dare to affirm that Christ as man is not a substance?"
I answer that, In evidence of this question we must know that two heresies have arisen with regard to the mystery of the union of the two natures in Christ. The first confused the natures, as Eutyches and Dioscorus, who held that from the two natures one nature resulted, so that they confessed Christ to be "from" two natures (which were distinct before the union), but not "in" two natures (the distinction of nature coming to an end after the union). The second was the heresy of Nestorius and Theodore of Mopsuestia, who separated the persons. For they held the Person of the Son of God to be distinct from the Person of the Son of man, and said these were mutually united: first, "by indwelling," inasmuch as the Word of God dwelt in the man, as in a temple; secondly, "by unity of intention," inasmuch as the will of the man was always in agreement with the will of the Word of God; thirdly, "by operation," inasmuch as they said the man was the instrument of the Word of God; fourthly, "by greatness of honor," inasmuch as all honor shown to the Son of God was equally shown to the Son of man, on account of His union with the Son of God; fifthly, "by equivocation," i.e. communication of names, inasmuch as we say that this man is God and the Son of God. Now it is plain that these modes imply an accidental union.
But some more recent masters, thinking to avoid these heresies, through ignorance fell into them. For some conceded one person in Christ, but maintained two hypostases, or two supposita, saying that a man, composed of body and soul, was from the beginning of his conception assumed by the Word of God. And this is the first opinion set down by the Master (Sent. iii, D, 6). But others desirous of keeping the unity of person, held that the soul of Christ was not united to the body, but that these two were mutually separate, and were united to the Word accidentally, so that the number of persons might not be increased. And this is the third opinion which the Master sets down (Sent. iii, D, 6).
But both of these opinions fall into the heresy of Nestorius; the first, indeed, because to maintain two hypostases or supposita in Christ is the same as to maintain two persons, as was shown above (A[3]). And if stress is laid on the word "person," we must have in mind that even Nestorius spoke of unity of person on account of the unity of dignity and honor. Hence the fifth Council (Constantinople II, coll. viii, can. 5) directs an anathema against such a one as holds "one person in dignity, honor and adoration, as Theodore and Nestorius foolishly wrote." But the other opinion falls into the error of Nestorius by maintaining an accidental union. For there is no difference in saying that the Word of God is united to the Man Christ by indwelling, as in His temple (as Nestorius said), or by putting on man, as a garment, which is the third opinion; rather it says something worse than Nestorius---to wit, that the soul and body are not united.
Now the Catholic faith, holding the mean between the aforesaid positions, does not affirm that the union of God and man took place in the essence or nature, nor yet in something accidental, but midway, in a subsistence or hypostasis. Hence in the fifth Council (Constantinople II, coll. viii, can. 5) we read: "Since the unity may be understood in many ways, those who follow the impiety of Apollinaris and Eutyches, professing the destruction of what came together" (i.e. destroying both natures), "confess a union by mingling; but the followers of Theodore and Nestorius, maintaining division, introduce a union of purpose. But the Holy Church of God, rejecting the impiety of both these treasons, confesses a union of the Word of God with flesh, by composition, which is in subsistence." Therefore it is plain that the second of the three opinions, mentioned by the Master (Sent. iii, D, 6), which holds one hypostasis of God and man, is not to be called an opinion, but an article of Catholic faith. So likewise the first opinion which holds two hypostases, and the third which holds an accidental union, are not to be styled opinions, but heresies condemned by the Church in Councils.
Reply to Objection 1: As Damascene says (De Fide Orth. iii, 26): "Examples need not be wholly and at all points similar, for what is wholly similar is the same, and not an example, and especially in Divine things, for it is impossible to find a wholly similar example in the Theology," i.e. in the Godhead of Persons, "and in the Dispensation," i.e. the mystery of the Incarnation. Hence the human nature in Christ is likened to a habit, i.e. a garment, not indeed in regard to accidental union, but inasmuch as the Word is seen by the human nature, as a man by his garment, and also inasmuch as the garment is changed, for it is shaped according to the figure of him who puts it on, and yet he is not changed from his form on account of the garment. So likewise the human nature assumed by the Word of God is ennobled, but the Word of God is not changed, as Augustine says (Qq. 83, qu. 73).