0. I wanted to explore supposita in their difference from primary substances, but John the Commenter sidetracked me into the aporetics of primary substance. But it is a sidetrack worth exploring even if it doesn't loop back to the mainline. For it provides me more grist for my aporetic mill.
1. Metaphysics is a quest for the ultimately real, the fundamentally real, the ontologically basic. Aristotle, unlike his master Plato, held that such things as this man and that horse are ontologically basic. What is ontologically basic (o-basic) is tode ti, hoc aliquid, this something, e.g., this concrete individual man, Socrates, and that concrete individual donkey. Such individuals are being, ousia, in the primary sense. And so Socrates and his donkey can be called primary beings, or primary substances. Asinity there may be, but it can't be ontologically basic.
This is clearly the drift of Aristotle's thinking despite the numerous complications and embarrassments that arise when one enters into the details.
(If you think that there is 'substance' abuse in Aristotelian and scholastic precincts, I sympathize with you. You have to realize that 'substance' is used in different senses, and that these senses are technical and thus divergent from the senses of 'substance' in ordinary language.)
2. But of course every this something is a this-such: it has features, attributes, properties. This is a datum, not a theory. Socrates is a man and is excited by the turn the dialectic has taken, and this while seated on his donkey. Man is a substance-kind, while being excited and being seated are accidents. (Let us not worry about relations, a particularly vexing topic when approached within an Aristotelian-scholastic purview.) Setting aside also the difficult question of how a secondary substance such as the substance-kind man is related to Socrates, it is safe to say that for Aristotle such properties as being excited and being seated are theoretically viewed as accidents. So conceptualized, properties are not primary beings as they would be if they were conceptualized as mind-independent universals capable of existing unexemplified. Accidents by definition are not o-basic: If A is an accident of S, then A exists only 'in' S and not in itself. A depends on S for its existence, a mode of existence we can call inherence, while S does not depend for its existence on A.
3. So much for background. Now to the problem. Which is ontologically basic: Socrates together with his accidents, or Socrates taken in abstraction from his accidents?
What I want to argue is that a dilemma arises if we assume, as John the Commenter does, that Socrates taken together with his accidents is an accidental unity or accidental compound. A simple example of an accidental compound is seated-Socrates. Now I won't go into the reasons for positing these objects; I will just go along with John in assuming that they are there to be referred to.
Seated-socrates is a hylomorphic compound having Socrates as its matter and being seated as its form. But of course the matter of the accidental compound is itself a compound of prime matter and substantial form, while the form of the accidental compound is not a substantial form but a mere accident. The accidental compound is accidental because seated-Socrates does not exist at all the same times and all the same worlds as Socrates. So we make a tripartite distinction: there is a compound of prime matter and substantial form; there is an accident; and there is the inhering of the accident in the substance, e.g., Socrates' being seated, or seated-Socrates.
As Frank A. Lewis points out, accidental compounds are "cross-categorical hybrids." Thus seated-Socrates belongs neither to the category of substance nor to any non-substance category. One of its constituents is a substance and the other is an accident, but it itself is neither, which is why it is a cross-categorical hybrid entity.
The dilemma arises on the assumption that Socrates together with his accidents is an accidental compound or accidental unity, and the dilemma dissolves if this assumption is false.
a. Either (i) Socrates together with his accidents is a primary substance or (ii) Socrates taken in abstraction from his accidents is a primary substance.
b. If (i), then Socrates is an accidental compound and thus a "cross-categorical hybrid" (F. A. Lewis) belonging neither to the category of substance nor to any non-substance category. Therefore, if (i), then Socrates is not a primary substance.
c. If (ii), then Socrates is not a concretum, but an abstractum, i.e., a product of abstraction inasmuch as one considers him in abstraction from his accidents. Therefore, if (ii), then Socrates is not a primary substance. For a primary substance must be both concrete and completely determinate. (These, I take it. are equivalent properties.) Primary substances enjoy full ontological status in Aristotle's metaphysics. They alone count as ontologically basic. They are his answer to the question, What is most fundamentally real? Clearly, Socrates taken in abstraction from his accidents is incompletely determinate and thus not fully real.
d. On either horn, Socrates is not primary substance.
What say you, John?