a) Unity of the substance-accident composite.
The distinction
between substance and accidents is a distinction of
reality, not just of the mind. The nearest evidence for the
distinction is that accidents, some of them at least, can
change and even be totally corrupted without inflicting a
change of substance. It can also be urged that some accidents
run counter to substance, a circumstance that implies
a real distinction between the two modes of being. Of its
very nature quantity, for example, entails divisibility, whereas
substance works in the opposite direction, making for
unity.
Yet the distinction has been objected to on the ground that it impairs (if it does not destroy) the unity of the concrete being; and if you allow that, it becomes practically impossible to give a satisfactory account of the changes we see in things. For, supposing the real distinction, substance (it is contended) is sealed off from accidents; consequently, changes that occur must be regarded as coming from extrinsic factors only, surface modifications upon a substrate the while and forever inert. Substance, then, might still be the peg on which changes must be hung, though the moderns would soon remove the peg and (they thought) keep the changes withal.
The foregoing criticism is typical of a wide segment of modern philosophy. Yet the criticism, to repeat a point made earlier, does not hit at substance but at pseudosubstance. Substance is represented as an inert substrate, and the real distinction is construed as isolating and insulating substance from accidents, and thus as disrupting the unity of being. But the truth of the matter is that accidents, while really distinct from substance, are not thereby prevented from combining with it to constitute a single, unified being. And if you ask how so, the answer is that accidents do not have independent existence; they "inhere" or (if the term be allowed) "inexist" in the subject. It is the concrete being that exists, a substantial reality that finds completion in its accidental modalities, all in a unified whole. So, too, it is the concrete being, the individual, that changes and acts: the man that thinks, the fire that burns, etc. Actions, it is a Scholastic axiom, are the supposit'sactiones sunt suppositorum. Consequently, it is completely off the mark to picture substance as one knows not what, an inert entity concealed behind a cloak of accidents which, cloak-like, come on and off mechanically. The concrete individual, the actual existent, though really multiple as to its principles is yet truly one, and acts as one, doing whatever it does from all that it is.
b) Individuation of material substance. Substance (first substance, obviously) can exist only as individual. Since there are many individuals, the metaphysician wants to know what it is that renders them distinct from each other. In the case of spiritual substances (which are pure forms), what accounts for their being distinct is their very form or essence; pure forms, this means, are individuated of themselves, of their very nature. Consequently, of such forms or substances there can only be one for any given nature; which is to say, each angel is a different species - quot angeli tot species.
Material substances tell a different story. There we find not only many species but also many individuals of the same species, which makes them formally identical. Consequently, not differing in formal or specific perfection, they must be differentiated or individuated on some other ground. St. Thomas agrees with Aristotle that the root principle of this individuation is matter. And why matter? Because, among other things, the principle that individuates substantial being must be of the substantial order. The choice then narrows down to matter or form; not being form (since material substances do not differ formally), it must be matter, prime matter. But for matter to perform this function it needs to be determined by the accident of quantity; hence the principle of individuation is not matter pure and simple but, in the accepted phrase, "matter marked by quantity, "materia signata quantitate."
St. Thomas explains why the intervention of quantity is necessary. Form is individuated by being received into this distinct and determined matter; but matter becomes divisible, hence distinguishable, through quantity. Consequently, there is no distinction of material being except through matter already under dimensions of some kind, which is matter quantified. However, the quantification in question, St. Thomas continues, does not entail definite terminations or determined dimensions of matter, but only dimensions considered without fixed limits. "From these interminate dimensions," he concludes, "results this matter, signate matter; and as such it individuates form. And thus from matter arises numerical diversity in the same species." 11
c) The problem of subsistence. With the mysteries of faith, of the Incarnation in particular, came a problem about substance that had escaped philosophers, the vexing problem of subsistence. But though undiscovered by the philosopher, the problem concerns him as well as the theologian. Subsistence involves the notion of supposit and person. Supposit, in the present context, means the substantial, subsistent individual; if the supposit is a rational being, the designation for it is "person." The problem, then, is this: in the existent individual, is there a real distinction between supposit or person on the one hand and individual nature or essence on the other? If the distinction is real, there must be something over and above individual nature, some formality or modal being which accounts for the autonomy and incommunicability that separate one existent substance from every other.
Most commentators of St. Thomas, at least since Cajetan, are for the real distinction. Substance, they explain, needs to be determined or terminated in the order of concrete autonomy by a special formality called "subsistence," which is conceived as a substantial mode. The mode, or subsistence, brings it about that a given nature belong to this individual only and cannot belong to another, (which is) that it be incommunicable. This additional entity is thought necessary because without it essence, though capable of determining and hence limiting existence to a specific nature, cannot account for the independence, the autonomy, that accompanies the existence of the nature. In all, the concrete subject of the created order stands forth as an individual nature completed by the distinct substantial mode, subsistence, whereupon ensues the ultimate completion (or perfection), existence proportionate to the nature.12