Objection 1. It would seem that Christ is not one, but two. For Augustine says (De Trin. i, 7): "Because the form of God took the form of a servant, both are God by reason of God Who assumed, yet both are Man by reason of the man assumed." Now "both" may only be said when there are two. Therefore Christ is two.
Objection 2. Further, where there is one thing and another there are two. Now Christ is one thing and another; for Augustine says (Enchiridion xxxv): "Being in the form of God... He took the form of a servant... being both in one; but He was one of these as Word, and the other as man." Therefore Christ is two.
Objection 3. Further, Christ is not only man; for, if He were a mere man, He would not be God. Therefore He is something else than man, and thus in Christ there is one thing and another. Therefore Christ is two.
Objection 4. Further, Christ is something that the Father is, and something that the Father is not. Therefore Christ is one thing and another. Therefore Christ is two.
Objection 5. Further, as in the mystery of the Trinity there are three Persons in one Nature, so in the mystery of Incarnation there are two natures in one Person. But on account of the unity of the Nature, notwithstanding the distinction of Person, the Father and Son are one, according to John 10:30: "I and the Father are one." Therefore, notwithstanding the unity of Person, Christ is two on account of the duality of nature.
Objection 6. Further, the Philosopher says (Phys. iii, text. 18) that "one" and "two" are predicated denominatively. Now Christ has a duality of nature. Therefore Christ is two.
Objection 7. Further, as accidental form makes a thing otherwise [alterum] so does substantial form make another thing [aliud] as Porphyry says (Praedic.). Now in Christ there are two substantial natures, the human and the Divine. Therefore Christ is one thing and another. Therefore Christ is two.
On the contrary, Boethius says (De Duab. Nat.): "Whatever is, inasmuch as it is, is one." But we confess that Christ is. Therefore Christ is one.
I answer that, Nature, considered in itself, as it is used in the abstract, cannot truly be predicated of the suppositum or person, except in God, in Whom "what it is" and "whereby it is" do not differ, as stated in I, 29, 4, ad 1. But in Christ, since there are two natures, viz. the Divine and the human, one of them, viz. the Divine, may be predicated of Him both in the abstract and in the concrete, for we say that the Son of God, Who is signified by the word Christ, is the Divine Nature and is God. But the human nature cannot be predicated of Christ in the abstract, but only in the concrete, i.e. as it is signified by the suppositum. For we cannot truly say that "Christ is human nature," because human nature is not naturally predicated of its suppositum. But we say that Christ is a man, even as Christ is God. Now God signifies one having the Godhead, and man signifies one having manhood. Yet one having manhood is differently signified by the word "man" and by the word "Jesus" or "Peter." For this word "man" implies one having manhood indistinctly, even as the word "God" implies indistinctly one having the Godhead; but the word "Peter" or "Jesus" implies one having manhood distinctly, i.e. with its determinate individual properties, as "Son of God" implies one having the Godhead under a determinate personal property. Now the dual number is placed in Christ with regard to the natures. Hence, if both the natures were predicated in the abstract of Christ, it would follow that Christ is two. But because the two natures are not predicated of Christ, except as they are signified in the suppositum, it must be by reason of the suppositum that "one" or "two" be predicated of Christ.
Now some placed two supposita in Christ, and one Person, which, in their opinion, would seem to be the suppositum completed with its final completion. Hence, since they placed two supposita in Christ, they said that God is two, in the neuter. But because they asserted one Person, they said that Christ is one, in the masculine, for the neuter gender signifies something unformed and imperfect, whereas the masculine signifies something formed and perfect. on the other hand, the Nestorians, who asserted two Persons in Christ, said that Christ is two not only in the neuter, but also in the masculine. But since we maintain one person and one suppositum in Christ, as is clear from 2, 2,3, it follows that we say that Christ is one not merely in the masculine, but also in the neuter.
Reply to Objection 1. This saying of Augustine is not to be taken as if "both" referred to the predicate, so as to mean that Christ is both; but it refers to the subject. And thus "both" does not stand for two supposita, but for two words signifying two natures in the concrete. For I can say that "both, viz. God and Man, are God" on account of God Who assumes; and "both, viz. God and Man," are Man on account of the man assumed.
Reply to Objection 2. When it is said that "Christ is one thing and another," this saying is to be explained in this sense--"having this nature and another." And it is in this way that Augustine explains it (Contra Felic. xi), where, after saying, "In the mediator of God and man, the Son of God is one thing, and the Son of Man another," he adds: "I say another thing by reason of the difference of substance, and not another thing by reason of the unity of person." Hence Gregory Nazianzen says (Ep. ad Chelid. ci): "If we must speak briefly, that of which the Saviour is, is one thing and another; thus the invisible is not the same as the visible; and what is without time is not the same as what is in time. Yet they are not one and another: far from it; for both these are one."
Reply to Objection 3. This is false, "Christ is only man"; because it does not exclude another suppositum, but another nature, since terms placed in the predicate are taken formally. But if anything is added whereby it is drawn to the suppositum, it would be a true proposition--for instance, "Christ is only that which is man." Nevertheless, it would not follow that He is "any other thing than man," because "another thing," inasmuch as it refers to a diversity of substance, properly refers to the suppositum. even as all relative things bearing a personal relation. But it does follow: "Therefore He has another nature."
Reply to Objection 4. When it is said, "Christ is something that the Father is"; "something" signifies the Divine Nature, which is predicated even in the abstract of the Father and Son. But when it is said: "Christ is something that is not the Father"; "something" signifies, not the human nature as it is in the abstract, but as it is in the concrete; not, indeed, in a distinct, but in an indistinct suppositum, i.e. inasmuch as it underlies the nature and not the individuating properties. Hence it does not follow that Christ is one thing and another, or that He is two, since the suppositum of the human nature in Christ, which is the Person of the Son of God, does not reckon numerically with the Divine Nature, which is predicated of the Father and Son.
Reply to Objection 5. In the mystery of the Divine Trinity the Divine Nature is predicated, even in the abstract of the three Persons; hence it may be said simply that the three Persons are one. But in the mystery of the Incarnation both natures are not predicated in the abstract of Christ; hence it cannot be said simply that Christ is two.
Reply to Objection 6. Two signifies what has duality, not in another, but in the same thing of which "two" is predicated. Now what is predicated is said of the suppositum, which is implied by the word "Christ." Hence, although Christ has duality of nature, yet, because He has not duality of suppositum, it cannot be said that Christ is two.
Reply to Objection 7. Otherwise implies diversity of accident. Hence diversity of accident suffices for anything to be called "otherwise" simply. But "another thing" implies diversity of substance. Now not merely the nature, but also the suppositum is said to be a substance, as is said Metaph. v, text. 15. Hence diversity of nature does not suffice for anything to be called "another thing" simply, unless there is diversity of suppositum. But diversity of nature makes "another thing" relatively, i.e. in nature, if there is no diversity of suppositum.
Source: Summa Theologica (New Advent)