Commentaria in Summam contra Gentiles, Liber IV

The fifth error ¹ was that of those who, wishing to avoid the aforesaid position, said that from the soul and flesh in the Lord Jesus Christ one substance was constituted, namely a man, and that this man was united to the Word, not in nature, but in person. But because he is a certain individual substance, which is to be a hypostasis or supposit, they indeed say that in Christ the hypostasis of that man is one thing and that of the Word of God is another, but that there is one person of both, by reason of which unity the Word of God is predicated of that man, and he himself of the Word of God; so that the meaning is, The Word of God is man, that is, The Person of the Word of God is the person of the man, and conversely. And by this reasoning they say that whatever is predicated of the Word of God can be predicated of that man, and conversely, yet with a certain reduplication, so that when it is said, God suffered, the meaning is, The man who is God on account of the unity of person, suffered; and so of the rest.

I. St. Thomas argues against this position, showing that this position necessarily falls into the error of Nestorius.

And he argues first thus. Every hypostasis of human nature is a person. Therefore, if from the sole union of soul and body a hypostasis was constituted in Christ, it follows that from the same union a person was constituted. And thus in Christ there will be two persons. Which belongs to Nestorian impiety.

Concerning what St. Thomas says, that a person is a certain part of a hypostasis, it must be noted that this is understood of a subjective part. For hypostasis is something more common than person: since hypostasis is said of every complete subsistent, in whatever nature it may be; while person is said of a subsistent in a rational nature only. Hence every person is a hypostasis: but not every hypostasis is a person, but only a hypostasis of a rational or intellectual nature.

Secondly. The hypostasis of the Word of God is the same as the person. Therefore, if the hypostasis of the Word of God is not the hypostasis of that man, neither will His person be the person of that man. The opposite of which they say.

Thirdly. Granted that a person were other than a hypostasis, this could not be unless because it adds some property over and above the hypostasis: for it can add nothing pertaining to substance, since hypostasis is the most complete thing in the genus of substance, which is called primary substance. Therefore the union will have been made only according to some accidental property. Which also comes down to the error of Nestorius.

Fourthly. In the Letter of Cyril to Nestorius, approved in the Synod of Ephesus, and in almost all the synodal writings, this is ascribed to the error of Nestorius, who posited two hypostases in Christ. Therefore etc.

Fifthly. Damascene, in Book III, attributes this error to Nestorius, when he says: From two perfect natures we say that the union was made, not prosopic of God (that is personal), as the enemy of God, Nestorius, says. Therefore etc.

II. Sixthly. It follows that there are two supposits in Christ. Because hypostasis and supposit are the same: since of primary substance, which is hypostasis, all other things are predicated, namely universals in the genus of substance, and accidents, as is said in the Categories.

It must be noted, according to the mind of St. Thomas, III Sent., d. VI, q. 1, a. 1 ², that this noun "supposit" signifies the particular nature in relation to the common nature, inasmuch as it subsists in it. But this noun "hypostasis" signifies the particular nature as it underlies the properties of the nature. On this account he said that hypostasis and supposit are the same, because of primary substance, which is hypostasis, universals of the genus of substance and accidents are predicated: for from the first it has the name supposit, but from the second the name hypostasis.

Seventhly. If the Word and that man differ in supposit, those things which are said of Christ in the divine Scriptures, namely divine and human, will have to be taken separately. Which is contrary to the sentence of Cyril, approved in the Synod. — The consequence is proved. Because then, when that man is supponated, the Word of God will not be supponated, nor conversely. But when the supposits are distinct, namely of the Word and of the man, it is necessary that those things also which are said of them be distinguished: since divine predicates do not belong to the supposit of the man except on account of the Word, nor conversely.

It must be observed that the reason why divine and human predicates are said of the man, and the same are said of God, is because by the name Christ God and by the name Christ man the same supposit is supponated, which is the supposit of the human and of the divine nature. For when human things are said of God Christ, the truth is spoken, because Christ God supponates for the supposit of the human nature, which is also the supposit of the divine nature. When also divine things are said of Christ man, the truth is spoken, because the divine supposit is supponated, which is also the supposit of the human nature. And thus the unity of supposit brings it about that of the same Christ both divine and human things are said at the same time. If therefore that unity of supposit is taken away, divine things will not be able to be said of Christ man, but only of Christ God; and human things will not be able to be said of Christ God, but only of Christ man. And this is the foundation of this argument.

III. Eighthly. Those things which belong to the Word of God by nature would not be said of that man except through a certain association in one person. For this is what that interposed reduplication signifies, when they are expounded: That man created the stars, that is, The Son of God, who is that man. For thus it will be expounded: That man is God, that is ³: The Word of God, who is that man, is God. But Cyril condemns expressions of this kind. Therefore etc.

Ninthly. It follows that that man cannot belong to the person of the Word of God except through the assumption by which he was assumed by the Word. But this is foreign to the right sense of the faith: as is clear from the statements in the Synod of Ephesus. Therefore etc.

It must be observed that, although it is true of the human nature that it does not belong to the person of the Word except through assumption by the Word, nevertheless of that man who is the hypostasis and supposit of the man in Christ, this is not true. For the hypostasis of the man does not belong to the person of the Word because it was assumed by the Word, but because it is the very hypostasis and person of the Word; since, according to the right faith, the hypostasis of the human and of the divine nature in Christ is the same.

Tenthly. It follows that Christ is simply two, and one only secundum quid: which is to dissolve and divide Jesus. And thus He will not be simply a being: since each thing is a being to the extent that it is one. — The consequence is proved. Because things that are many in supposit are simply many, nor are they one except secundum quid.

How Christ is one person and one thing simpliciter, and cannot be called two, St. Thomas abundantly shows in the Tertia, q. XVII, a. 1, and III Sent., d. VI, q. II, a. 1.