Chapter 41 — How we are to understand that the Son of God was incarnate

In order to reply to these objections, we must begin by going into the question a little more deeply. While Eutyches maintained that the union of God with man was effected in the nature, whereas Nestorius held that it was effected neither in the nature nor in the person, the Catholic faith teaches that it was effected in the person, and not in the nature. And so it would seem necessary to know first of all what is meant by being united in the nature, and by being united in the person.

The word ‘nature’ has various meanings. It may mean either the birth of a living being, or the principle of generation and movement, or matter and form. It is also sometimes used to signify a thing’s essential definition, which contains all that belongs to the species of that thing; thus we say that human nature is common to all men, and so on. Accordingly, those things are united in nature which go to form the complete species of a thing; thus, for instance, the soul and body are united together to form the species of an animal; and, generally speaking, whatsoever things are the parts of a species.

Now it is impossible for anything extraneous to be united in unity of nature to a species that is already complete without doing away with the species. For, since species are like numbers, in which either the addition or the subtraction of unity changes the species, if any addition be made to an already complete species, the result must be another species. Thus, if you add sensibility to a substance that is only animate, you have another species, because animal and plant are different species. On the other hand, something not belonging to the integrity of the species may be found in an individual of another species; for instance, ‘white’ or ‘clothed’ in Socrates or Plato, or a sixth finger, and so forth. Consequently, nothing hinders things from being united in an individual that are not united in one complete species: for instance, human nature, whiteness, and music in Socrates, and whatsoever things may be united together in one subject. And since an individual substance is called a ‘hypostasis,’ and an individual rational substance is called a ‘person,’ it is reasonable to say that all these are united together in the hypostasis, or person. Hence it is possible for things not to be united in the nature, and yet united in the hypostasis or person.

Accordingly, hearing that the union of God and man in Christ is a fact, heretics, disregarding the path of truth, have explained this fact in various ways. Some deemed this union to be like the union of things in one nature. Such were Arius and Apollinaris, who contended that the Word supplied, in Christ’s body, the place of the soul or mind. Such also was Eutyches, who held the existence of the two natures of God and man before the Incarnation, but of only one after the Incarnation.

But their view involves an utter impossibility. For it is evident that the nature of the Word, from all eternity, was most perfectly complete, and wholly incorruptible and unchangeable. Therefore, it is impossible for anything extraneous to the divine nature, such as human nature or a part of it, to be joined to the Word in unity of nature.

Others, realizing the impossibility of this view, adopted the contrary position. For when a thing of a certain nature receives the addition of something that does not belong to the integrity of its nature, this would seem either to be an accident, such as whiteness or music, or to bear an accidental relationship to that thing, such as a ring, clothes, a house, and the like. Seeing, then, that human nature is joined to the Word of God, and yet does not belong to the integrity of his nature, they concluded that human nature was united to the Word accidentally. And since it is evident that it cannot be an accident in the Word of God—both because in God there cannot be any accidents, as we have proved, (Bk. 1, ch. 23.) and because human nature, through being in the genus of substance, cannot be the accident of anything—it seemed to follow that human nature was joined to the Word not as an accident, but as bearing an accidental relationship to it. Therefore, Nestorius maintained that Christ’s human nature was the temple of the Word, as it were, so that the union of the Word with human nature would be reduced to a kind of indwelling. And since a temple is individually distinct from one who dwells in it, and since individuality in human nature is personality, he concluded that human nature and the Word had distinct personalities, so that the Word and the man were two persons.

In order to avoid this difficulty, others imagined that human nature was so assumed that, properly speaking, it could have no personality. They said, in effect, that the soul and body, in which the integrity of human nature consists, were assumed by the Word in such a way that the soul was not united to the body so as to form a substance; they said this lest they should be forced to admit that the substance thus formed was a person. They maintained that the Word is united to the soul and body, as though these were related to it accidentally, even as clothes are related to the one who is clothed, somewhat following in the footsteps of Nestorius.

Since all this has been refuted above, (Ch. 34.) we must hold that the union of the Word with man is such that neither are the two natures fused into one; nor does the Word stand related accidentally to human nature, as a substance—a man for instance—to something extraneous, as, for instance, his house or his clothes. But we hold that the Word subsists in human nature as made his own nature by the Incarnation, so that the body is truly the body of the Word of God, and the same with the soul; and that the Word of God is truly man.

And though man cannot perfectly explain this union, we shall nevertheless endeavor as best we can to say something for the building up of faith, so as to defend the Catholic faith from the attacks which unbelievers direct at this mystery.

In all created things nothing bears so great a resemblance to this union as the union of soul and body. Greater still would be the resemblance, as Augustine says, (Contra Felician.) were there but one intellect in all men, as some indeed have maintained. If this were true, we should have to say that the already existing intellect would become united to the human concept in such a way that the two together would form one person, just as we say that the already existing Word was joined to human nature in one person. Hence, on account of this resemblance between the two unions, Athanasius says in his Creed: As a rational soul and flesh are one man, so God and man are one Christ.

Now the body is united to the rational soul both as the matter and as the instrument of the soul. But the above resemblance does not regard the former mode of union. For thus there would be one nature made of God and man, since matter and form are the proper constituents of the specific nature. The resemblance, therefore, regards the union of soul with the body as its instrument. In fact, the statements of the early doctors are in agreement with this, who considered the human nature in Christ to be an instrument of the divinity even as the body is the instrument of the soul.

For the body and its parts are the instruments of the soul otherwise than extraneous instruments. Thus a man’s spade is not so much his own instrument as his hand is, since many can work with that spade, whereas that hand is employed for the special work of that man’s mind. Hence a man’s hand is an instrument united and proper to him, whereas the spade is distinct from him and common to many. Now let us apply this to the union of God with man. All men in comparison with God are instruments of his works: God is at work in you, both to will and to work for his good pleasure (Phil 2:13), as the Apostle says. Now other men are employed by God as extraneous and separate instruments, so to speak, since he moves them not only to their own proper operations, but also to those that are common to all rational creatures, such as understanding the truth, loving what is good, and doing what is right. But human nature was assumed by Christ so that it might be an instrument in works belonging to God alone, such as cleansing from sins, enlightening our minds by grace, and perfecting us unto eternal life. Accordingly, Christ’s human nature is compared to God as a proper and united instrument, as the hand is compared to the soul.

Nor is it out of keeping with the usual course of nature that a thing be naturally the proper instrument of that which is not its form. Thus the tongue, as the instrument of speech, is the proper instrument of the understanding, and yet, as the Philosopher proves, (Aristotle, On the Soul, bk. 3.) the understanding is not the form of any part of the body. In like manner, there may be an instrument that does not belong to the specific nature, and yet belongs to the individual on the part of the matter, as, for instance, a sixth finger, or something of the kind. In this way, therefore, nothing prevents our saying that human nature by its union with the Word became, as it were, the Word’s instrument, not separate but united. Yet human nature does not belong to the nature of the Word, nor is the Word its form; but it does belong to the person of the Word.

We must not, however, look for perfect resemblance in the above examples. We must realize that the Word of God was able to unite himself to human nature in a manner far more sublime and penetrative than that of the union of the soul with any proper instrument whatsoever, especially since that he is united to the whole human nature through the intellect. And, although the Word of God by his power penetrates all things as preserving and upholding them, it is possible for him to be united in a much more sublime and ineffable manner with intellectual creatures inasmuch as they are able properly to enjoy him and participate in him.