Ramesh Ponnuru has responded to me at the Corner.
There's been some confusion here, and it's my fault. I've been using "human" when I should be saying "person". So while I don't think that the fetus is some different species than human, I also don't believe that it's a person. (Additionally, I think a case can certainly be made that a developing organism that has not been born, or perhaps is not viable, is not in fact a member of that species in a meaning relevant to our discussion.)
Here's the bigger problem: "The virtue of using conception as the dividing line, in my view, is not so much that it is non-arbitrary as that it is, well, true."
That's certainly been in keeping with the tenor of the comments, where (in a gratifyingly civil conversation) many people have been making conflicting statements about what's just true, or about what's self-evident, etc. That seems to be a recurring theme in our abortion discussion. To some people, abortion just is murder. To others, a fetus just isn't a human person. To some, the fetus just has rights. And on and on. All of these things are not only true, to the people who believe them, but are self-evident. So what Ponnuru finds obviously true I find obviously untrue. The debate over abortion is also marked by people claiming to have found the magic bullet, some definition or evidence-- usually scientific-- that, to them, solves the debate, once and for all. The problem is that these notions always end up begging the question somewhere or other. When someone says "See, we detected these kinds of brain patterns at this fetal age, therefore this baby has a consciousness, therefore you can't abort it after this point," they think all of that flows logically. But someone else says "Brain patterns don't really differentiate consciousness, and anyhow consciousness doesn't mean you have adult rights." And so on. Scientific answers are constantly being proposed to ethical or philosophical question. Sadly, we can't use science to get us out of this mess.
I think the postmodern concept is useful here. First, I think that here we may have arrived at true incommensurability. This divide might not be solved. Second, I think we would do well to jettison notions of truth and instead operate according to principles of pragmatics and use. Without a transcendent authority to tell use where personhood begins, or whether personhood entails an absolute right to not be aborted, we aren't going to get at the truth about whether or not abortion is moral. There is no truth about abortion, only what various people think about the issue, and so we should attempt to craft a pragmatic vision of abortion that is necessarily dependent on appeals to popularity. This is bound to be unsatisfying. No one is particularly moved by appeals to popular consensus-- "slavery was popular, too!"-- but in the end, that's what will rule the day. Not to say you don't argue with the consensus. I'm more sympathetic to late-term abortion than the average person. But I think we should give up on magic bullet answers to the question of abortion. Too many committed and honest people have too many inherently contradictory notions about what is "simply true" for me to have faith in any objective truth about the morality of abortion.