For this reason, a popular way to go about giving a substantial definition of personhood is to identify which entities one is already directly morally concerned about, and find any combination of criteria which kick out that same list and no more.
If this approach sounds scandalously circular, let me point out that it can still fail! It may very well be the case that one's intuitive feelings of 'this is a person' and 'that is not a person' can't be neatly justified by a set of principles, even if any consistent set of principles will do (beyond sheer intuition affirmation). So even this very ad hoc exercise can shake up intuitions and motivate one to accept a reformed definition of personhood, which is what happened to me.
A Proposed Personhood
Rather than work up to my own take on personhood, I'll present it and then defend it.
A person is an entity which has had conscious experience and desires, and is still capable of conscious experience and desires.Or, a shortened version which I'm stipulating as taking the same meaning as the first version:
A person is an entity which has been sentient, and is still capable of sentience.In catchier form:
Personhood begins with sentience; personhood ends with the loss of the capacity for sentience.Hopefully no one will claim this overall criterion is completely in the wrong neighborhood. I'm not calling the Grand Canyon a person or questioning the personhood of blog readers. But I do expect most will think it too exclusive or too inclusive about some important classes of entities.
The Basic Justification
By 'conscious experience' I mean having a first-person point of view. Inner experience. The thing David Chalmers tries to draw out as the 'hard problem' of consciousness.1 And by 'desires' I include pain and happiness, along with the notions of fulfillment, frustration, well-being, love, wishing, preferring, etc. Having any of these counts as having some desires. By putting conscious experience together with desires, we are talking about an entity to whom things matter. And entities to whom things matter are the proper targets of direct moral concern.
Though I would quibble with his vocabulary and intended scope, I think Kant was on a parallel track when he wrote:
Beings whose existence depends not on our will but on nature's, have nevertheless, if they are irrational beings, only a relative value as means, and are therefore called things; rational beings, on the contrary, are called persons, because their very nature points them out as ends in themselves, that is as something which must not be used merely as means, and so far therefore restricts freedom of action (and is an object of respect). These, therefore, are not merely subjective ends whose existence has a worth for us as an effect of our action, but objective ends, that is, things whose existence is an end in itself;2 (emphasis added)Essentially, there is an all-important difference between making use of non-sentient entities to fulfill our desires, and using sentient entities to fulfill our desires. What is morality if it isn't taking the desires of others — or others with desires — into account?
'Too Limited!' Objections
It doesn't count sufficiently brain-damaged human beings!
Suppose a human's brain were entirely removed and destroyed, and the rest of the body kept alive by advanced, non-sentient machines. Would you count what's left as a person?
Now, there may be important questions about whether a given patient has lost all capacity for sentience. It may be ethically sound to play it safe when we're not sure, but extreme cases like total brain removal and destruction demonstrate the principle.
It doesn't count early stages of unborn human life!
That's correct, because we have good reason to believe an embryo has never had a first-person inner experience. It is partially analogous to the former example of a brainless adult body, except that it will likely start a new sentient existence in the future.
What about temporarily unconscious humans?
The above definition covers humans in dreamless sleep or in a cryogenic state. The difference between killing an embryo and killing a sleeping adult is, respectively, that of preventing a personal life from ever starting and that of permanently ending a personal life.
'Not Limited Enough!' Objections
It counts non-humans!
If you believe dogs have a first-person experience and can suffer when hurt, it shouldn't be difficult to accept that how we treat dogs is morally relevant for this very reason. Does a lobster feel pain like we do when it is boiled alive? I don't know, but I hope the answer would make a difference to you.
Also, peaceful aliens aren't going to land and share their secrets if we maintain a humans-only attitude like that!
It counts later stages of unborn human life!
That's also correct, because we have good reason to believe that from (roughly) 20 weeks and on, a developing human has first-person consciousness and the ability to suffer.
It counts sentient beings who lack rational thought, or moral sense, or awareness of self identity, or the concept of time, or language, etc!
These are all popular personhood criteria, which can be intuitively rejected in single move. Imagine an adult human who lacks any one of these by birth or by accident, but is still sentient. Are you willing to write off this entity as a person?
What About Metaethics?
This personhood definition should be compatible with a variety of metaethical views, including but not limited to my own.
Whatever your deep view on the nature of morality, it comes down to whether you think it matters how you treat fellow sentient beings. Wait; scratch that. It matters regardless of what you think because it matters to them.
1. See http://consc.net/papers/facing.html
2. From Fundamental Principles of the Metaphysics of Morals.