In the last post, I explained that Nestorians believe that a complete individual human nature is indiscernible from an individual human person.
Monophysites also take this idea very seriously. In fact, the Monophysite takes very seriously the more general claim that a complete individual nature of any kind is indiscernible from the corresponding individual that belongs to that kind (for instance, a complete individual cow-nature just is the individual cow in question). So if there are two natures in Christ, then there will be two individuals that correspond to each of those natures.
But the Monophysite does not want to say that there are two persons in Christ, so he will insist that there is just one nature in Christ. That way, there will just be one person. But since Christ is both human and divine, this one nature must be a special hybrid of divinity and humanity.
The strongest form of Monophysitism would claim that this hybrid Christness-nature has all divine properties, and all human properties. This, however, is incoherent, for it would amount to two persons as well. After all, having all the divine properties is sufficient for membership in God’s-kind, and having all human properties is sufficient for membership in human-kind. There would, then, still be two natures, which contradicts the initial claim that there is just one (allegedly) hybrid nature.