I was thinking of a somewhat mathematical approach to the whole "one nature from two natures" slogan of the Anti-Chalcedonians, although I cannot claim to be a professional mathematician.
It seems to me that, since the Father and the Son share the same Divine Nature (with the Holy Spirit, too), we can say something like the following.
If we let a=the Divine Nature of the Logos and b=the Divine Nature of the Father, then a=b.
Now let c=Christ's human nature.
If, as the Anti-Chalcedonians assert, our Lord has one nature from two natures, then that is adding a+c to get a single composite nature that is the sum of the two.
However, if a=b, then a+c=b+c!
Has the Father acquired a human nature?
If not, then does the Son now possess a nature different from that of the Father and the Holy Spirit?
Clearly, since that is not the case, the union of the Divine and human natures of Christ has not produced a composite nature.
Since the Son is different from the Father not in nature but in hypostasis, the union of the Divine and human in Christ is to be found there, and the two natures must remain distinct, unmixed, and unconfused. They are inseparable at the hypostatic level in the One Person of our Lord and Savior Jesus Christ, but they are not a sum that produces a third nature that makes the Son's nature different from that of the Father and the Holy Spirit.
Although Anti-Chalcedonians protest that they do not confuse or mingle the two natures, how can that be true when they derive one nature for our Lord from two natures and say that "after the union it is impossible to speak of the two natures?"
If, as some of them protest, by nature they mean hypostasis, then they have the one Divine Hypostasis of Christ the Word as the sum of two other hypostases! If nature=hypostasis, what else can "one nature from two natures" mean?
Deriving the one composite Hypostasis of our Lord from two other hypostases is just what the Nestorians did!
I don't believe any other conclusions can be derived from the Anti-Chalcedonian phrase "one nature from two natures."
Now if that is not really what the Anti-Chalcedonians mean, why do they continue to say it?
Why not say, with St. Leo and the Council of Chalcedon, "one Hypostasis in two natures?"
That statement preserves the Orthodox doctrine of the inseparable union of the Divine and human in the One Person of Christ but maintains the distinction between the Divine Nature our Lord shares with the Father and the Holy Spirit - and which is impassible - and the human nature which our Lord shares with us.
Our Lord Jesus deifies our human nature in His Divine Person, but not by somehow altering the Divine Nature He shares with the Father and the Holy Spirit.
Is all that clear, or is it confusing?
