Instead of parsing everything each of us has to say, how about offering your definitive definitions.
Yes. And all are essentially the same. They mark the beginning point of an individual with the real potential to become an adult human being, not a secondary capability or a theoretical potential.
Instead of parsing everything each of us has to say, how about offering your definitive definitions.
I'm simply trying to make everyone's arguments stronger. If a well-meaning but ill-equipped pro-lifer gets into a debate with a pro-abortion opponent, they could lose an argument and the opinions of anyone else who was paying attention. The pro-life side generally has its heart in the right place but often makes arguments that are some variation on, "That the unborn are people is self-evident, why don't you just admit it?" That doesn't win debates. You need to understand why that is so and why other people disagree.
My own criteria? As background, any reasonable criteria for personhood must be applicable to situations beyond the abortion debate and be somewhat compatible with people's intutive judgements concerning both real and fictional situations. The fact that people would think of Scooby Doo as a "person" and might think of killing Scooby Doo, if he were real, as "murder" is important to consider because situations like this (and the Star Trek example I suggested earlier) allow us to more clearly see how we migth apply various criteria used for the unborn to adults if they were in a similar state of dependency, motionlessness, or unconsciousness.
That said, I think you've got two rational choices for picking a criteria that will include human adults and exclude animals, yet doesn't exclude potential sentient aliens (or fictional beings like Scooby Doo that people would naturally think of as "persons" if they were real) or necessarily include the brain dead (though it could), include corpses, or include human cells that are clearly not human individuals. The criteria that clearly seperates humans from animals and Bambi from a real dear is a human-like awareness or "sentience". Your two choices are to pick (A) when that sentience is actually present or (B) when that sentience can be present in the future.
The first point to consider is that future capabilities are an integral part of how we judge events. Murder is wrong, not because of the pain it causes but because it is irreversible. It also robs a person of their future. This becomes clear if you think about various types of killings. If one could resurrect a dead person, is their any doubt that their murder would not be any more terrible than the sum of the non-fatal effects of the murder (pain, loss of freedom or time, etc.)? And if we didn't consider the future potential of a person and what was taken from them, wouldn't we look upon the old people dying every day in nursing homes as being just as tragic as the deaths of young people who die in car accidents or plane crashes? So it seems clear to me that what you take from a person matters which means that what they have ahead of them matters. In some ways, it matters more than their current condition (one might find it understandable to put a person who was dying painfully out of their misery but would probably not find it understandable to put another person, suffering equal pain but who was not going to die, out of their misery).
All that said, the problem with picking actual capability (A) is clear. It excludes infants (for those who think that humans are adequately mentally distinguished from animals before the age of about 2, I suggest this paper on the language capabilities of humans and chimps -- it discusses the inadequacy of chimpanzee language capabilities but also touches on the inadequacies of early human language capabilities in comparison) and it also excludes people who could temporarily lose their sentience but could recover it. Neither of these fits well with how people commonly react to either of these situations.
The better criteria is to look at the future potential of an organism and judge its present worth by what it can become, if simply given a reasonable chance (sufficient nutrition and a non-fatal environment), which is what we are expected to provide for children until they become more self-sufficient. This criteria recognizes infants as "persons" and explains why "perminent" is an important feature of the "brain dead" criteria for clinical death. It also addresses a whole host of hypothetical fantasy situations in a satisfactory way (e.g., my earlier Star Trek example). Once this criteria is chosen, it logically follows all the way back to the point where the individual became independent and reasonably capible of becoming a sentient adult. That could be either fertilization or cloning. In the case of twins, I see no paradox with one individual becoming two. Simply look at science fiction examples of replication machines is you need an adult example of one person becoming two at some intermediate point to wrap your mind around.
Now, you can (and people do) try to draw some other line based on some criteria to determine personhood, somewhere between fertilization and birth. For those criteria, I have two simple questions. "What does this criteria have to do with personhood?" and "If I apply this criteria to animals, aliens, or hypothetical examples, will it produce resonable results?" Suppose you pick a heartbeat, or neural activity, or myelination, or quickening or whatever. Animals may meet all of those criteria yet they will never be persons. And none of those criteria address the root criteria of sentience or real potential (that a fertilized egg doesn't implant does not address its real individual potential up to that point any more than an abandoned baby's failure to live addresses its real individual potential up to that point). And if you want to argue that any of these criteria only matters in the context of the developing human, it is easy enough to isolate them out with hypothetical imaginary examples using adults which demonstrates their weakness as criteria.
You'll note that I purposely avoid discussion of "souls" as a critera. If someone wants to raise that criteria, I need them to tell me what a "soul" is and how I can tell if an organism has one or not. If they can't do that, then the definition is essentially subjective. Yes, it may be reasonable to think about souls or worry about them but if someone can't demonstrate them and how to determine of they are present or not, they are a wholly worthless criteria for an objective discussion.
That's not to say that I don't have any respect for religious beliefs in this matter. I do. But if that's the basis for your pro or anti-abortion beliefs, then you first have to explain your religious views to me and convince me that they are correct before you even bother to start in with the abortion issue. Otherwise, dismissing religious abortion arguments is as simple as saying, "I don't believe it. Prove it."