Is it permissible for a Catholic to believe in Miaphysitism?

The OED defines "miaphysite" as:

Of or relating to the doctrine that in the person of Jesus there is either a single divine nature, or one inseparable nature, partly divine, and partly (subordinately) human

Both of these senses are heretical because Jesus doesn't have one nature (as also monophysites believe), nor does He have "one inseparable nature" that is "partly divine, and partly (subordinately) human." He has two natures that are each fully (not partly) human and divine.

The Council of Chalcedon (451 AD) condemned monophysitism and miaphysitism (μονο and μία both mean "one"):

Therefore, following the holy Fathers, we all teach that with one accord we confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and the same perfect in human nature, true God and true man, the same with a rational soul and a body, consubstantial with the Father according to divine nature, consubstantial with us according to the human nature, like unto us in all things except sin…

The short answer is that, correctly understood, Miaphysitism can be compatible with the Catholic Faith.

In order to see how this is so, it is important to understand carefully what Catholics understand by “Miaphysistism.”

This position refers to a formula attributed to St. Cyril of Alexandria, intended to defend the so-called “communion of properties” in Christ:

μία φύσις τοῦ θεοῦ λόγου σεσαρκωμένη

one nature (physis) of the Word of God made flesh (sesarkomene)

(Briefly, the doctrine of the communion of properties, or communicatio idiomatum, means that the properties or actions of Jesus that proceed from his Divine Nature may rightly be ascribed to him as man, and vice versa. For example, we may rightly say both that God died on the Cross, and that Jesus the man created the universe. This communion arises because of the hypostatic union, since all actions are ultimately ascribed to the hypostasis, not the nature. See also Catechism of the Catholic Church [CCC] 466.)

It should be noted that St. Cyril (c. 376–444) made this formula before the Council of Chalcedon (451), and before the terminology carefully distinguishing “nature” (physis) and “Person” (hypostasis) had stabilized.*

Hence, if the term physis here corresponds roughly to the Chalcedonian concept of hypostasis, then this formula can certainly be interpreted in an orthodox manner.

The Magisterium, in fact, has solemnly affirmed that Cyril’s formula may still be used. In the Second Council of Constantinople, the following canon is found:

If someone confesses that a union of natures, divine and human, has taken place, or speaks of one incarnate human nature of the Word of God (μίαν πύσιν τοῦ Θεοῦ λόγου σεσαρκωμένην), but does not understand these formulations as in the teachings of the Holy Fathers, … let him be anathema” (Denzinger-Hünermann no. 429, my translation).

In referring to the “Holy Fathers,” the Council clearly means St. Cyril of Alexandria, who did not at all espouse a Eutychian or Apollinarian Monophysitism (both of which effectively deny the full humanity of Jesus).

If Miaphysitism is understood according to this precise formulation, then it is perfectly orthodox and sanctioned by the Magisterium (so long as we understand that the Council of Chalcedon used physis in a different manner).

Hence, one could say that the key difference between Miaphysitism (correctly understood) and Monophysitism is that, whereas the latter denies some aspect of Christ’s humanity, the former respects his full and perfect humanity.

^{* The terms physis and hypostasis come ultimately from Aristotle—although the Alexandrian fathers were highly influenced by Platonism—and, in philosophical terms, have roughly the same meaning. Therefore, exactly how these terms are employed in theology is conventional. Cyril was writing before physis came to signify that which answers the question “What is it?” and hypostasis came to signify the personal subject that answers the question “Who is it?”}