What word besides "nature" would you use to describe what we Chalcedonians call the human and divine natures of Christ? I am not trying to start an argument. I just would like to know what other term than "nature" could be used to express the doctrine that Christ is both human and divine.
I don't think words suffice, but if you follow along the article from monachos by Fr. Timothy Thomas, you will find an interesting tendency in St. Severus. Hypostasis today is defined in a manner much differently than then. Fr. V.C. Samuel also goes through this in some detail in his monumental book, The Council of Chalcedon Re-Examined
. St. Severus defines hypostasis as a real existence. Not every hypostasis is a person.
Let's use a very simple example of a rock. This rock's ousia would be "rockness." When "rockness" exists, it exists as a rock, which is a hypostasis. Nature can mean either an abstract essence, an ousia, or a real existence, a hypostasis.
For St. Severus, to describe the full reality of Christ's humanity, he called it an hypostasis. However, because the eternal Word of God became incarnate and made this humanity His very own, the humanity is called a "non-self-subsistent hypostasis". (non-self-hypostatic hypostasis). In other words, the humanity of Christ is real, but does not exist on its own. Its existence depends on the hypostasis of the Word, a self-subsistent hypostasis (an existence that exists on its own). Nevertheless, the Word of God incarnate is ONE hypostasis, which mean it is one real existence. When we say "one nature", it is used in the sense that it is "one existence".
So, for St. Severus, he says that the humanity of Christ is "hypostatic", that is literally, it "really exists", body, soul, spirit, all properties of humanity all exist truly. When the Word became flesh, he became REAL flesh.
Later theologians who defined hypostasis with a different definition as synonymous with prosopon made a theory of "enhypostasia", which goes pretty much against the definitions St. Severus defined. For later theologians, it is correct for them to say the humanity has no hypostasis, because it has no prosopon. But St. Severus said that all prosopa has hypostases, but not all hypostases have prosopa (see the example of the rock). Thus, when a student of St. Severus hears that the humanity has no hypostasis, in his mind, he thinks "the humanity has no existence", ironically
falling into the thoughts of docetism. Of course, no body made the mistake of docetism, but this is what Fr. V.C. Samuel's analysis if using the thought and definitions. Likewise when a student of, let's say St. John of Damascus hears St. Severus say that the humanity of Christ is hypostatic, they ironically
accuse the man who was accused with Eutyches of the same heresy a "Nestorian".
I'd say for Chalcedonians there's also a nuanced definition with the word "nature". Nature seems to me, and correct me if I'm wrong, a real existence based off an ousia that is "inseparable" in its basic element. In other words, in the ears of those who heard "one nature", it seemed to them a mixture, a new ousia had to be made in order for this nature to be "one". But of course, the definition of nature for the other side is "a real existence" or more accurately "a state of existence". Therefore, to them when the state of existence is "two", to them they think two separate independent hypostases are present. So, this became a war of definitions. Each side trying to justify his predecessor's tradition defined terms in a way that was consistent in its own system and leads polemically to condemn the other system.
For St. Cyril, the problem lies in the flexibility of the use of the term "nature". Because he knew that nature can be used as "ousia" and used as "hypostasis", he used it very liberally, and it needed to be understood in its context. Interestingly enough, many of those in the Antiochian side were accusing St. Cyril and the council of Ephesus of docetism for his use of the word nature, and they felt he used it inconsistently. His monumental work with John of Antioch shows that he did not allow misunderstandings get in the way, nor did he enforce in it to accept a council, but the faith of the letter he saw in it the faith of the council of Ephesus without verbal acceptance. He even allowed the veneration of Theodore of Mopsuestia and Diodore of Tarsus to continue, despite the fact that he refuted their writings as forerunners to Nestorianism.
Unfortunately 20 years later, something went wrong, and here we are.
But I believe if we avoid ambiguity today and are far from the constraints of imperial pressure, and are asked in humility "what do you mean when you say..." and the meaning is Orthodox, then we shouldn't be afraid of unity in the faith today. It goes through pains when someone has to rely on some sort of strict formula to define the faith. I think today, we need not use terms like "nature" or others to confuse people. We need to define the terms we use before we use them. But I think it suffices to just use the terms "humanity" and "divinity". When I read about both families of Orthodoxy, I believe in both of them carries this one faith:
The One Word of God after the incarnation is both fully man and fully God, or fully human and fully divine. I believe in the One Word of God, the same One Word of God who was begotten of the Father before all ages, this same One Word of God became incarnate of the Holy Spirit and Virgin Theotokos. The One Word of God by whom all things were created became man. The same One Word of God who created man in His image and likeness took on the name of Jesus. The same One Word of God who sends His Holy Spirit to anoint prophets and kings became anointed as the Christ. The same begotten not created One Word of god became created. The same One Word of God who is co-essential with the Father became co-essential with all of humanity in truth and perfection, in body, soul, spirit, will, energy, any and all properties of humanity except sin (which is after all not an essential property of humanity). The Source of all goodness and righteousness, the same One Word of God while without sin became a sin offering, as the Lamb of God, that He may take away the sins of the world, and grant us the Life from His eternal divinity.
I think when we contemplate on the good things both our families have to offer to the same One Christological faith, we can go on for pages that would be more pleasant than the arguments of language.