Now, when we say that men have one nature, it must be understood that we do not say this with the body and soul in mind, because it is impossible to say that the soul and the body as compared to each other have one nature. Nevertheless, when we take a number of human hypostases, all of these are found to admit of the same basis of their nature. All are made up of a soul and a body, all share the nature of the soul and possess the substance of the body, and all have a common species. Thus, we say that several different persons have one nature, because each person has two natures and is complete in these two natures, that is to say, the natures of the soul and of the body.
In the case of our Lord Jesus Christ, however, it is impossible to have a common species for there never was, nor is, nor ever will be another Christ of divinity and humanity, in divinity and humanity, the same being perfect God and perfect man. Hence, in the case of our Lord Jesus Christ, one cannot speak of one nature made up of divinity and humanity as one can in the case of the individual made up of soul and body. In this last case we have an individual, but Christ is not an individual, because He does not have a predicative species of Christness. It is precisely for this reason that we say that it was of two perfect natures, the divine and the human, that the union was made. It was not made by mixing, or mingling, or blending, or compounding . . . neither was it apparent nor relative, nor by dignity or harmony of will or equality in honor or identity of name or complaisance . . . Rather, it was by composition – hypostatically, that is to say – without change or mingling or alteration or division or separation. And we confess one Person of the Son of God incarnate in two natures that remain perfect, and we declare that the Person of His divinity and His humanity is the same and confess that the two natures are preserved intact in Him after the union. We do not set each nature apart by itself, but hold them to be united to each other in one composite Person. For we say that the union is substantial; that is to say, true and not imaginary. We do not, however, define the substantial union as meaning that the two natures go to make up one compound nature, but as meaning that they are truly united to each other into one composite Person of th Son of God, each with its essential difference maintained intact. Thus, that which was created remained created, and that which was uncreated, uncreated; the mortal remained mortal and the immortal, immortal; the circumscribed remained circumscribed and the uncircumscribed, uncircumscribed; the visible remained visible and the invisible, invisible.
– Two Natures in One Person, III.3. Emphasis mine.