I need to answer three questions. This post addresses only the first.
1. What is the difference between an Aristotelian primary substance and a supposit (hypostasis, suppositum)?
2. Is there any non-theological basis for this distinction?
3. If the answer to (2) is negative, is the addition of suppposita to one's Aristotelian ontology a case of legitimate metaphysical revision or a case of an ad hoc theoretical patch job? According to Marilyn McCord Adams, "Metaphysical revision differs from ad hoc theoretical patching insofar as it attempts to make the new data systematically unsurprising in a wider theoretical context." ("Substance and Supposits," p. 40)
The First Question
By 'substance' I mean an Aristotelian primary substance, an individual or singular complete concrete entity. Among the characteristics of substances are the following: substances, unlike universal properties, cannot be exemplified or instantiated; substances, unlike accidents, cannot inhere in anything; substances, unlike heaps and aggregates, are per se unities. Thus Socrates and his donkey are each a substance, but the classical mereological sum of the two is not a substance.
Now what is a supposit? Experts in medieval philosophy — and I am not one of them, nota bene — sometimes write as if there is no distinction between a substance and a supposit. Thus Richard Cross: "Basically a supposit is a complete being that is neither instantiated or exemplified, nor inherent in another." ("Relations, Universals, and the Abuse of Tropes," PAS 79, 2005, p. 53.) And Marilyn McCord Adams speaks of Socrates and Plato as "substance individuals" and then puts "hypostases or supposits" in apposition to the first phrase. (PAS 79, 2005, p. 15)
My first question, then, is: Is there any more-than-verbal difference between a substance and a supposit, and if so, what is it?
One answer that suggests itself is that, while every substance has a supposit, some substances have alien supposits. That is, some substances are their own supposits, while others are not their own supposits, but have alien supposits. (I take the phrase 'alien supposit' from Adams, p. 31 et passim.) A substance has an alien supposit if and only if it is not its own supposit. I understand Aristotle to maintain or at least be committed to the proposition that every (primary) substance is essentially its own supposit. (I rather doubt that the Stagirite ever raised the question of alien supposition.) If so, then no substance is possibly such as to have an alien supposit. If alien supposition is metaphysically or broadly logically possible, however, then we have a ground for a more-than-terminological distinction between substances and supposits. Whether the converse of this conditional holds is a further question. For it may be that there is a ground for the distinction even if alien supposition is not possible.
Incarnation, Trinity, and the separated soul's survival between death and resurrection are theological examples of alien supposition. Whether there are non-theological examples is a further, and very important question, one the answer to which has consequences for questions (2) and (3) above.
The Incarnation is an example of alien supposition as I will now try to explain.
The orthodox view is that God the Son, the second person of the Trinity, the Word, becomes man in Jesus of Nazareth. Although the Word becomes flesh and dwells among us as we read in the NT, the Word does not merely assume a human body, nor does it acquire a universal property, humanity; the Word assumes a particularized human nature, body and soul. The eternal Word assumes or 'takes on' a man, an individual man, with an intellectual soul and and animal body. 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.)
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). For if there are two primary substances of a rational nature, the Word and Christ, then there are two persons each with his own individualized nature, contra Chalcedonian orthodoxy, according to whch there is exactly one person in two natures. The solution to the tetrad is to deny (d), the very natural Aristotelian assumption that every substance is its own supposit. 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 itself a supposit.
Let me now say a bit about the Trinity. Here too a problem looms that can be cast in the mold of an aporetic tetrad.
a. A person is a (primary) substance of a rational nature. (Boethian definition)
e. There are exactly three divine persons, Father, Son, Holy Ghost . (Rejection of 'Quaternity')
f. The individualized nature of God 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.
Again, the tetrad is inconsistent, and again the solution is to reject (d) by saying that, while the individualized divine nature is a primary substance, it is not one that supposits itself: it has three alien supposits, Father, Son, and Holy Ghost.
The Son is thus the alien supposit of both God's divine nature and Christ's human nature.
My first question concerned the difference between a substance and supposit. My tentative answer is that while only substances can be supposits, there are substances that are not their own supposits nor are they supposits for anything else, an example being the individualized human nature of Christ.
Is there a non-theological basis for the distinction? if not, then the suspicion arises that the distinction is purely ad hoc, crafted to save tenets of orthodox Christian theology. But this is a question for another occasion.
What's wrong with ad-hocery?
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