Here is another of the scholastic manuals I pulled off my shelf: Fernand van Steenberghen, Ontology (Nauwelaerts Publisher, Brussels, 1970, tr. Moonan). A paragraph from p. 278 supports my thesis that the distinction between primary substance and suppositum is an ad hoc device invented for a theological purpose, a device for which there is no independent philosophical warrant:
4. The problem of subsistence or personality. This problem was inserted into metaphysics for the benefit of theology, as is quite plain, in order to prepare the way for a satisfactory explanation of the theological mystery of the incarnation, the question of knowing how and why the human nature of Jesus Christ does not constitute a human person. But this problem is extraneous to philosophy and must remain so, for from the metaphysical point of view, there is no reason for distinguishing individual nature and individual. It is therefore contrary to any sane method to ask in ontology on what conditions an individual nature might not be a suppositum (or person, where it is an intelligent nature that is in question.)
According to the Chalcedonian definition (A.D. 451), God the Son and Jesus Christ are one person, not two. And yet this one person has two distinct existing individual natures, the one divine and uncreated, the other human and created. Now an existing individual nature is a primary substance. So what we have are two numerically distinct primary substances that are yet one and the same. On the face of it, this is a contradiction: two substances cannot be one substance. This is the prima facie evidence of the impossibility of the Incarnation doctrine as understood in the Chalcedonian definition.
In a separate thread, Dr. Novak writes, "In order that a doctrine is rationally acceptable, the absence of evidence of impossibility (contradiction) is enough, evidence of absence of impossibility (contradiction) is not needed." To me it is clear that there is no absence of evidence of impossibility in this case. There is positive evidence of impossibility and that evidence is that two primary substances cannot be one primary substance.
This is why there is a problem about rational acceptability. Since there is evidence of impossibility, that evidence has to be shown to be merely apparent if the doctrine is to be shown to be rationally acceptable. One way to attempt to secure this end is by making a distinction between primary substance and metaphysical supposit. This distinction allows one to say that there can be a primary substance that is not a supposit, and thus not a rational supposit or person. Accordingly, the man Jesus is an existing primary substance but not a person. There is exactly one person, and that person is the Second Person of the Trinity.
This solution to the problem -- the problem of how two primary substances can be one and the same -- requires the distinction between supposit and primary substance. Now if there were some non-theological case in which this distinction could be seen to figure, then the distinction would have independent warrant and we would be home free: we would have a satisfactory solution. Unfortunately, however, as Van Steenberghen points out, there is no reason independent of theology for drawing the distinction in question.
This amounts to saying that the solution is ad hoc. But then the rational acceptability of the doctrine has not been demonstrated. Why not? Well, if the distinction is crafted for the sole purpose of solving the problem in question, then the distinction is just as problematic as the original problem.
Of course, it is very difficult to get a dogmatist to appreciate any of this, for what he will do is simply repeat his formulations. But doing so does nothing to show the rational acceptability of their content; all it succeeds in showing is that the dogmatist is a dogmatist.
Please note that my claim is not that the Incarnation doctrine is rationally unacceptable; my claim at the moment is simply that there is a very serious problem about the rational acceptability of the doctrine and that this problem cannot be given a satisfactory solution by making a distinction between primary substance and (metaphysical as opposed to logical) suppositum.