Summa III, Question 2

Question 2. The mode of union of the Word incarnate


Article 1. Whether the Union of Incarnate Word took place in the nature?


It seems that, from two natures, one has come.


To make this question clear we must consider what is “nature.”


“Nature is what informs a thing with its specific difference,” But we are now speaking of nature as it signifies the essence, or the “what-it-is,” or the quiddity of the species. It is not possible that God Incarnate happened in this way, because God’s eternal nature cannot change. God, “mixed” with human nature, would imply change to God’s eternal nature.


The Divine nature being infinite, it would consume the human nature; there could be no mixture, but the Divine Nature alone would remain.

Fr. LaGrange says, The argument of St. Thomas may be reduced to the following syllogism. There are only three possible ways for the union to take place in the nature, namely: 1. by the composition of things that are perfect in themselves and that remain perfect; 2. by the mixture of things perfect in themselves that have undergone a change; 3. by the union of things imperfect in themselves that have been neither mixed nor changed.

But these ways are incompatible. Therefore it is impossible for the union to take place in the nature.


Article 2. Whether the union of Incarnate Word took place in the Person?



God is not distinct from His nature.

Since no individual is “human nature” but each is a person, Christ’s “human nature” would be attached to “personality” and thus the union cannot take place in the divine person.


On the contrary, We read in the Synod of Chalcedon “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.”


Thomas answers that, Person has a different meaning from “nature.” 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 Incarnation is altogether done away with, and Christian faith wholly overturned.

Thomas will often show us that a distinction and a difference are not [always] the same thing. This is one doctrine where this understanding will appear again and again.


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.


Article 3. Whether the union of the Word Incarnate took place in the suppositum or hypostasis?


Christ’s human nature is an individual substance but not an individual subject.  Mary is Theotokos, the Mother of God.  She would merely be the mother of His human nature if the human nature was an individual subject.  “This subject” would be what was born of Mary, and this would destroy the entire basis for belief in the Word Incarnate and the entire Christian faith. The Person of Christ then MUST be of two natures but one subject, one Person, and this is the Person born of the Virgin.


Article 4. Whether after Incarnation the Person or Hypostasis of Christ is composite?


Here, again, what we are speaking of it and in what way we are speaking of it differ.  Christ is altogether simple as the One Divine Person.  There is but one subsisting Being in Christ, but, as Damascene says, “In the Lord Jesus Christ we acknowledge two natures, but one hypostasis composed from both.”


“He is said to be a composite person, insomuch as one being subsists in two.”


Article 5. Whether in Christ there is any union of soul and body?


It would seem that, if there is a union of soul and body in Christ, this creates a separate hypostasis, and this could not be.


However, unless there is a union of soul and body in Christ, He does not have a real human nature, because to human nature belongs the composite of body and soul. The soul is the form, the act, of the body, and both are required and required to exist in this way to call something human. Any other view destroys the truth of Christ’s humanity.


From the replies to objections, it is most important to note that the concern that this resulted in a human person, united to the Person of Christ, caused some to reject this understanding, not realizing that “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.”


Article 6. Whether the human nature was united to the Word of God accidentally?


It would seem that the body of Christ was an instrument of Christ, and joined accidentally to Him.


But something predicated accidentally is not of the substance but a quality or quantity or such of that substance, and cannot stand on its own apart from it, such as whiteness with a “something that is white.”  But human nature is not like this.


Again, heresies professing either one nature in Christ or two persons in Christ have resulted of not correctly understanding this union.


The Catholic faith 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. “Since the unity may be understood in many ways…the Holy Church of God…confesses a union of the Word of God with flesh, by composition, which is in subsistence.”


The Second Person of the Trinity has existed from all eternity.  When a human nature became his, the nature remained unchanged, but was absorbed into the existence of His eternal Person.


I am adding here what has been very helpful to me in understanding this, from the pen of Fr. John Hardon, S.J.:


An individual nature receives the quality of incommunicability which describes a person from its own act of existence, something quite distinct from the kind of nature it has. Why, for example, is the human nature I possess not communicable to any other individual of the human family? Precisely because I exist as a complete, autonomous substance. Therefore, if a human nature lacked what is normally (except for a sublime miracle) its natural complement, namely actual existence, it would not be a human person.

This is Thomas’ explanation of what occurred at the Incarnation. The humanity of Christ had all that was required for the perfection of a human nature–body and soul, faculties and emotions–all that we have completely. Yet it came into existence and remains so for eternity not by its own natural act of existence (as happens with everyone else) but by the infinite subsistence of the Second Person of the Trinity. Christ is therefore a divine Person because His act of existence, which identifies personality, is not human but divine.

Article 7. Whether the union of the Divine nature and the human is anything created?


All relationships of God to creatures exist in the creature, and not in God per se.  This is something St. Thomas discusses extensively in Disputed Questions on the Power of God, Question 7, and it is relevant to understand his answer here.


The union of the Second Person to His human nature is not eternal, but began in time, and “Whatever has a beginning in time is created.” The union of Christ’s divinity and His humanity is a relation, which is a really existent thing, but not an eternally existent thing.  Therefore, it is created.


Article 8. Whether union is the same as assumption?


After laying forth the objections, Thomas says, On the contrary, The Divine Nature is said to be united, not assumed.


The union of the Divinity and humanity of Christ is permanent, but the assumption of a human nature by Christ is a completed action.  Therefore, they are not the same.  The union, we may say, occurred between the human and the divine, or between the divine and human.  We cannot, however, say that the human assumed the divine, but only that the divine assumed the human. Thomas states that “assumption means a taking to oneself from another.” This can only happen in one direction between what is human and what is divine.


Article 9. Whether the union of the two natures in Christ is the greatest of all unions?


It is objected that “what is united falls short of the unity of what is one, since what is united is by participation, but one is by essence.” And also that “from the union of soul and body in us there arises what is one in person and nature; whereas from the union of the Divine and human nature there results what is one in person only. Therefore the union of soul and body is greater than that of the Divine and human natures”


The reply is simply that we must view this in respect to “in what” these two are united, and they are united in the eternal Person of Christ.”  This gives it preeminence over all other unions.


In replying to the objections, this view is taken:


“For the unity of a Divine Person is an uncreated and self-subsisting unity,… And hence in this respect the union of Incarnation is higher than numerical unity by reason of the unity of the Divine Person”


“The unity of the Divine Person is greater than the unity of person and nature in us.”


It all comes down the where the union takes place, and that is in the Divine Person.


Article 10. Whether the union of Incarnation took place by grace?


Grace, as a free gift “accidentally” (in the metaphysical sense) joined to men belongs to all men that are given this grace.  But grace belonging to one as personal being belongs exclusively to Christ. There is not a habitual grace, a created grace, in the union of Christ to a human nature, but by uncreated grace.  This belongs not to anyone else, as created grace belongs to the saints and all those in a state of grace, but to Christ alone among those with a human nature, because He is a divine uncreated Person.


Article 11. Whether any merits preceded the union of Incarnation?


“Not by the works of justice which we have done, but according to His mercy He saved us, by the laver of regeneration.”


No merits could have preceded the union of Christ to His humanity, because the union, as stated above, is one of uncreated grace. Christ’s merits, as a Person with a human nature, all followed His Incarnation, and so no merits from Him could be said to happen leading to the Union.  The principle of grace cannot fall under merit, because grace is the cause of any possibility of merit. So it cannot even be the foreseen merits prior to the Incarnation, since those merits presuppose the Incarnation.


Article 12. Whether the grace of union was natural to the man Christ?


Grace is natural to the man Christ, since from the beginning of His conception the human nature was united to the Divine Person, but not owing to his human nature, but only to the Divine Person of Christ.


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