The philosophical notion of ‘person’ is not simply equivalent to ‘human being.’ Persons, for philosophers, are those entities that should have rights and responsibilities under the law–so, currently, this might exclusively be human beings, but it could some day include space aliens or self-conscious robots or perhaps genetically modified animals. I mention this up front for those unfamiliar with the philosophical use of the term, so there’s no confusion, and so it’s understood that I’m not suggesting we treat the severely brain-damaged in any but the most caring possible way.
Suppose you have a severely brain-damaged child who never develops language, motor control, etc. Basically, a child born in a vegetative state. There are questions about the person-status of such beings. It’s been pointed out that many animals are more intelligent, more conscious, etc., than such a child. This child meets almost none of the standard criteria for personhood
Marya Schechtman holds that such a child enters “person-space” and is treated as a person, and therefore acquires personhood, because we expect of human beings that they will develop into persons*. “Person-space,” is the institutional and practical space inhabited by humans. An animal might burrow into our house, but it isn’t in person-space, because we don’t clothe it, name it, give it a bed, care for it, and do all the things we do for infants.
However, if we bring a dog home, we might just name it, clothe it, give it a bed, etc. I’m currently a little unclear on why, in Schechtman’s account, such a dog isn’t a person, since it has entered person-space in being treated as a person and receiving the trappings of personhood. One answer she gives is that it is expected that a human child will acquire the characteristics of person; that is, reflective consciousness, language, etc, whereas there are no such expectations for a dog. However, I don’t imagine there are such expectations for the severely brain damaged child, so I find this a little unsatisfying, but I’m open to there being a good explanation of the difference that I haven’t understood or which is forthcoming in future writings.
But Schechtman makes a strong point: there are two parts to personhood: one is having a place in person-space, and the other is the internal set of capacities for personhood (consciousness, language, etc.) Obviously, no one would develop language and most of the other person-characteristics without person-space. Nor would there be person-space without entities that had these characteristics.
But why should the very brain-damaged human child be a person, whereas a chimp that is raised in a house, given clothes, a name, a bed, and maybe can even sign a few words is not granted personhood? The chimp seems to have both (a) entered person space and (b) be in possession of at least some, and certainly more, of the characteristics of a person than the vegetative child.
So a thought experiment: suppose that the severely brain-damaged child is tested at, say, age 9, and it is found out that this is not a human child. It looks like a human, but when tested genetically, it is found to be a non-human. We can imagine this is like Davidson’s “swamp man,” except this is a non-thinking creature, a sort of human-shaped plant or fungus with no mental function.
The fungus been accepted into person space without person-characteristics on the assumption that it’s a human being, and human beings are expected to develop into persons. But this is a fungus; if we knew that, we might never have treated it as a person, given it a space in person-space, dressed it, etc.
Upon discovering this, I don’t know what the parents would do. Would they continue to love and care for the fungus-child? Would they suddenly reject it? I can certainly imagine that either might be the case, and that different people would respond differently to the same circumstance. I think what this indicates is that treating the severely brain-damaged child as a person is simply a convention (and perhaps a very good one!), and not a reflection of some intrinsic quality. If we simply decide that all redheads are non-persons, it’s obvious we’re making a mistake: their personhood will impinge upon us as they lobby for rights, engage in speech acts, organize socially, etc. Not so with the child in the vegetative state.
I think we can have all kinds of positive attitudes towards the child because it’s our child, because the parent has an attachment to offspring, because it looks like us, came from us, needs care, etc, and that none of this necessitates granting personhood to the non-conscious/vegetative infant. I’m not exactly sure what’s gained by saying that the severely brain-damaged child is a person whereas a chimp raised in a home is not. I think this may just reflect the fact that we, human beings, use “person” to mean “human being.” But the philosophical term “person” is obviously not meant to have the same admission criteria as “human being.” It may well be that the terms are co-extensive, but if their definitions aren’t distinct then the philosophical term isn’t doing much work.
One assessment I might add is the following: if there are two paths to personhood, as per Schechtman, one being admission into person-space (this is somewhat like, but much richer than, Dennett’s “stance” criterion), the other being that one possesses person-characteristics like reflective consciousness, empathy, language, second-order volitions, etc. (making this latter list has been the task of most philosophers of personhood, and I can leave it vague), then these might correspond to two aspects of the forensic concept of a person. Whatever criteria we use for personhood, once we’ve established that an entity is a person we grant that entity certain rights, and also certain responsibilities. It might be that entry into person-space is our basis for judging the rights (we cannot harm the severely brain-damaged child, the child must be fed and cared for, etc.) and having the person-capacities is the basis for responsibilities (we certainly aren’t going to hold the severely brain-damaged child culpable for any crime, for example). So we have a social component, person-space, which is rights-granting, and an individual component, person-capacities, which is responsibility-producing. In fact, if we think about our person-criteria in terms of capacities, these are mostly capacities that allow us to assess guilt or distribute praise or blame.
*Marya Schechtman, “Personhood and the Practical,” Theoretical Medicine and Bioethics, 31:271-283, 2010
Popular Metaphysics 
I think there’s one possible criterion that you don’t consider: it could be that having the potential to develop person-characteristics *permanently* qualifies an entity(/organism/object/whatever) for personhood, even if that potential is later removed.
At some point in its development, even if only in utero, your brain-damaged child had the potential to develop a full set of person characteristics, and that can’t be said of the chimp. Even if that potential is now gone because of the brain damage, it’s still a relevant fact about the child.
I think that’s a reasonable criterion, but it’s clearly not the operative criterion, that is, that’s not actually why people treat extremely brain-damaged children as people.
Also, you have to throw in some sort of clause to note that, indeed, a mutant chimp could in fact develop person qualities but we just never expect such a thing to happen, and further, that a damaged sperm or egg carrying certain genetic defects already is ruled out of personhood, so that the potential is never present even in the zygote formed by such gametes, and yet a human child born from such a pairing would still be treated as a person.
Anyway, these may be interesting thought-cases, but I’m sort of more interested in why we actually treat the extremely brain-damaged child as a person. I think it has more to do with this being our child, coming from us, and because we see that other parents have children that become persons, and that human children are generally expected to become persons (this is basically what Marya Schechtman says…I’m maybe coming closer to her viewpoint here.) Still, I think this is a convention, and there are indeed historical examples of peoples who didn’t necessarily expect their offspring to become persons, and would leave damaged newborns to die of exposure. In those case, the child never entered person-space, and was not given person-status, and this was because there was no expectation of them becoming persons.
Well, I think you established in this post that there are multiple operative criteria; I think mine could be one of them, or at least it’s not clear to me why it couldn’t. But I agree it’s not a major one. If I had to pick just one, I’d go with the “coming from us” part. I’m sure you know the exercise in linguistic philosophy (I don’t know where it comes from exactly) in which it proves surprisingly difficult to define a simple thing like a dog; you start out by saying it has four legs and fur, but then so do other animals of course …OK, four legs and fur and it barks …but then what about a mute dog, isn’t it still a dog? What if it lost a leg? …and so on. In the end the only real answer is the recursive one, that it’s a dog because its parents were dogs. Maybe the same thing is going on here.
O, absolutely, I think your criterion is perfectly good, I just wanted to point out some other options that are available, and other ways to conceive of the problem that don’t demand that we bestow personhood on all humans.