Chris gave an interesting response to my posts on personhood, and I would like to respond in turn.
As Chris notes, the dispute is not the sort of dispute that can be decided by empirical facts. In fact, it is a dispute over how we should regard the empirical facts (any empirical facts that might be put forward) in the first place. I would go further and point out that the dispute is not an ethical one either. When we have decided that X is a person, we have not provided an answer to the question of what our moral responsibilities to X may be. It is clear, for instance, that our moral responsibilities are not the same for every person we encounter; they vary from person to person, as they must if they are to keep the person in mind. It is also clear that when we have decided that X is not a person, we have not provided an answer to the question of what our moral responsibilities to X may be.
The reason personhood is a concept relevant to ethics is that, despite providing no answers of itself and on its own, it turns out to be immensely useful. Ethical questions are immensely difficult; to reason about them we need a way to set them in order. In a sense, this is true of every discipline. Every discipline has its topoi, its loci communes, concepts and maxims that help to simplify, to guide, to order. This is true of ethics, as well; but the loci communes are especially important in ethics, because ethics deals with so many contingent factors. Person, in the course of its historical existence in theological, metaphysical, and moral disputes, has gathered about itself a wide array of commonplaces that help us to simplify moral reasoning, diagnose moral situations, and put forward general guidelines. When new situations arise, we can take them as a template and modify them in the relevant ways.
If I understand the position elaborated by Chris aright, it is actually very similar to my own. On my view, while we call human beings 'organisms' and 'persons' for different reasons, all human organisms are persons. Caleb in one of the comments to one of my posts asked where I set the cut-off point, and the answer is this: I set it where the whole thing breaks down, where it simply becomes impossible to talk about this as human without equivocation. This is a strong view; its advantage is that it simplifies the background. With some exceptions, the view Chris articulates will cover much the same ground. I think this is required if the higher-brain death position is to avoid becoming a humunculus theory, in which the person is just part of an organism and never the organism itself.
It's a view that has considerable plausibility. But we need to look not merely at initial plausibility; we need to see how it affects our moral reasoning. After all, the primary reason that the issue of personhood is relevant to cases of higher-brain death in the first place is that the notion of a person has forensic qualities. It conjures up a topics, in the Aristotelian sense of the term: a set of concepts and principles that make argument of a certain sort possible. And so we can ask, "What does this view do to our topics for the relevant moral issues?" And the only answer I can see is that it annihilates them.
To be sure, it does not, strictly speaking, leave us with nothing. What the position does is give us a set whose members are all human organisms but not human persons. Our personal commonplaces must be set aside. And what we are left with is very murky. What we have in this case, understood in this way, are human beings (in a straightforward sense) that are alive (again, in a straightforward sense of the term) but are not persons. We have no topics for this. We do not know what our responsibilities to such a thing should be, any more than we know what our responsibilities to a manticore or chimera should be. Our personal commonplaces are set aside. We cannot apply our commonplaces about corpses, despite the oxymorons I've read (e.g., "animate corpses"); if we were dealing with corpses, properly speaking, no issues would arise, because the sort of questions we have to ask about those who have experienced higher-brain death cannot arise in the case of corpses. You do not fret about whether a corpse should die; if you can rationally do so, you are not dealing with a corpse. We cannot simply apply the commonplaces we use to guide moral reasoning about animals - first, because our present commonplaces in that area are minimal, to say the least, and don't bear much stretching; second, because they do not take into account the human factor; third, because they do not take into account the fact that this organism was a person. Each of these seems relevant; and even if they turn out not to be, this can't be assumed a priori. So there is only one thing that seems available to us: to go back through our entire moral reasoning about human beings and, bit by bit, pry apart those aspects of it that consider the human being qua human organism and those that consider the human being qua person.
In other words: this move does not, as it is, help us to understand what we should do, or are morally allowed to do, in the case of those who have undergone higher-brain death. It does not simplify; it complicates. It does not point the way to answers nearby; it requires us to say that there are no such answers - we have to go back to the beginning and work it through from scratch. This is one reason I don't understand the complacency of those who usually put forward this view (in one form or another). Their approach should result not in complacency but in a mad scramble: we are faced with an entire field, right here, right now, with regard to which we are morally in the dark. Everything is up in the air, and the morally conscientious person has to make a mad dash to think things out from the beginning.
I have seen no such recognition of the moral responsibility the position brings; the position is often put forward as if it answered the ethical questions, when in fact it merely introduces harder questions. But this additional set of complications might end up being worthwhile if we had reason to believe it were the way of moral progress. I see no reason to think that it is. Perhaps a constrasting example might help.
Some people have proposed that we need to expand our concept of personhood to include animals other than human beings. Now, this actually requires quite a considerable revision of massive areas of our moral reasoning. But these proponents have in their favor the fact that their way is a plausible line of moral progress. Obviously our personal topics does not apply straightforwardly to nonhuman animals; but there is no need for it to do so. Just as, when people stretched the concept of personhood so that it included God and angel and man, no one was committed to the view that each of these is a person in exactly the same way; so no one is committed on this view that animals are persons, as we understand the term now, in a straightforward way. What it does, however, is hold open to us the possibility that we can begin to be more morally responsible, more compassionate, more just, by starting to look at ways in which our personal commonplaces can be extended and adapted to the case of other animals. One is reminded of the early Franciscans - Francis and the birds, Anthony and the fishes. One is reminded of Buddhist ethics. And so forth. There is promise here. But I see none of this in the case of this qualification.
The higher-brain death view, then, appears to do no real rational work: it furthers no general principle, simplifies no process of reasoning, clarifies no actual situation. In fact, over a particular area it complicates and obscures everything. Overcoming this requires a type of rational inquiry in which most of its proponents show no interest whatsoever. It exhibits no marks that by taking it seriously we might become more just, more compassionate, or more responsible than we currently can be. In light of all this, I would need to see an excellent rational justification for it before I could in good conscience regard it as anything other than what it seems to me on the surface to be: rationally dubious and potentially dangerous gerrymandering. Such is my view, anyway.