Summa Th. III EN Qu.2
Now we must consider the mode of union of the Incarnate Word; and, first, the union itself; secondly, the Person assuming; thirdly, the nature assumed.
Under the first head there are twelve points of inquiry:
(1) Whether the union of the Word Incarnate took place in the nature?
(2) Whether it took place in the Person?
(3) Whether it took place in the suppositum or hypostasis?
(4) Whether the Person or hypostasis of Christ is composite after the Incarnation?
(5) Whether any union of body and soul took place in Christ?
(6) Whether the human nature was united to the Word accidentally?
(7) Whether the union itself is something created?
(8) Whether it is the same as assumption?
(9) Whether the union of the two natures is the greatest union?
(10) Whether the union of the two natures in Christ was brought about by grace?
(11) Whether any merits preceded it?
(12) Whether the grace of union was natural to the man Christ?
Objection: 1. It would seem that the Union of the Word Incarnate took place in the nature. For Cyril says (he is quoted in the acts of the Council of Chalcedon, part ii, act. 1): "We must understand not two natures, but one incarnate nature of the Word of God"; and this could not be unless the union took place in the nature. Therefore the union of the Word Incarnate took place in the nature.
2. Further, Athanasius says that, as the rational soul and the flesh together form the human nature, so God and man together form a certain one nature; therefore the union took place in the nature.
3. Further, of two natures one is not denominated by the other unless they are to some extent mutually transmuted. But the Divine and human natures in Christ are denominated one by the other; for Cyril says (quoted in the acts of the Council of Chalcedon, part ii, act. 1) that the Divine nature "is incarnate"; and Gregory Nazianzen says (Ep. i ad Cledon.) that the human nature is "deified," as appears from Damascene (De Fide Orth. iii, 6,11). Therefore from two natures one seems to have resulted.
On the contrary It is said in the declaration of the Council of Chalcedon: "We confess that in these latter times the only-begotten Son of God appeared in two natures, without confusion, without change, without division, without separation---the distinction of natures not having been taken away by the union." Therefore the union did not take place in the nature.
I answer that To make this question clear we must consider what is "nature." Now it is to be observed that the word "nature" comes from nativity. Hence this word was used first of all to signify the begetting of living beings, which is called "birth" or "sprouting forth," the word "natura" meaning, as it were, "nascitura." Afterwards this word "nature" was taken to signify the principle of this begetting; and because in living things the principle of generation is an intrinsic principle, this word "nature" was further employed to signify any intrinsic principle of motion: thus the Philosopher says (Phys. ii) that "nature is the principle of motion in that in which it is essentially and not accidentally." Now this principle is either form or matter. Hence sometimes form is called nature, and sometimes matter. And because the end of natural generation, in that which is generated, is the essence of the species, which the definition signifies, this essence of the species is called the "nature." And thus Boethius defines nature (De Duab. Nat.): "Nature is what informs a thing with its specific difference,"---i.e. which perfects the specific definition. But we are now speaking of nature as it signifies the essence, or the "what-it-is," or the quiddity of the species.Now, if we take nature in this way, it is impossible that the union of the Incarnate Word took place in the nature. For one thing is made of two or more in three ways. First, from two complete things which remain in their perfection. This can only happen to those whose form is composition, order, or figure, as a heap is made up of many stones brought together without any order, but solely with juxtaposition; and a house is made of stones and beams arranged in order, and fashioned to a figure. And in this way some said the union was by manner of confusion (which is without order) or by manner of commensuration (which is with order). But this cannot be. First, because neither composition nor order nor figure is a substantial form, but accidental; and hence it would follow that the union of the Incarnation was not essential, but accidental, which will be disproved later on (Article ). Secondly, because thereby we should not have an absolute unity, but relative only, for there remain several things actually. Thirdly, because the form of such is not a nature, but an art, as the form of a house; and thus one nature would not be constituted in Christ, as they wish.Secondly, one thing is made up of several things, perfect but changed, as a mixture is made up of its elements; and in this way some have said that the union of the Incarnation was brought about by manner of combination. But this cannot be. First, because the Divine Nature is altogether immutable, as has been said (I 9,1 I 9,2), hence neither can it be changed into something else, since it is incorruptible; nor can anything else be changed into it, for it cannot be generated. Secondly, because what is mixed is of the same species with none of the elements; for flesh differs in species from any of its elements. And thus Christ would be of the same nature neither with His Father nor with His Mother. Thirdly, because there can be no mingling of things widely apart; for the species of one of them is absorbed, e.g. if we were to put a drop of water in a flagon of wine. And hence, since the Divine Nature infinitely exceeds the human nature, there could be no mixture, but the Divine Nature alone would remain.Thirdly, a thing is made up of things not mixed nor changed, but imperfect; as man is made up of soul and body, and likewise of divers members. But this cannot be said of the mystery of the Incarnation. First, because each nature, i.e. the Divine and the human, has its specific perfection. Secondly, because the Divine and human natures cannot constitute anything after the manner of quantitative parts, as the members make up the body; for the Divine Nature is incorporeal; nor after the manner of form and matter, for the Divine Nature cannot be the form of anything, especially of anything corporeal, since it would follow that the species resulting therefrom would be communicable to several, and thus there would be several Christs. Thirdly, because Christ would exist neither in human nature nor in the Divine Nature: since any difference varies the species, as unity varies number, as is said (Metaph. viii, text. 10).
Reply to Objection: 1. This authority of Cyril is expounded in the Fifth Synod (i.e. Constantinople II, coll. viii, can. 8) thus: "If anyone proclaiming one nature of the Word of God to be incarnate does not receive it as the Fathers taught, viz. that from the Divine and human natures (a union in subsistence having taken place) one Christ results, but endeavors from these words to introduce one nature or substance of the Divinity and flesh of Christ, let such a one be anathema." Hence the sense is not that from two natures one results; but that the Nature of the Word of God united flesh to Itself in Person.
2. From the soul and body a double unity, viz. of nature and person---results in each individual---of nature inasmuch as the soul is united to the body, and formally perfects it, so that one nature springs from the two as from act and potentiality or from matter and form. But the comparison is not in this sense, for the Divine Nature cannot be the form of a body, as was proved (I 3,8). Unity of person results from them, however, inasmuch as there is an individual subsisting in flesh and soul; and herein lies the likeness, for the one Christ subsists in the Divine and human natures.
3. As Damascene says (De Fide Orth. iii, 6,11), the Divine Nature is said to be incarnate because It is united to flesh personally, and not that It is changed into flesh. So likewise the flesh is said to be deified, as he also says (De Fide Orth. 15,17), not by change, but by union with the Word, its natural properties still remaining, and hence it may be considered as deified, inasmuch as it becomes the flesh of the Word of God, but not that it becomes God.
Objection: 1. It would seem that the union of the Incarnate Word did not take place in the person. For the Person of God is not distinct from His Nature, as we said (I 39,1). If, therefore, the union did not take place in the nature, it follows that it did not take place in the person.
2. Further, Christ's human nature has no less dignity than ours. But personality belongs to dignity, as was stated above (I 29,3, ad 2). Hence, since our human nature has its proper personality, much more reason was there that Christ's should have its proper personality.
3. Further, as Boethius says (De Duab. Nat.), a person is an individual substance of rational nature. But the Word of God assumed an individual human nature, for "universal human nature does not exist of itself, but is the object of pure thought," as Damascene says (De Fide Orth. iii, 11). Therefore the human nature of Christ has its personality. Hence it does not seem that the union took place in the person.
On the contrary We read in the Synod of Chalcedon (Part ii, act. 5): "We confess that our Lord Jesus Christ is not parted or divided into two persons, but is one and the same only-Begotten Son and Word of God." Therefore the union took place in the person.
I answer that Person has a different meaning from "nature." For nature, as has been said (Article ), designates the specific essence which is signified by the definition. And if nothing was found to be added to what belongs to the notion of the species, there would be no need to distinguish the nature from the suppositum of the nature (which is the individual subsisting in this nature), because every individual subsisting in a nature would be altogether one with its nature. Now in certain subsisting things we happen to find what does not belong to the notion of the species, viz. accidents and individuating principles, which appears chiefly in such as are composed of matter and form. Hence in such as these the nature and the suppositum really differ; not indeed as if they were wholly separate, but because the suppositum includes the nature, and in addition certain other things outside the notion of the species. Hence the suppositum is taken to be a whole which has the nature as its formal part to perfect it; and consequently in such as are composed of matter and form the nature is not predicated of the suppositum, for we do not say that this man is his manhood. But if there is a thing in which there is nothing outside the species or its nature (as in God), the suppositum and the nature are not really distinct in it, but only in our way of thinking, inasmuch it is called "nature" as it is an essence, and a "suppositum" as it is subsisting. And what is said of a suppositum is to be applied to a person in rational or intellectual creatures; for a person is nothing else than "an individual substance of rational nature," according to Boethius. Therefore, whatever adheres to a person is united to it in person, whether it belongs to its nature or not. Hence, if the human nature is not united to God the Word in person, it is nowise united to Him; and thus belief in the Incarnation is altogether done away with, and Christian faith wholly overturned. Therefore, inasmuch as the Word has a human nature united to Him, which does not belong to His Divine Nature, it follows that the union took place in the Person of the Word, and not in the nature.
Reply to Objection: 1. Although in God Nature and Person are not really distinct, yet they have distinct meanings, as was said above, inasmuch as person signifies after the manner of something subsisting. And because human nature is united to the Word, so that the Word subsists in it, and not so that His Nature receives therefrom any addition or change, it follows that the union of human nature to the Word of God took place in the person, and not in the nature.
2. Personality pertains of necessity to the dignity of a thing, and to its perfection so far as it pertains to the dignity and perfection of that thing to exist by itself (which is understood by the word "person"). Now it is a greater dignity to exist in something nobler than oneself than to exist by oneself. Hence the human nature of Christ has a greater dignity than ours, from this very fact that in us, being existent by itself, it has its own personality, but in Christ it exists in the Person of the Word. Thus to perfect the species belongs to the dignity of a form, yet the sensitive part in man, on account of its union with the nobler form which perfects the species, is more noble than in brutes, where it is itself the form which perfects.
3. The Word of God "did not assume human nature in general, but 'in atomo'"---that is, in an individual---as Damascene says (De Fide Orth. iii, 11) otherwise every man would be the Word of God, even as Christ was. Yet we must bear in mind that not every individual in the genus of substance, even in rational nature, is a person, but that alone which exists by itself, and not that which exists in some more perfect thing. Hence the hand of Socrates, although it is a kind of individual, is not a person, because it does not exist by itself, but in something more perfect, viz. in the whole. And hence, too, this is signified by a "person" being defined as "an individual substance," for the hand is not a complete substance, but part of a substance. Therefore, although this human nature is a kind of individual in the genus of substance, it has not its own personality, because it does not exist separately, but in something more perfect, viz. in the Person of the Word. Therefore the union took place in the person.
Objection: 1. It would seem that the union of the Word Incarnate did not take place in the suppositum or hypostasis. For Augustine says (Enchiridion xxxv, xxxviii): "Both the Divine and human substance are one Son of God, but they are one thing [aliud] by reason of the Word and another thing [aliud] by reason of the man." And Pope Leo says in his letter to Flavian (Ep. xxviii): "One of these is glorious with miracles, the other succumbs under injuries." But "one" [aliud] and "the other" [aliud] differ in suppositum. Therefore the union of the Word Incarnate did not take place in the suppositum.
2. Further, hypostasis is nothing more than a "particular substance," as Boethius says (De Duab. Nat.). But it is plain that in Christ there is another particular substance beyond the hypostasis of the Word, viz. the body and the soul and the resultant of these. Therefore there is another hypostasis in Him besides the hypostasis of the Word.
3. Further, the hypostasis of the Word is not included in any genus or species, as is plain from I 3,5. But Christ, inasmuch as He is made man, is contained under the species of man; for Dionysius says (Div. Nom. 1): "Within the limits of our nature He came, Who far surpasses the whole order of nature supersubstantially." Now nothing is contained under the human species unless it be a hypostasis of the human species. Therefore in Christ there is another hypostasis besides the hypostasis of the Word of God; and hence the same conclusion follows as above.
On the contrary Damascene says (De Fide Orth. iii, 3,4,5): "In our Lord Jesus Christ we acknowledge two natures and one hypostasis."
I answer that Some who did not know the relation of hypostasis to person, although granting that there is but one person in Christ, held, nevertheless, that there is one hypostasis of God and another of man, and hence that the union took place in the person and not in the hypostasis. Now this, for three reasons, is clearly erroneous. First, because person only adds to hypostasis a determinate nature, viz. rational, according to what Boethius says (De Duab. Nat.), "a person is an individual substance of rational nature"; and hence it is the same to attribute to the human nature in Christ a proper hypostasis and a proper person. And the holy Fathers, seeing this, condemned both in the Fifth Council held at Constantinople, saying: "If anyone seeks to introduce into the mystery of the Incarnation two subsistences or two persons, let him be anathema. For by the incarnation of one of the Holy Trinity, God the Word, the Holy Trinity received no augment of person or subsistence." Now "subsistence" is the same as the subsisting thing, which is proper to hypostasis, as is plain from Boethius (De Duab. Nat.). Secondly, because if it is granted that person adds to hypostasis something in which the union can take place, this something is nothing else than a property pertaining to dignity; according as it is said by some that a person is a "hypostasis distinguished by a property pertaining to dignity." If, therefore, the union took place in the person and not in the hypostasis, it follows that the union only took place in regard to some dignity. And this is what Cyril, with the approval of the Council of Ephesus (part iii, can. 3), condemned in these terms: "If anyone after the uniting divides the subsistences in the one Christ, only joining them in a union of dignity or authority or power, and not rather in a concourse of natural union, let him be anathema." Thirdly, because to the hypostasis alone are attributed the operations and the natural properties, and whatever belongs to the nature in the concrete; for we say that this man reasons, and is risible, and is a rational animal. So likewise this man is said to be a suppositum, because he underlies [supponitur] whatever belongs to man and receives its predication. Therefore, if there is any hypostasis in Christ besides the hypostasis of the Word, it follows that whatever pertains to man is verified of some other than the Word, e.g. that He was born of a Virgin, suffered, was crucified, was buried. And this, too, was condemned with the approval of the Council of Ephesus (part iii, can. 4) in these words: "If anyone ascribes to two persons or subsistences such words as are in the evangelical and apostolic Scriptures, or have been said of Christ by the saints, or by Himself of Himself, and, moreover, applies some of them to the man, taken as distinct from the Word of God, and some of them (as if they could be used of God alone) only to the Word of God the Father, let him be anathema." Therefore it is plainly a heresy condemned long since by the Church to say that in Christ there are two hypostases, or two supposita, or that the union did not take place in the hypostasis or suppositum. Hence in the same Synod (can. 2) it is said: "If anyone does not confess that the Word was united to flesh in subsistence, and that Christ with His flesh is both---to wit, God and man---let him be anathema."
Reply to Objection: 1. As accidental difference makes a thing "other" [alterum], so essential difference makes "another thing" [aliud]. Now it is plain that the "otherness" which springs from accidental difference may pertain to the same hypostasis or suppositum in created things, since the same thing numerically can underlie different accidents. But it does not happen in created things that the same numerically can subsist in divers essences or natures. Hence just as when we speak of "otherness" in regard to creatures we do not signify diversity of suppositum, but only diversity of accidental forms, so likewise when Christ is said to be one thing or another thing, we do not imply diversity of suppositum or hypostasis, but diversity of nature. Hence Gregory Nazianzen says in a letter to Chelidonius (Ep. ci): "In the Saviour we may find one thing and another, yet He is not one person and another. And I say 'one thing and another'; whereas, on the contrary, in the Trinity we say one Person and another (so as not to confuse the subsistences), but not one thing and another."
2. Hypostasis signifies a particular substance, not in every way, but as it is in its complement. Yet as it is in union with something more complete, it is not said to be a hypostasis, as a hand or a foot. So likewise the human nature in Christ, although it is a particular substance, nevertheless cannot be called a hypostasis or suppositum, seeing that it is in union with a completed thing, viz. the whole Christ, as He is God and man. But the complete being with which it concurs is said to be a hypostasis or suppositum.
3. In created things a singular thing is placed in a genus or species, not on account of what belongs to its individuation, but on account of its nature, which springs from its form, and in composite things individuation is taken more from matter. Hence we say that Christ is in the human species by reason of the nature assumed, and not by reason of the hypostasis.
Objection: 1. It would seem that the Person of Christ is not composite. For the Person of Christ is naught else than the Person or hypostasis of the Word, as appears from what has been said (Article ). But in the Word, Person and Nature do not differ, as appears from I 39,1. Therefore since the Nature of the Word is simple, as was shown above (I 3,7), it is impossible that the Person of Christ be composite.
2. Further, all composition requires parts. But the Divine Nature is incompatible with the notion of a part, for every part implicates the notion of imperfection. Therefore it is impossible that the Person of Christ be composed of two natures.
3. Further, what is composed of others would seem to be homogeneous with them, as from bodies only a body can be composed. Therefore if there is anything in Christ composed of the two natures, it follows that this will not be a person but a nature; and hence the union in Christ will take place in the nature, which is contrary to Article .
On the contrary Damascene says (De Fide Orth. iii, 3,4,5), "In the Lord Jesus Christ we acknowledge two natures, but one hypostasis composed from both."
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.
Reply to Objection: 1. And thereby the solution to the first is clear.
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.
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.
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 Articles ,3.
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.
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 (Ph 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 (Article , ad 2).
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 (Lc 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."
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.
Objection: 1. It would seem that the human nature was united to the Word of God accidentally. For the Apostle says (Ph 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.
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.
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 ). Hence the human nature must have accrued accidentally to the Son of God.
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 ). 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).
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.
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.
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.
Summa Th. III EN Qu.2