The viability criterion is significant in that it's the basis of current federal case law on abortion since Roe v. Wade, but Vocab correctly notes that viability changes with the availability of technology, and that doesn't seem like a feature that should be relevant to whether one is a person. On the other hand, it is relevant to the notion of dependence--pre-viability is a time where, if you do grant that a fetus is a person, it's a person that is dependent for its existence upon another person. This raises questions of when it is morally permissible for a person upon whom another is dependent for their life to sever that dependence. Judith Jarvis Thomson's argument on abortion, which I referred to earlier in my response to part 1 of Vocab's essay, presents the following scenario:
You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist's circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. The director of the hospital now tells you, "Look, we're sorry the Society of Music Lovers did this to you--we would never have permitted it if we had known. But still, they did it, and the violinist is now plugged into you. To unplug you would be to kill him. But never mind, it's only for nine months. By then he will have recovered from his ailment, and can safely be unplugged from you." Is it morally incumbent on you to accede to this situation? No doubt it would be very nice of you if you did, a great kindness. But do you have to accede to it? What if it were not nine months, but nine years? Or longer still? What if the director of the hospital says. "Tough luck. I agree. but now you've got to stay in bed, with the violinist plugged into you, for the rest of your life. Because remember this. All persons have a right to life, and violinists are persons. Granted you have a right to decide what happens in and to your body, but a person's right to life outweighs your right to decide what happens in and to your body. So you cannot ever be unplugged from him."My intuition is that in this scenario, it is morally supererogatory to remain connected to the violinist--it is not a moral requirement. The problem with this scenario is that it isn't quite analogous to pregnancy except in case of rape. If one gave voluntary consent to be connected to the violinist to save his life, it seems that one would have a moral duty to see it through. That raises the question of what constitutes "voluntary consent" with respect to pregnancy, which may occur accidentally or unintentionally despite use of contraception, for example. And note again that this scenario only applies in the case where personhood is taken as given, which I've been arguing is definitely not the case in early stages of a pregnancy.
The argument from wantedness, like the argument from viability, doesn't appear to be offer a criterion of personhood, but it is of course relevant to the overall abortion debate. Bringing into being persons who are not wanted and aren't going to be cared for is something that should be avoided, since the odds are not good for children in such circumstances. A controversial argument in Steven Levitt and Stephen Dubner's book Freakonomics is that there's a correlation between abortion rates and declining crime rates--i.e., the authors argued that a consequence of the unavailability of abortion is more unwanted children who become criminals. If that argument is correct (and I personally wouldn't bet on it), that's a form of evidence in favor of the availability of legal abortion, though I don't think it trumps a personhood argument. [NOTE (added Nov. 24, 2012): Levitt and Dubner's argument is thoroughly debunked in chapter 3 of Steven Pinker's The Better Angels of Our Nature: Why Violence Has Declined (pp. 119-121). Freakonomics in general is found to be filled with errors in a review in the American Scientist by Andrew Gelman and Kaiser Fung.]
Vocab quotes from a book by abortion doctor Suzanne Poppema about her own abortion, in which she says to her embryo, "I’m very sorry that this is happening to you but there’s just no way that you can come into existence right now." He identifies this as "confused logic," since clearly the embryo already exists. I agree with Vocab that she has written this statement in an apparently confused way, but it could be made coherent if she had written of the embryo developing into a person or of a person coming into existence, which is probably what she meant to imply.
Continue to part five.