Under current law, women have the right to kill their children in the womb, at any stage of pregnancy and for almost any reason; but in most jurisdictions, if somebody kills her while she's pregnant, the killer can be and sometimes is charged with two homicides. Thus, an action protected as a right for the mother counts as a felony if anybody else does it. (I'm just waiting for a case in which the father of a child is charged with a form of homicide for having assaulted the mother, thus having inadvertently caused the death of their unborn child, while she happened to be on the way to getting an abortion. "News of the Weird," indeed.) Such legal incoherence persists, and is widely accepted, because there is no agreement about the moral status of the unborn child. Some hold, and others deny, that such a child is a person, entitled to that moral respect which persons owe to persons merely as such. That dissensus is why "choice"—for the mother, and only for the mother—is the mantra of those who defend the status quo. In the absence of consensus about the moral status of the unborn child, it is left up to the mother to decide what to do with "her own body"; for it's a lot easier to get political agreement about the extent of her moral autonomy than about her duties, if any, to her child. That doesn't make the resulting legal regime any more coherent, of course; but it does at least highlight a question that admits of an answer. The question is: how can it be rationally decided whether, and if so when, the unborn child is a person entitled to the respect owed to persons merely as such?
As a resource for answering that question, Robert P. George and Christopher Tollefsen have produced a splendid tome entitled Embryo: A Defense of Human Life (Doubleday, 2008). In it, they make a case for the personhood of the embryo that does not rely on any distinctively religious premises, but on the basis of reason alone. That has been done before, to be sure; but I've never seen it done as clearly and convincingly. One can rationally disagree with them, as does William Saletan; but their arguments show that one doesn't need revealed religion to make a reasonable case that the embryo is an individual human being and thus a person. That becomes all the clearer in their reply to Saletan. If such a case can be widely disseminated, it could have enormous political significance in fact as well as in theory.
Notice that I said "reason alone." I did not say "science alone." Science cannot tell us what, if anything, counts as a person or what the rights of persons as such are. The concept of personhood is irreducibly philosophical, even if our understanding of it relies crucially on intuitions and experiences which are pre-philosophical as well as pre-scientific. Science can only give us data that make it reasonable, or not, to count some biological entity as a person. But there is pretty broad agreement among ordinary folk that entities which count as individual human beings, as distinct from mere cells or organs of other human beings, are persons and thus, unlike sperm or egg cells, merit some sort of moral respect. Of course there's not such a clear consensus that innocent persons have an absolute right not to be killed, and that is more of a problem than many pro-lifers seem to realize. But the main intellectual focus appears to be still more basic; for there is as yet no public consensus even on the question whether human embryos are individual human beings. So even though the basic issues are philosophical, debate among the most rigorous protagonists typically hinges on how much scientific support can be mustered for the proposition that human embryos are individual human beings.
That's the question on which George and Tollefson focus much of their book, and their answer is "A lot." It is also on that question that Saletan focuses his critique. I've already provided the link to his review; you can read it for yourself. I believe that George and Tollefson successfully rebut the specifically scientific arguments that Saletan adduces. But I also believe they are making a tactical error. Ironically, their rebuttals of Saletan's arguments also show what the error is.
They show, convincingly enough, that the scientific facts Saletan cites fail to establish that the embryo is not an individual human being. The way they show it is to expose the philosophical problems Saletan gets himself into with his objections. They then provide a cogent argument that Saletan, and by the same token each of us, was once an embryo. If you were once an embryo, then there is spatial-temporal continuity between the embryo you were and the individual human being you are now. That makes it reasonable to hold that the embryo you were was an individual human being. And all that in turn makes it reasonable to hold that the embryo you were was the same person you are now—if you hold that spatio-temporal continuity between biological entities A and B suffices for identity between A and B; if you define 'person' roughly as Boethius did, as "an individual substance of a rational nature"; and if you hold that each and every individual member of the species Homo sapiens is a person in that sense. Now I share those premises with George and Tollefsen, who also summarize philosophical arguments for them. But people who, for philosophical reasons, don't share those premises, aren't going to find the "scientific" arguments of George and Tollefsen persuasive. They might, at the end of the day, be willing to concede that it is reasonable on scientific grounds to hold that human embryos are individual members of the species, but at the same time be unwilling to concede that all such entities are persons meriting the moral respect due to persons merely as such. That is the line long taken by George's Princeton colleague Peter Singer, who sees nothing wrong in principle with infanticide. He's operating with a different conception of personhood, and hence of the moral status of persons, from the classical one on which George and Tollefsen rely and which they defend. Hence, he hardly bothers debating the science of the matter. Admittedly, most "pro-choice" Americans are unwilling to go that far; but that only shows that many of them haven't thought through how far they're willing to go.
That's why it is hardly dispositive, even though true, to say as George and Tollefsen do:
Is the human embryo a whole living member of the species Homo sapiens—a human being—in the earliest stage of his or her natural development? We say yes, that is exactly what a human embryo is; he says no. The question is not metaphysical or religious, but rather scientific.
The science is important for defending a premise necessary for the philosophical argument. The premise is that embryos are individual human beings "in the earliest stage of natural development." One might quibble at the margins about whether science can demonstrate that as opposed to merely making it reasonable to hold it; but either way, the difficulty is that the nub of the argument is philosophical, and the science cannot settle such an issue one way or another. I see the same sort of problem with Intelligent-Design advocacy. The scientific facts cited by ID advocates make it reasonable to believe that the universe is designed to be roughly the way it is; but science can never establish any such belief just by itself, and there's always the lurking possibility that natural facts which are now scientifically inexplicable won't remain so. What then becomes of the argument? It's best to keep it philosophical to begin with.
That said, the strength of the George-Tollefsen approach is that it attends carefully to a question people are inclined to attend to, and shows that pro-lifers have more to gain than lose by that. I just wish they wouldn't give out the impression that that's where the main work lies. They actually do much else of the needed work; but that's not what you're likely to hear about unless you earn your living at this sort of thing.