Whether Christ Is One or Two?
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.
Obj. 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.
Obj. 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.
Obj. 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.
Obj. 5: Further, as in the mystery of the Trinity there are three Persons in one Nature, so in the mystery of the 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.
Obj. 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.
Obj. 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 the First Part (Q. 29, A. 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.