Tertullian, On the Flesh of Christ 13

The flesh was made soul so that the soul might be shown. Was the flesh, then, also made soul so that the flesh might be manifested? If flesh is soul, it is no longer soul but flesh. If soul is flesh, it is no longer flesh but soul. Wherever flesh is and wherever soul is, one of the two has been turned into the other. Indeed, if neither exists while each becomes the other, it is utterly perverse: naming flesh, we would understand soul; signifying soul, we would interpret flesh. Everything risks being taken for what it is not and losing what it is, because it is being taken otherwise, if it is named otherwise than it is. The fidelity of names is the salvation of properties. Even when qualities change, they take possession of their words. For instance, clay, once baked, takes the name of earthenware; it no longer shares the name of its former kind, because it no longer shares that kind itself. Therefore the soul of Christ, made flesh, cannot fail to be what it has been made, and not be what it was — namely, something else now made. And since we have used the closest example, we will press it further. The earthenware from clay is certainly one body and one word — namely, of that one body. Nor can it be called clay; for what it was no longer exists, and what does not exist does not adhere at all. So too the soul made flesh is uniform, solidified: the whole is a singularity, an undivided substance. In Christ, however, we find soul and flesh expressed in simple, bare words — that is, soul as soul, flesh as flesh; nowhere soul as flesh or flesh as soul, which is how they would have to be named if they were so blended. Instead, each substance is pronounced separately by Him, distinctly soul and distinctly flesh, precisely to mark the distinction of two natures. "My soul is deeply grieved, even to death" (Matthew 26:38), He says, and: "The bread that I will give for the life of the world is my flesh" (John 6:51). Now, if soul had been flesh, there would be one fleshy soul or one animate flesh in Christ. But since He distinguishes the kinds — flesh and soul — He shows two. If two, then not one; if not one, then neither soul-flesh nor flesh-soul, for one such blend is a single thing. Unless He also carried a separate soul apart from the one that was flesh, and a separate flesh apart from the one that was soul. But if there is one flesh and one soul — that one sorrowful to death, that one bread for the world's salvation — the count of two substances, distinct in their kind, stands firm, excluding the single hybrid species of soul-flesh.