First, it is asked¹ whether this union was brought about in the person or in the nature. It seems that it was in the nature.

Therefore, it should be known that the name “nature” is taken from being born. Hence, nature, or “about to be born,” was first said of the actual birth of living things, namely, of plants and animals. Next, the name nature was transferred to the principle of the aforementioned nativity. And since the principle of this kind of nativity is interior, the name “nature” was further extended to signifying a more interior principle of motion, according to what is said in book 2 of the Physics, that nature is a principle of motion in which motion is essential, not accidental.¹ Since natural motion especially in generation is terminated at the essence of a species, we may say further, the essence of a species, which a definition signifies, is called a nature. Hence, Boethius also says in On the Two Natures that nature is the specific difference informing each and every thing.² And it is in this way that nature is taken here.

However, in order to understand what a person is, it must be considered that if there is some thing in which there is nothing other than the essence of the species, the essence of the species itself will be subsisting individually through itself. Thus, in a thing of this kind, nature and suppositum would be really the same, differing only in account; that is, the thing is called a nature insofar as it is the essence of the species, but it is called a suppositum insofar as it subsists through itself.

If there were anything within a thing beyond the essence of the species, which the definition signifies, it would be something other, either an accident or the matter of an individual. The suppositum will then not be entirely the same as that nature, but it will be constituted by an addition to the nature, as is most evident in those things which are composed of matter and form. And what was said concerning the suppositum must be understood concerning the person in a rational nature, since a person is nothing other than a suppositum of a rational nature according to what Boethius says in the book On the Two Natures, that person is an individual substance of a rational nature.¹

Hence, it is obvious that nothing prevents some things being united in a person which are not united in the nature. For an individual substance of a rational nature can have something which does not pertain to the nature of a species, and this is united to it personally, not naturally. Therefore, in this manner it must be understood that the human nature was united to the Word of God in the person, not in the nature. For if it does not pertain to the divine nature, nevertheless it pertains to his person, insofar as the person of the Word joined human nature to himself by assuming it. But concerning the manner of this kind of conjunction doubt and discord occurs. For we see in creatures that something comes to another in two ways, namely, accidentally and essentially.

Therefore, Nestorius, and Theodorus of Mopsuestia before him, posited that the human nature was conjoined to the Word accidentally, namely, according to the indwelling of grace, positing that the Word of God had been united to the man Christ as if by dwelling in him as in his temple. But we see that every substance accidentally conjoined to another retains its own proper singularity separately, as when clothes are put on by man or a house contains an inhabitant. Hence, it follows that man will have a proper singularity which is his personality. Therefore, according to Nestorius it followed that the person of man in Christ was a distinct person from the person of the Word, and that one was the son of man and the other the Son of God. Hence, the Blessed Virgin was not acknowledged as the mother of God, but the mother of a man. But this is entirely absurd.

First, indeed, this is because Sacred Scripture speaks in one way concerning men, in whom the Word of God dwelled through grace, and in another way concerning Christ. For concerning other men, it says that the Word of the Lord was brought to a given prophet, but concerning Christ it says, the Word was made flesh (John 1:14), that is, a man, as if the Word itself were personally a man.

Second, this is because the Apostle in the letter to the Philippians (Phil 2:7) calls this union an emptying of the Son of God. But it is clear that the indwelling of grace is not adequate to the account of emptying. Otherwise, emptying would belong not only to the Son, but also to the Father and to the Holy Spirit, concerning which the Lord says, he shall abide with you, and shall be in you (John 14:17); and concerning himself and the Father, we will come to him, and will make our abode with him (John 14:23). Therefore, on account of this and many other things, the aforesaid error was condemned in the Council of Ephesus.¹

Certain men, holding with Nestorius that human nature had come to the Word accidentally, wanted to avoid the duality of persons which Nestorius posited, positing that the Word assumed a soul and a body to itself not united to each other, so that a human person would not be produced from a soul and a body. But from this a greater absurdity follows, that is, that Christ was not truly a man, since the account of a man consists in the union of a soul and a body. And thus, this error was also condemned under Alexander III in the Council of Tours.¹

But others took the other side, positing that human nature was joined to the Word essentially; so that it is as if one nature or essence was produced out of the divine and human nature. And on this point indeed Apollinaris made three claims, as Pope Leo says in a certain letter to those living in Constantinople.¹ First, he posited that a soul was not united to Christ, but that the Word had come to the flesh in place of the soul. As a result, one nature was produced from the Word and the flesh, just as in us one nature is produced from the soul and the body. In this doctrine Apollinaris also followed Arius. But since the evangelical Scriptures explicitly speak about the soul of Christ, according to that passage of John 10:18, I have power to lay down my soul, he fell into the second opinion, so that he even posited that there was a sensitive soul in Christ, but not a rational soul; rather, he posited that the Word took the place of the intellect in the man Christ.

But this is unfitting, because according to this the Word did not assume a human, but a bestial nature, as Augustine argued against him in the Book of Eighty-Three Different Questions.¹ His third teaching was that the flesh of Christ was not taken from a woman but made from the Word changed into flesh and even conversely. But this is most impossible, because the Word of God, since he is truly God, is completely immutable. For these doctrines of Apollinaris were condemned in the Council of Constantinople,² as was that of Eutyches, who followed his third teaching, in the Council of Chalcedon.³

So, therefore, if the union was not made in the person, but only according to inhabiting, as Nestorius claimed, nothing new happened in the Incarnation of Christ. But that the union occurred in the nature, as Apollinaris and Eutyches claimed, is completely impossible. For since the species of things are like numbers in which added and subtracted units vary the species, as is said in book 8 of the Metaphysics,¹ it is impossible that whatever nature is complete in itself should receive the addition of another nature.² Or if it did receive, it would not be the same nature, but another one. But the divine nature is most complete. Similarly, even the human nature has the completion of its own species. Hence, it is impossible that one nature should be joined to another in a natural union. Even if it were possible, then surely that which was produced from both would be neither a human nature nor a divine nature; and thus, Christ would be neither man nor God, which is unfitting. Therefore, it remains that the human nature was united to the Word neither accidentally, nor essentially, but substantially, insofar as substance signifies a hypostasis, and hypostatically or even personally.

But there is no example of this union in created things nearer than the example Athanasius posits, of the union of the rational soul to the body.¹ This is not, indeed, in the way that the soul is the form of the body, because the Word cannot be a form in matter, but in the way that the body is the instrument of the soul—not, indeed, an extrinsic and foreign instrument, but an instrument that is its own and conjoined. Hence, Damascene said that the human nature is the implement of the Word.² However, we would find a closer example, as Augustine says in Contra Felicianus,³ if we suppose, as many want to, that there is a generic soul in the world, which, forming matter to its uses, makes one person of itself.

Nevertheless, all examples of this kind are deficient. For the union of an instrument is accidental, but this is a certain singular union above every mode of union known to us. For just as God is goodness itself and his own being, so also he is unity itself through his essence. Thus, as his power is not limited to these modes of goodness and being which are in creatures, but is able to make new kinds of goodness and being unknown to us, so also he was able to make a new kind of union through the infinity of his own power, in order for human nature to be united to the Word personally, but not accidentally, although no adequate example for this can be found in creatures. Hence, Augustine, speaking of this mystery in the epistle To Volusianus, said, if a reason is sought, it is not wonderful; if an example is demanded, it is not unique. We must grant that something is possible for God, which we confess that we cannot investigate; for in such things the entire reason for the thing made is the power of the maker.¹ Dionysius says in the second chapter of On the Divine Names, according to us Jesus is a divine composition, that is a union, and he is ineffable by any word and unknown to the mind, and also to the first of the most worthy angels himself.²

Second, it is asked whether there is only one hypostasis or suppositum in Christ or two.¹ It seems that there is not only one.

But in the first place, those who posited this were ignorant of the proper word. For a hypostasis is nothing other than an individual substance, which is also signified by the name “suppositum.” Now Boethius says, in the book On the Two Natures, that a person is an individual substance of a rational nature.¹ So, therefore, it is obvious that there cannot be a hypostasis of a rational nature unless it is a person. Now, it is clear that the human nature is a rational nature; hence, if there is a proper hypostasis or proper suppositum of the human nature in Christ beyond the hypostasis or suppositum of the Word of God, it follows that there is a proper person of the human nature in Christ beyond the hypostasis of the Word. Thus, this position would not differ from the position of Nestorius.

Second, this is because, if it is granted that the person adds to a hypostasis in a rational nature some property pertaining to dignity, just as all wearing a mask are said to be like one having some dignity,¹ it would follow that the union of the human nature to the Word was not brought about except in something accidental, namely, in some property pertaining to dignity, which is what Nestorius also posited. Hence, it must be known that this heresy was condemned in the fifth Council carried out at Constantinople, where it is read, if anyone tries to introduce two subsistences into the mystery of Christ or to argue that two persons are said to be one person according to dignity and honor and adoration, as Theodorus and Nestorius wrote foolishly, let them be anathema. For the Holy Trinity did not receive a person or subsistence cast away in the one incarnation of the holy Trinity from the Word of God.²

Therefore, in order that it may be known what must be conceded and what denied in these matters, it is necessary to consider which of the names pertain to individuation, whether they are names of first imposition, such as “person” and “hypostasis,” which signify things themselves, or whether they are names of second imposition, such as “individual,” “suppositum,” and others of this kind, which signify the intention of individuality. Some of these pertain only to the genus of substance, such as “suppositum” and “hypostasis,” which are not said of accidents, and person in a rational nature, and also a thing of nature according to the understanding of St. Hilary.¹ But others pertain to individuation in every genus, such as “individual,” “particular,” and “singular,” which are also said of accidents.

Now, it is proper to a substance that it subsists through itself and in itself, but it is the character of an accident to be in another. Thus, those names which pertain to the individuation of a substance only have a place in those things which subsist through themselves and in themselves. For this reason also, these names are not said of the parts of substances, since they are not in themselves but in a whole, although they are not in a subject. Yet the names pertaining to individuation as much in substances as in accidents can fittingly be said of those things. For it cannot be said that this hand is a person, or a hypostasis, or a suppositum, although it can be said that it is some particular, singular, or individual. For even if a hand pertains to the genus of substance, yet because it is not a completed substance, subsisting in itself, it is not called a hypostasis, or a suppositum, or a person.

Thus, therefore, since the human nature in Christ does not subsist separately through itself but exists in another, that is, in the hypostasis of the Word (indeed, not as an accident in a subject, nor properly as a part in a whole, but through an ineffable assumption), thus the human nature in Christ can indeed be called some individual or particular or singular, yet it cannot be called a hypostasis or suppositum, just as it cannot be called a person. Hence, it remains that in Christ there is only one hypostasis or suppositum, namely, the divine Word.

Third, it is asked whether Christ is one or two in the neuter.¹ It seems that he is two in the neuter.

But certain authors, positing one person in Christ, posited two supposita or hypostases, one of the Son of God and another of the son of man.¹ Hence, although they did not say that Christ is two in the masculine because of the unity of person, still they said that he was two in the neuter because of the duality of supposita. But because this opinion too is contrary to the truth of the faith, as was said above, thus, leaving aside this opinion, we must consider whether, supposing that in Christ there is one hypostasis and one suppositum, Christ ought to be called two in the neuter or one.

For a clear understanding of this, we must consider that what has unity is called “one” denominatively, just as a white thing is what has whiteness, or what is a subject for it. For the same reason, “many” is said denominatively from a multitude, and two from a duality. But since one is convertible with being, so according as it is an accidental and a substantial being, something is called one or many either according to an accidental form or according to a substantial form. Indeed, something which is a subject for diverse accidental forms is called many, either successively or simultaneously, according to the accidental forms. It is successively, indeed, as when Socrates sitting is different from Socrates standing; hence, Socrates, insofar as he is first standing and later sitting, is many things successively. It is simultaneously, on the other hand, as when Socrates, insofar as he is white and musical, is many.

However, a two-footed animal, which is predicated of Socrates, is one and not many, since one of them is compared to the other as potency is to act, as is said in Metaphysics 8.¹ Now, white and musical are not related to each other in this way;² thus, Socrates, insofar as he is white and musical, is many, indeed not simply, but in a certain respect.³ This is because it is according to accidents that something is said to be in a certain respect, and not simply. But something that is a being is called one or many simply according to substance. But according to the Philosopher in Metaphysics 5, substance is said in two ways, namely the suppositum, which is not predicated of another; and the form, or nature of the species, which is predicated of the suppositum.⁴ Indeed, in mere creatures this is one and many at the same time. For the essence of different supposita is not one in number, nor again is some one suppositum found in pure creatures having two natural substances. But this singular is in Christ,⁵ and the first type of singular is in the three divine persons.⁶ Therefore, it is clear that Christ can in some measure be called one, since he is one suppositum; and in some way many, or two, because he has two natures—much more so than Socrates, of whom one is predicated insofar as he is one in subject; many, inasmuch as he is white and musical.

But we should consider which of these is said simply and which in a qualified way. Therefore, it must be known that something is said to be such a thing simply and properly which is such according to itself. But something is said to be such according to itself is according to a whole, more than according to a part, because a part is not the same as a whole simply. Now, the term “itself,” since it is reciprocal, is a relative term of identity. Thus, what belongs to something according to the whole, belongs to it more simply than what belongs to it only according to a part. Hence, if it is appropriate for something to belong to another according to both whole and part, then if it belongs to it only according to the part, it is said to belong to it in a certain respect, and not simply, just as if an Ethiopian who has white teeth is called white. It is quite otherwise for something which can be attributed only according to a part, as when someone is called curly simply if he has curly hair. But it is clear that the suppositum is signified through the manner of a whole, but the nature through the manner of a formal part, as is obvious from the aforesaid. Thus, although one and many can be referred both to the suppositum and the nature, it is clear that if some one suppositum has many substantial natures, it will be one simply and many in a certain respect. A sign of this is that those things which differ in suppositum and are one in that which pertains to the nature essentially, are, indeed, many simply, but one in genus or species. Thus, from the contrary, if one suppositum has many natures, it will be one simply and many in a certain respect. Since, therefore, Christ is one suppositum having two natures, he will be one simply and two in a certain respect.

Fourth, it is asked whether there is only one being in Christ.¹ And it seems that there is not.

Now, we should consider that there are some forms by which something is a being, not simply, but in a certain respect, and such are all accidental forms. But there are some forms by which a subsisting thing has being simply, because, obviously, they constitute the substantial being of the subsisting thing. Now, in Christ the subsisting suppositum is the person of the Son of God, which is sustained simply through the divine nature, but it is not sustained simply through the human nature; for the person of the Son of God existed before he assumed humanity, nor was he increased or perfected through having assumed a human nature. But the eternal suppositum is sustained through the human nature insofar as it is this man. Thus, just as Christ is one simply on account of the unity of the suppositum, and two in a certain respect on account of the two natures, so he has one being simply on account of the one being of the eternal suppositum. But there is also another being of this suppositum, not insofar as it is eternal, but insofar as it became a man in time. That being, even if it is not an accidental being, because man is not accidentally predicated of the Son of God, as was said above,¹ nevertheless is not the principal being of its suppositum but its secondary being. Now, if there were two supposita in Christ, then each suppositum would have its own principle being. Thus, there would be a two-fold being in Christ simply.

Fifth, it is asked whether in Christ there is only one activity.¹ It seems that there is.

One way is from the perspective of the acting subject. In this way, the action’s unity or plurality is considered according to number. This is just as any other accident has numerical unity or plurality from the subject’s perspective. For this is how Socrates’s sight or hearing is different in number from Plato’s sight or hearing.

In another way, we can consider an action’s unity or plurality from the perspective of the principle by which the agent acts; and in this way, the action is said to be one or many according to species. Thus, sight and hearing are activities differing in species.

Now, an action comes from an agent according to the account of the power by which it acts. Nor is it a problem that actions receive their species from their objects, because determinate powers look toward determinate objects. Nevertheless, we should consider that, if the power which is the principle of an action should be moved by another, higher power, the activity which comes from it is not only an action but also a passion, namely, insofar as it comes from a power which is moved by a higher one. Now, in man all the powers of the sensitive part are in a way moved by the will as by a certain first principle. Thus, to hear, to see, to imagine, to desire, and to be angry are not only actions, but also certain passions coming from the will’s motion, namely, insofar as a man goes towards them from his own will. Therefore, though in one man there seem to be many actions differing in species according to diverse potencies and habits, nevertheless, because they all come from the will’s one first action, we say there is one action of one man. In this way, if one craftsman were to work with many tools, we would say one man has one activity.

Because of this, therefore, some have asserted that there is only one activity in Christ, since in him human nature is subject to and moved by the divine activity. Thus, human nature’s action has more the account of a passion in comparison with the divine. Hence, because of the unity of the divine action, they said that there is only one action in Christ.

But this was irrationally said for two reasons.

First, this was irrationally said because some powers do not have ownership of their own act in such a way that they are moved by a superior to the extent that they themselves do not act but are more acted upon. Hence, the Philosopher says in Ethics 4 that a sense is not the principle of any action.¹ But a power which does have ownership of its own act, namely, the will, is moved by something higher, namely, God, in such a way that it is not only acted upon but also acts. Since, according to his human nature, Christ has the will’s created potency, just as he has a created intellect—because he lacks nothing pertaining to the perfection of human nature—consequently, the motion of the human will in Christ will be an action and not merely a passion. Otherwise, he could not merit according to human nature.

Second, it was irrationally said because a thing’s activity has species and unity from the first principle pertaining to the same nature. Thus, the will, from which all human actions have unity, is a certain intrinsic principle of human nature. But no actions have unity by being traced back to some first principle of a different nature; otherwise, it would follow that all things have one action because there is one first principle moving everything, namely, God. In this way, therefore, even if the human nature in Christ is moved by the divine, nevertheless, there must still be two actions because the two natures are distinct.

Therefore, from the position that there is one action in Christ, it follows that he would have only one nature and one will. Thus, this position is tantamount to the heresy condemned by the sixth synod.¹