Objection 1. It would seem that the human nature was united to the Word of God accidentally. For the Apostle says (Philippians 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 (Article 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 (Article 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).
Reply to Objection 2. Whatever accrues after the completion of the being comes accidentally, unless it be taken into communion with the complete being, just as in the resurrection the body comes to the soul which pre-exists, yet not accidentally, because it is assumed unto the same being, so that the body has vital being through the soul; but it is not so with whiteness, for the being of whiteness is other than the being of man to which whiteness comes. But the Word of God from all eternity had complete being in hypostasis or person; while in time the human nature accrued to it, not as if it were assumed unto one being inasmuch as this is of the nature (even as the body is assumed to the being of the soul), but to one being inasmuch as this is of the hypostasis or person. Hence the human nature is not accidentally united to the Son of God.
Reply to Objection 3. Accident is divided against substance. Now substance, as is plain from Metaph. v, 25, is taken in two ways: first, for essence or nature; secondly, for suppositum or hypostasis--hence the union having taken place in the hypostasis, is enough to show that it is not an accidental union, although the union did not take place in the nature.
Reply to Objection 4. Not everything that is assumed as an instrument pertains to the hypostasis of the one who assumes, as is plain in the case of a saw or a sword; yet nothing prevents what is assumed into the unity of the hypostasis from being as an instrument, even as the body of man or his members. Hence Nestorius held that the human nature was assumed by the Word merely as an instrument, and not into the unity of the hypostasis. And therefore he did not concede that the man was really the Son of God, but His instrument. Hence Cyril says (Epist. ad Monach. Aegyptii): "The Scripture does not affirm that this Emmanuel," i.e. Christ, "was assumed for the office of an instrument, but as God truly humanized," i.e. made man. But Damascene held that the human nature in Christ is an instrument belonging to the unity of the hypostasis.
Source: Summa Theologica (New Advent)