Jonathan Prejean, one of the most intelligent Catholic apologists on the block, posted some correspondence with Ramond Maxwell Spiotta at Crimson Catholic yesterday, and I found all of it fascinating but the discussion of hypostasis, being, and substance particularly so. The word "substance" has taken on a rather restricted, ahistorical meaning, principally due to the historical accident of the rise of a new form of empiricism in the 16th and 17th centuries that led to the final dominance of materialism in Western intellectual history.
These days when one hears the term "substance" one thinks almost automatically of something material. "What is the substance in that beaker?" "What is this substance on my shoe?" "The surface of the water was coated with an oily substance." But the identification of "substance" with "matter" is an accident of history, dating from the Scientific Revolution when the dominant materialism assumed that everything that exists is material. The word "substance", philosophically, just means "something that exists as a metaphysical unity", and if you're not a materialist you might be willing to endorse the idea that non-material things exist as metaphysical unities, things like God, or one's own soul, or numbers. But if you're a materialist, obviously, only material things can exist as metaphysical unities (indeed, most materialists also chuck the notion of a metaphysical unity) and so only material things can be substances and so when one says "substance" one means "some kind of material thing."
The English word "substance" is derived from the Latin term substantia, which in turn is derived from sub, "under", and stare, "to stand". A substance, then, is something that "stands under" something else. This becomes clearer, if only a little so, when we note that the Latin term is itself a translation of a Greek word with the same meaning, hupokeimenon, from hupo, "under", and keimenai, "to lie" or "be situated". That which lies under something else, according to Greek metaphysics, is the thing in itself, and that which it underlies are the thing's accidental properties. The Greeks, particularly Plato and Aristotle, were interested in giving a metaphysical explanation of change, particularly the problem of how it is that objects, principally objects like human beings but also things like tables, rocks, bodies of water, etc., can at one and the same time be in a process of change, that is, becoming different, and yet remain the same thing through time. For example, a human being is conceived as a single cell, grows into a fetus, is born an infant, grows into a toddler, a child, an adolescent, an adult, and then dies. At each of these various times the human is both very different than it was before and will be in the future but also the same thing that it was right from the beginning and will remain right until the end.
Early Greek philosophers had been fascinated by this state of affairs. Heraclitus, a Greek speaking philosopher living in Ephesus (on the west coast of what is today Turkey) found the process of change itself to be the most fundamental phenomenon in the kosmos, and he argued, in effect, that to be anything at all just is to be some sort of process or other. By contrast, the Eleatic philosopher Parmenides (Elea is in southern Italy) maintained that being implies stasis: how can something be said to be if it is engaged in some sort of process of changing? To change is to go, for example, from one state, call it S, to a different state that we may call not-S, and that latter state, not-S, is inherently a kind of non being, that is, it is the non-being-of-S. Since it is, on this account, a kind of non-being rather than a kind of being, Parmenides held that it is meaningless to talk about it, hence we can never speak meaningfully about the sorts of processes that Heraclitus held were the fundamental constituents of the world. Instead there are only beings. Or rather, there is only Being: Parmenides also held that all being is one, since if we were to differentiate one being from another we would thereby be admitting again the possibility of non-being. For example, to say that being A is a different being than being B is to say that B is the-non-being-of-A, which Parmenides holds is meaningless. So everything is one.
Into this mix came the Platonists (we may include Aristotle and his school under this rubric). Plato, famously, held, with Parmenides, that the beings (in Greek, ta onta) of the world do not change: they are eternally what they are, and they are what they are without reference to the being of any other thing. Hence they are ontologically prior. These beings Plato often refers to as ousiai, a noun form derived from the Greek verb to be. They are literally "the beings", then, but the word ousia is usually translated as "substance", so the ousiai are what Plato holds to be the substances of the kosmos, the things that are the metaphysical unities.
What are these beings, according to Plato? We get some indications of what he thinks they are in dialogs such as Phaedo, Republic, Symposium, Parmenides, and Sophist, but Plato nowhere argues for their existence directly and nowhere lays out a formal theory of being. Instead we must garner what little information we can from these sources and cobble together a theory for him. One clue comes from the fact that Plato speaks of these beings as paradeigmata, paradigms or examples, and as eidê, images or forms (the word eidos comes from a Greek verb meaning "to see" or "to look"--the beings literally are the shape, or form, of things, what they "look like"). What this seems to mean to him is that in spite of the fact that things like human beings and chairs and dogs and oak trees undergo many observable changes, there is still something that they continue to be like. A particular human, for example, whether she is tall or short, light or dark, blond or brunette, is a thing that is derived from a kind of blue print, the paradeigma of all human beings, what we may call The Human Itself. All other particular human beings are also derived from this example, and according to Plato it is the example, the paradeigma or eidos, form, that is the genuine being in this picture. The particular human being is not a being in this most important, Platonic sense, but a derivative thing, a shadow or an image of a being.
Plato was not a materialist, so for him it was not puzzling to say that the most real beings of the kosmos are non-material entities that serve as blueprints for the material entities that we observe all around us. Indeed, on his view, the things that we call beings, the particular humans, horses, plants, etc., are in some sense not really beings at all, and that is extremely puzzling to modern sensibilities, hence there are few Platonists out there these days other than among professional philosophers (and perhaps some mathematicians).
Platonism was among the dominant philosophical schools during the early centuries of the Christian era, and as Christians became better educated and started thinking and writing about their religion, they naturally turned to the language of the philosophical schools for their theological language. Indeed, much early Christian theology is clearly influenced by Platonism in its metaphysics and moral theory. This fact is not always congenial to all Christians these days, but it is undeniable, and has come about as close as anything can come to being a proven fact in the scholarship bearing on the Patristic period. In particular, the Greek Fathers borrowed much of the terminology of Neoplatonism (which, in spite of its name, is really an attempt to return to the original form of Platonism of Plato's own school) in their writings, where we find such terms as ta onta, ousia, hupokeimenon, and eidos deployed in pretty much the same way that any ordinary Platonist would deploy them. Some Christians are willing to admit the borrowing of terms from Neoplatonism but deny that Christian metaphysics are in any way Platonic, but that is mostly ideological posturing: the similarities are striking and very real, even though there are also some obvious differences.
Central to any Platonic theory of substance, from Plato through Aristotle to Plotinus and beyond, is the notion of substance as cause, a notion that has disappeared entirely from 21st century materialist theories of substance (to the extent that there are such theories). On Plato's account, a substance causes particular things to be the things that they are: the form of The Human Itself, which is an immaterial entity, stands in a causal relation to all the particular humans out there such as Socrates or Gregory or Thomas. Each of them is human by virtue of "participating" somehow in the Form, having a "share" of it, to use Plato's term (methexis). Plato nowhere explicates this notion sufficiently to suit most philosophers, and I confess I find it rather mysterious. I think Aristotle may have agreed with me, because in his hands the notion of substance as cause is rather straightforwardly explanatory: we can explain a substance by talking about its formal, final, efficient, and material causes, all of which are nothing more than ways of explaining where the substance in question comes from (for example, the "efficient cause" of a substance is simply who or what brought it into existence; the "material cause" is what matter it is made out of, etc.). If I were to categorize myself in this way, I would have to say that I am more of an Aristotelian Platonist than a Platonic Platonist, but perhaps things are confusing enough around here already without getting into that.
One of the most important importations (phew) into Christianity from Neoplatonism is the notion of a hupostasis. This is a rather interesting word, because it is quite close, etymologically, to the Latin word substantia: it comes from the Greek hupo, meaning "under", and stasis, "standing", and yet it was not used by the earliest Platonists to mean "that which stands (or lies) under processes of change". As we have seen, the word hupokeimenon was used for that. Instead it referred to the more classically Platonic idea of that which stands under the outer appearance, that is, it refers to the inner, objective reality of some entity. The use of the term developed over time; in some writers it is used interchangeably with ousia, in others it develops a more technical sense: Person; but there is often disagreement about how best to employ it. John Philoponus, a sixth century Christian philosopher and commentator on Aristotle, famously argued that the word basically means the same thing as "nature" and so it doesn't make any sense to say that the Son, being one Person, also has two natures. (It's worth noting that Philoponus makes this argument by appealing to an ambiguity in the term "nature", but I will not enter into that debate here.)
I can't lay claim to being any kind of expert metaphysician, but I am quite impressed by folks like Jonathan Prejean who have such admirable talents at making these difficult and obscure matters seem clear and obvious, and who, in addition, often do so in the context of apologetics, a valuable ministry in itself.