This recent excursion into the philosophy of The School is proving to be quite fascinating, and I thank Dr. Novak et al. for their stimulation. I should say that I have read thousands upon thousands of pages of scholastic material, from Aquinas to Zubiri, from Maritain to Marechal, over the past 40 years, so it is not as if I am a complete stranger to it; I do confess, however, to finding some of it mumbo-jumbo and lacking in the sort of analytic rigor that we broadly analytic types prize. To get a better handle on the notion of suppositum ('supposit' in English), this morning I pulled down from the shelf a number of scholastic manuals.
Let us first turn to George P. Klubertanz, S. J., Introduction to the Philosophy of Being, 2nd ed. (Meredith Publ., 1963). Back in the day, when Catholic colleges were Catholic as opposed to catholic, this textbook was inflicted upon many a bored undergraduate in required courses. In those days, philosophy was taught systematically; this was before and during Vatican II, before and while the rot set in (if rot it was) and before chaos descended, the kind of chaos that issued in the Vagina Monologues being presented at the University of Notre Dame. (To cop a riff from Dennis Prager, there is no coward like an academic coward, and the abdication of authority on the part of university officials from the 60s on is something to marvel at.)
But I digress. According to Klubertanz, "The first substance is the singular substance which exists. When we want to designate the being precisely as an existing, substantial, complete individual, we call it a 'supposit.'" (251) He goes on to say that a supposit is a "complete individual" and therefore not something common to many in the manner of a secondary substance. Nor is a supposit an integral part, or an essential part, of a substance. Klubertanz gives the example of the body of a living thing as an example of an essential part of it -- presumably because a living thing cannot exist without a body -- and the example of a hand as an example of an "integral part." Klubertanz gives no rigorous definition of the latter phrase, but I surmise that an integral part is a part that is not essential to the whole of which it is a part. Thus a primary substance such as a particular man can exist without a hand.
Klubertanz further remarks that "the property of a supposit is 'incommunicability.'" (251) This is the property of being a complete individual, not common to many, and not an essential or integral part. This incommunicability might be what Lukas Novak has been referring to with his 'suppositality.'
Klubertanz usefully advises: "Distinguish the supposit as spoken of in metaphysics (the incommunicable first substance) from the supposit spoken of in logic (the ultimate subject of predication)." (251) That is surely an important distinction that every clear-thinking philosopher is bound to make whether or not he clothes the distinction in this particular terminology. We can predicate properties of anything we can talk about or single out in thought; thus anything can be a logical supposit. But not everything can be a metaphysical supposit. "The supposit in logic may be a thing (Peter, a tree); it may be a collection (three men, a flock); it may be an abstraction (humanity) or a being of reason (nothing)." (251) Peter and the tree under which he is sitting are both logical and metaphysical supposits. The Three Stooges, by contrast, is a logical but not a metaphysical supposit.
"The traditional definition of a supposit is 'a distinct subsistent individual in a particular nature.'" (251) It is subsistent in that it "exists with its own proper and proportionate act of existing." Klubertanz then asks, "What then is the difference between an individual thing and a [metaphysical] supposit?" I was about to ask the same question myself. An individual thing is presumably the same as a first or primary substance. So the question is, What is the difference between a primary substance (an Aristotelian prote ousia) and a metaphysical supposit? Why do we need the term 'supposit'? What does it add? Here is Klubertanz's answer:
Ordinarily and naturally, an individual being has its own proper act of existing, even though there is a real distinction between the nature or essence and the act of existing. But the term "individual thing" only connotes an act of existing, while the term 'supposit" means "having its own proper act of existing." A techinical term sometimes used for "supposit" is the word "hypostasis." (252)
This is not at all helpful. I cheerfully grant that an individual thing or primary substance is composed of essence and existence and that this is a real distinction. I also grant that each individual thing has its own act of existing, one that is proper to it. Now unless an individual thing can have an act of existing that is NOT proper to it, there is no justification for the distinction between individual thing and supposit. But how can an individual thing not have an act of existing proper to it? How can it be that, while in general each first substance has its own act of existing, there are exceptions to this, first substances that do not have their own act of existing?
It is at this point that theological dogma enters the picture and influences the background general ontology. Orthodox doctrine has it that Christ is one, not two: it is not as if there are two primary substances, the Second Person of the Trinity and the man Jesus. There is exactly one primary substance having two distinct natures, one divine, the other human. There are two natures but one person. A person, as Klubertanz points out, is "a supposit in a rational nature," "a rational supposit." (252) So what we have in Christ is one rational supposit supporting two distinct individual natures, one divine, the other human. Since Christ is a divine person, he cannot also be a human person, for then there would be two persons, one God the Son, the other an ordinary man. But orthodoxy has it there is only one person supporting two singular (not universal) natures. Since the singular natures are precisely singular, each has an act of existing. Now if each nature has its OWN act of existing, an act of existing proper to it, then we would have two rational supposits, two persons. Since there is exactly one person, exactly one rational supposit, the singular human nature of Christ does not have its own act of existing. It has an act of existing all right, but not its OWN act of existing. As Klubertanz puts it, "the Divine Person supplies the esse to the human nature; hence Christ's human nature exists, but not with the esse proper to such a nature." (252)
The question I asked was, What is the difference between a primary substance and a metaphysical supposit? Klubertanz's answer, in effect, is that a primary substance has an act of existing while a metaphysical supposit has its own proper act of existing. Now from the point of view of general ontology (metaphysica generalis) this is a very bad answer, a scholastic Spitzfindigkeit. Why? Because, as Klubertanz himself says, a primary substance "ordinarily and naturally" has its own proper act of existing. From the point of view of general ontology, it makes no sense to say that each primary substance has its own act of existing and then to allow an exception, namely a primary substance that does not have its own act of existing.
The exception is made to accommodate a piece of theology, namely, the orthodox Incarnation doctrine. And so the distinction between primary substance and supposit is open to the charge of being ad hoc. The Latin phrase means 'to this' and suggests that the distinction has no independent support and is a mere invention pulled out of thin air to render coherent an otherwise incoherent, or not obviously coherent, theological doctrine.
If our question as philosophers of religion is whether the Incarnation doctrine is rationally acceptable, then is hard to see how it can shown to be such by the use of a distinction which has no independent support, a distinction which is crafted for the precise purpose of saving the doctrine in question. To rebut this objection from ad hocness, someone will have to explain to me that and how the primary substance-supposit distinction has independent warrant. Is there some clear non-theological case in which the distinction surfaces?
Another troubling aspect of the scholastic literature on supposita is that suppositum and person seem to be both distinguished and conflated. For example, Klubertanz writes, "Hypostasis is another name for supposit or person." (254) This suggests that every supposit is a person. But earlier (252) a person is defined as a rational supposit, which suggests that there are supposits that are not persons. So which is it? Are all supposits persons? Or are there supposits that are not persons? If the latter, what would be an example of a non-rational supposit that was yet distinct from a primary substance?