In this installment, Peter Lupu, atheist, defends the logical coherence of the doctrine of the Trinity. My critical comments follow in blue.
It may be somewhat of an astonishment to those who know me well that I should venture to defend the doctrine of the Trinity. I am not a Christian; I am not religious; I am an atheist; and I have at least on one occasion privately expressed to Bill my reservations about the coherence of the Trinity doctrine. Nevertheless, there is a question here that deserves exploring. What is the question?
Bill asks: ". . . . is it unproblematic to say that God is one in respect of his nature (or substance) but three in respect of his Persons?"
The question, then, is not whether the Trinity doctrine is true tout court, but whether it is possible: i.e., whether it is a logically coherent doctrine. And despite all my sins listed in the opening paragraph, I wish to know the answer to this question. Our problem is that the following two doctrines appear to be logically inconsistent:
1) On the one hand, we have the doctrine of the absolute unity of God. Thus, if God exists at all, then God must be one absolutely unified individual: nothing can be identical to part of God without being identical to the whole of God. The principle of the absolute unity of God implies that for any two entities x and y, if x is identical to God and y is identical to God, then x and y are absolutely identical . So, clearly, the absolute unity of God is to be interpreted in terms of the notion of identity. In order to render this explicit I shall call this the Absolute Identity Doctrine (AID).
2) On the other hand, we have the doctrine of the Trinity. What is this doctrine? Well, this doctrine is also interpreted as involving essentially the notion of identity. How so? Well, the way the doctrine of the Trinity is interpreted is roughly along the following lines: there are three absolutely distinct entities (regardless of how they are named or described) x, y, and z such that each of these absolutely distinct entities x, y, and z is identical to God. So here the doctrine of Trinity involves essentially the notion of identity. I shall call this interpretation of the doctrine of Trinity the Identity Interpretation of Trinity (IT).
3) Of course, it should be immediately apparent that AID and IT cannot be both true. This is the paradox of the doctrine of the Trinity.
4) Father Gratry's solution (as cited in Bill's post) relies upon distinguishing between "absolute unity of nature" vs. "absolute trinity of persons". Thus, with respect to God's nature, God is absolutely identical to one and only one thing and, hence, absolutely unified. However, with respect God qua person, God is absolutely identical to three different persons. What sort of a distinction is being made here? It would be natural to interpret the distinction Father Gratry relies upon for his solution as presupposing a notion of relative identity. Relative to God's nature, God is one; relative to God as a person; God is many (i.e., three).
5) Bill does not buy Father Gratry's solution:
"Either the divine nature is an individual entity or it is not. If it is an individual (unrepeatable) entity, then each Person can be God only by being identical to God. But this implies that there is one person, not three. If, on the other hand, the divine nature is a multiply exemplifiable (repeatable) entity, then each Person can be God only by exemplifying the divine nature. This, however, implies that there are three Gods."
I suspect that Bill rejects Father Gratry's solution because he thinks that whether or not relative identity makes sense in general, it certainly makes no sense when it comes to God. If God's very nature is to be absolutely unified; absolutely one, then there cannot be some other respect in which God could be many. It is precisely this sort of relativity that is excluded by maintaining that God's unity is absolute by his very nature. Therefore, the possibility that there is some other respect in which God could be many is excluded by the very notion that God's unity is an inherent part of God's nature. What else could AID mean?
6) I think that Bill's intuitions are correct and I agree with him on this point. But perhaps there is a different way of interpreting Father Gratry's solution. Suppose we do not interpret Father Gratry's distinction between "absolute unity of nature" vs. "absolute trinity of persons" as a form of relative identity. Suppose that instead we interpret it as distinguishing between two senses of `is'. God's nature is such that for every two entities x and y, if x is identical to God and y is identical to God, then x and y must be identical. We do not need to reinterpret AID as relative identity. However, once we insist upon interpreting AID in this manner, we no longer can interpret IT as a claim about three absolutely distinct entities being identical to God: i.e., we simply cannot interpret IT as involving also the notion of identity. I suggest that we should not think about the doctrine of Trinity as involving identity at all. Rather we should interpret the `is' in the claims: person x is God; person y is God; and person z is God as the `is' of instantiation or rather as primary instantiation. While God is absolutely unified by his very nature, God has three primary instantiations in three absolutely distinct persons x, y, and z.
7) This proposal takes a page from Plato's solution to the problem of the One and the Many. By the very nature of forms, there is one and only one unified form of horsehood: there cannot be several (within Plato's metaphysics this is easily provable). However, the form of horsehood can have none, one, or many instances. Similarly, by God's very nature, there can be only one absolutely unified God. But God can be exemplified qua persons in several different ways. In this case I suggest that the proponents of the doctrine of the Trinity should maintain that there can be three and only three absolutely different primary instantiations of God qua person. So Father Gratry's distinction between "absolute unity of nature" vs. "absolute trinity of persons" can now be interpreted as a distinction between the sense in which God by his very nature is absolutely unified and hence necessarily one (analogous to Plato's forms) vs. the possible primary instantiations of God in the manner of persons of which there can be only three absolutely distinct such instantiations.
8) If we allow this distinction, then the Absolute Identity Doctrine and the Trinity Doctrine are logically compatible because while the former is about (absolute) identity, the later does not involve identity at all: the doctrine of Trinity is about the possible primary instantiations of God in the form of persons. Hence, the two doctrines do not conflict.
9) None of this proves, of course, that the doctrine of Trinity is true; only that it is possible.
The problem of the Trinity, to put it schematically, is to prove the consistency of the following set of propositions while avoiding tritheism:
a) P1 is numerically distinct from P2.
b) P2 is numerically distinct from P3.
c) P1 is numerically distinct from P3.
d) P1 is G.
e) P2 is G.
f) P3 is G.
If the 'is' in the last three propositions is the 'is' of identity, then a contradiction is easily derivable given the Absoluteness of Identity and the Transitivity of Identity. (Verify this for yourself.)
On Peter's suggestion as I understand it the solution lies in the direction of reinterpreting the 'is' as it occurs in the last three propositions as an 'is' of predication. Say what you want about Bill Clinton, he rendered a great service to philosophical logic by insisting that much depends on what the meaning of 'is' is. And he saved his hide to boot!
Reading the 'is' as the 'is' of predication avoids inconsistency but gives rise to a different problem. Suppose you take 'The Father is God' to mean 'The Father is divine' where the 'is' expresses predication and 'divine' picks out the property of divinity. Then what you are saying in effect is that the Father exemplifies or instantiates divinity. And similarly in the cases of the Son and the Holy Ghost. But if each exemplifies divinity, then each is a god, and the result is tritheism.
The trick is to maintain monotheism while also maintaining the distinctness of Persons. Can this be done without major surgery on standard logic? Perhaps it can be done if it can be maintained that identity is sortal-relative. For then one can say the following: 'The Father is the same god as the Son, but a different person,' 'The Son is the same god as the Holy Ghost, but a different person,' and 'The Father is the same god as the Holy Ghost, but a different person.' Thus we would avoid such inconsistent triads as the following: The Father = God; The Son = God; The Father is not identical to the Son.
But is the sortal-relativity of identity sound logical doctrine? That's a separate post.