I am still digesting the discussions in Prague. In this post I present part of the rambling and over-long paper I delivered, beefed up somewhat, in an attempt to formulate more clearly my main points.
The orthodox view of the Incarnation is that God the Son, the second person of the Trinity, the Word or Logos, becomes man in Jesus of Nazareth. Although the Word becomes flesh and dwells among us as we read in the New Testament, the Word does not merely assume a human body, nor does it acquire a universal property, humanity; the Word assumes a particularized or individualized human nature, body and soul. The eternal Word assumes or 'takes on' a man, an individual man, with an intellectual soul and an animal body. And it does this without prejudice to its divine nature. But now a problem looms, one that can be articulated in terms of the following aporetic tetrad:
a. A person is a (primary) substance of a rational nature. (Boethian definition)
b. There is only one person in Christ, the Word, the Second Person of the Trinity. (Rejection of the heresy of Nestorius, according to which in Christ there are two persons in two natures rather than one person in two natures, as orthodoxy maintains.)
c. The individual(ized) human nature of Christ is a primary substance of a rational nature.
d. Every (primary) substance is its own supposit, which implies that every substance of a rational nature has its own personhood.
The tetrad is logically inconsistent: any three limbs taken in conjunction entail the negation of the remaining one. Thus the conjunction (a) & (c) & (d) entails the negation of (b). The solution to the tetrad is to deny (d). One does this by maintaining that, while the individualized human nature of Christ is a substance, it is not a substance that supports itself: it has an alien supposit, namely, the Second Person of the Trinity. If the Incarnation as Chalcedonian orthodoxy understands it is actual, then it is possible. If so, alien supposition is possible, which straightaway entails a distinction between substance and supposit: while every substance has or is a supposit, not every substance has or is its own supposit. The individualized human nature of Christ is a supposited substance but is not a supposit.
Given the substance-supposit distinction, we can secure the coherence of both the Incarnation and Trinity doctrines. Christ is one person (one supposit) in two natures while God is one nature in three persons (three supposits).
In correspondence, Dennis Monokroussos writes, "(c) is unacceptable to the orthodox Christian. There are two natures in the Word, but not two primary substances." I admit that I should have said something in defence of (c). But I think it is clear that on orthodoxy the Son's assumption of human nature is the assumption of a particular(ized) human nature with all that that entails, namely, a particular human soul and a particular human body with the very materia signata that a human body must have to be a concrete physical entity. Thus, in the Incarnation the Son becomes one with a particular human concrete primary substance. It is not the case that the Son assumes human nature in the abstract, whether human nature as a universal or human nature as particularized but taken in abstraction from matter and existence. The Son of God become man, a man, a living, breathing, suffering man mit Haut und Haar, skin and hair. So, contra Monkroussos, there are two distinct primary substances, the Son, and the man Jesus. There are two individual natures and two individual primary substances. But there is, on orthodoxy, for soteriological reasons that needn't detain us, only one person, only one supposit of a rational nature.
The distinction between substance and supposit can now be explained as follows. Since there are primary substances that are their own supposits and primary substances that are not, to be a primary substance and to be a (metaphysical as opposed to logical) supposit are not the same. The man Jesus is not a primary substance that is its own supposit: it has an alien supposit, namely, the Second Person of the Trinity. (I borrow the phrase 'alien supposit' from Marilyn McCord Adams.)
The problem that needs solving is this. If there are two individualized natures, one divine, the other human, and both including rationality, then there are two persons (assuming the Boethian definition of person.) But orthodoxy requires that there be only one person. The contradiction is avoided in the time-honored manner by making a distinction, in this case the distinction between substance and supposit. The distinction allows that an individualized rational nature needn't be its own personal supposit.
The main point of my paper is that the substance-supposit distinction is ad hoc because crafted for the precise purpose of removing theological contradictions. What makes it ad hoc is that there are no non-theological examples of the distinction.
You might grant me that the distinction is ad hoc, but then ask: what is wrong with that? What is wrong with it is that it does not advance the project of understanding how the doctrines in question (Trinity and Incarnation) are rationally acceptable. If the theological doctrines are rendered intelligible by a distinction crafted for that very purpose, then we are turning in a very tight circle: the doctrines in question are intelligible because the substance-supposit distinction is valid, and the distinction is valid because the doctrines are intelligible. In other words, the doctrines and the distinction stand and fall together. Hence the distinction, which has no application apart from the theological doctrines, does nothing to show how the doctrines are possible or intelligible to our finite, discursive reason.
If my problem is to understand how it is possible that two individualized rational natures be one person, you are not helping me if you make a distinction the validity of which presupposes the possibility in question.
"Look, the Incarnation as orthodoxy understands it is actual; therefore it is possible: esse ad posse valet illatio."
To which I respond: the precise question is whether the doctrine can satisfy a necessary condition of rational acceptability, namely, freedom from contradiction. For if it is not free of contradiction, then it cannot be actual. If such freedom is purchased in the coin of a distinction that is as questionable as the doctrine it is meant to validate, then no progress is made.
Nothing I have said entails that the Incarnation is not actual. For our inablity to understand how it is possible does not entail that it is not possible. (Compare: our inability decisively to refute Zeno and demonstrate how motion is possible is consistent with motion's being actual.) One can make a mysterian move here: the Incarnation (and the Trinity) are actual, but our cognitive architecture is such as to prevent us from ever understanding how they are possible. What is unintelligible to us, might be intelligible to angelic intellects or to God.
Compare the mysterianism of Colin McGinn. He maintains that consciousness is wholly natural, a brain-function, but that our cognitive architecture is such as to prevent us from every understanding how it could be a brain function. That naturalism is true, he takes 'on faith,' relying (apparently) on the magisterium, the teaching authority of Science, while insisting (rightly in my opinion) that it is utterly unintelligible to us how meat could give rise to consciousness. How could meat mean? Gushing over the complexity of brain meat cuts no ice, to mix some metaphors.
On the other hand, if we cannot understand how X is possible, is that not some sort of reason for suspecting that it is not possible?