I want to draw somewhat more on the argument from "marginal cases," (AMC) which I discussed in the previous post. In doing so, I hope to make it clear why I use scare quotes. Much of my thinking in this area has been greatly influenced by Will Kymlicka.
When arguing against animal rights, people often claim that a certain number of qualities or capacities which humans alone possess are necessary for the foundation of rights. The AMC urges us to look across the diversity of the human population and realize that any quality or number or qualities would not be possessed by all humans, and thus could not justify "human rights" as conventionally conceived. Thus, the door is opened to considering non-humans as potentially eligible for fundamental moral rights.
There's another way for this argument to work. One could look not just across the diversity of the human population, but also across the diversity of experience in a single human life. It's clear in this case as well that there's a broad spectrum of qualities and capacities that an individual is going to go in and out of possessing, but again, we usually think of "human rights" as applying consistently over an entire person's life, and not co-varying with any particular traits.
One can try to fudge the metaphysics here a bit, and say that the fact that a human life will at some point possess the relevant characteristics, this is enough to justify the possession of rights. I think whatever metaphysical story one tries to tell here will be doomed; if someone is killed as a very young infant, there's no relevant sense in which it's future characteristics are morally relevant (at least not in any way that is different than the future characteristics of someone who is not yet conceived.) But the metaphysics here do get somewhat involved and tricky, which is why philosophers tend to focus on the "marginal cases" of the severely disabled (who could, in fact, never develop the relevant characteristics). The argument is nearly as effective in this limited form, and relies on less controversial premises.
However, because I think I'm right on this point, I dislike the term "marginal cases." If I'm right, everyone is at some point a "marginal" case, which makes them, of course, hardly marginal. I think acknowledging this also puts us in a more understanding relationship with the cognitively disabled, in which we conceive of them not as aberrations, but as individuals who possess a certain range of abilities on a broad spectrum.
After all, even if wanted to use rational agency as the foundation of rights, even typical adults are going to vary in their capacity for rational agency. This would seem to imply that those who are better at being rational agents deserve greater recognition of our rights. And again, it's not intuitive for us to conceive of our rights as varying to correspond to some moral measuring stick in this way.
All of which raises the question of what, in fact, is the necessary condition for the possession of rights. Another way to ask the question, to draw out what I think is the most intuitively plausible answer, is what quality would someone have to possess for us to start considering their interests (such that it could even make sense to protect such interest with a right)? Easily enough, it would seem, the answer is that they ought to at least have interests. Which, in other words, means they must have their own subjective experience of the world, that things can go better or worse for them, that they can feel (something like) pleasure or pain. That is, they must be sentient.
One might object that sentience is just another quality like any other, which will vary like any other, and is vulnerable to all the criticisms I laid out above. That's a great objection! However, I don't think we should have a problem denying any concern for a being that lacks sentience. That means, as I've said, that things cannot go better or worse for that being. There's an important sense in which it's not even right to call such a thing a being at all. And no one could be offended by what I've just said, because in denying rights to those lacking sentience, I'm not talking about anyone at all.
Also, you wouldn't get the problem of some people being more sentient than others, because it doesn't make sense to talk about being more or less sentient than someone else. I am, for example no more sentient than my dog or my mother. Once you pass the threshold of being a sentient being, there's nothing more to say about the matter.
The trickier complication is that there might be beings about whom it doesn't seem right to say they're sentient or that they're not sentient. Perhaps insects or mollusks fall in this group, in principle it's impossible to know. Our concept of sentience seems to be an either/or kind of deal(just like our concept of rights), either you haven't or you don't, but our knowledge of the world and of neurology suggests this is not the case. So I'm not quite sure what we ought to say about these cases on the edge (about which it might in fact be apt to use the term "marginal). Tentatively enough, it seems that if we can avoid using or harming beings that might be on the fringes of sentience, then we ought to do so.