Call in the philosophers
Not on Faith Alone - New York Times

Most Americans...believe that embryonic stem cell research may provide cures. They will demand that Congress act to realize that potential. If the president vetoes a bill that advances that potential, he will have to provide more than sincere religiosity to prove that human life exists as early as fertilization...The best way to test that proposition would be to employ a panel of respected scientists, humanists and religious leaders to consider testimony from bioscience experts describing when consciousness first appears, when viability outside the womb usually occurs, and how other religions treat the subject.

Does Bush seriously believe that personhood begins at conception or that stem cells have a serious right to life? I doubt it. The idea that it does is wrapped up with esoteric Aristotelian notions of "potentiality" that aren't on his radar.

The commonsense view is probably that personhood insofar as it entails a serious right to life is a matter of degree, and depends on characteristics like sentience and consciousness. There is my lab lying on the rug, wimpering and twitching as he dreams about retrieving ducks: he clearly has a serious right to life and so does any animal whose mental life is at least comparable.

Given our intuitive sliding scale I think all mammals and at least some birds, infants and late term fetuses have a serious right to life. By that intuitive criterion stem cells are out of the ballpark. Human/non-human is not the relevant cut: the intuitive distinction, and it's a fuzzy one, is between beings that are conscious and those that aren't, whatever consciousness comes to--and it is a matter of degree. That's why most Americans don't go along with the administration's line on either stem cell research or the Terri Schiavo case

Once again, the dogmatists on both sides are shooting themselves in their respective feet. No one, after watching pop-science documentaries about fetal development, seriously believes that birth is a morally significant threshhold: by digging in their heals on that pro-choice activists alienate potential supporters. No one seriously believes that stem cells have a serious right to life either and conservatives are now alienating potential supporters by insisting that they do.

So let's call in the philosophers to get clear about this. There's no philosophical consensus about personhood--least of all about this intuitive mini-theory I sketched. But on one thing we do agree, viz. that the less you claim the more plausible your position is. If you are seriously interested in supporting the interests of late-term fetuses you don't attach amendments about the rights of stem-cells and if you are seriously interested in protecting women's right to choose you don't make partial-birth abortion part of the package.