[This was a response I made in the excellent thread of conversation going on in the thread about Augustine's understanding of Justification. It actually moves over into Thomas, and so strays off topic slightly. However, this same worry has reappeared a few times in conversations I've had through the years, so I thought I would address it here for the inquiring minds.]
“Well I can’t speak to Augustine’s concept of nature (perhaps Gerald could say something). But I think Aquinas is quite right to deny that sin and grace are substantial predicates. But I’m fairly sure this is a confusion caused by the terminology, so I’ll try to explicate what I presume Tommy A is getting at. I apologize if the answer is a bit lengthy, but Thomas is working in an intellectual framework somewhat foreign to our own and it is necessary to present a bit of it to get at the heart of what you are objecting to.
A substance is an answer to the question: “What is this?” Suppose I point to Socrates and ask “What is this?” You respond, “Socrates is a human”. By giving this answer you have predicated the substance ‘humanity’ of Socrates.
But what makes Socrates a human being? According to Aristotle there should be some feature (or set of features, perhaps) which uniquely divide things into a hierarchy of classes and subclasses. Some things are bodies, some things are not bodies. Some bodies are living, some bodies are not. Some living bodies have the power of sensation, some do not. Some sensitive living bodies are rational and some are not. Now, if this collection of properties (rational, sensitive, living, corporeal) uniquely identifies this species of human beings as opposed to all other things in the world and is universally present in each and every human being, then you have found the “essence” of what it means to be a human being. Being a human being just means being a living, sensitive, rational body. Cease to be living and you cease to be a human being.
Now, the difference between ’substantial’ and ‘accidental’ predicates is that substantial predicates predicate the essence, but accidental predicates predicate something else. “Socrates is black” does not tell us about he essence of Socrates which is necessarily true of him. No, Socrates’s color is purely contingent because it has nothing to do with the kind of thing he is. If I point to Socrates and ask “What is this?” it is clear that “Black” is an insufficient answer.
Now, there are all different kinds of accidental predicates. Some accidents are qualities like color, shape, etc. One particular kind of accidents are called “states”. Examples, “Socrates is asleep,” “Socrates is wounded.” What Thomas is asserting is that Grace is a state (Habitus in Latin means “state”). If God gives you grace, then you are in grace. If he doesn’t, then you aren’t. If you think there is something like “being in a state of grace”, then you are already agreeing with what Thomas is articulating here.
But why should the state of grace be an accidental rather than essential property?
Suppose Socrates is a pagan and no pagans are in a state of Grace. I point at Socrates and ask “What is this?” The right answer is “A human being.” Now suppose that God moves Socrates’s will, and Socrates repents and is baptized. Now I point at Socrates and ask “What is this?” Again, the right answer is “A Human being.” If Grace is the essence of a human being, then nobody who lacks grace is really a human being. If, on the other hand, you can recognize that “sinful human beings” and “redeemed human beings” are still both human beings, then Thomas’s position poses no problem. Now, I’ve pushed some Protestants far enough that they would actually say that the unregenerate aren’t really people, but this result strikes me as so ludicrous that they can’t possibly mean it. (And it raises all sorts of troubling ethical and political problems.)
But if “sinful human beings” and “redeemed human beings” are both human beings, then it is clear that being in a state of grace or not is an accidental rather than an essential feature of human being.”