A student at Concord Christian Academy (Concrod, NH) made that claim during the Q&A after the assembly this morning. Given time constraints, I could only give him a brief reply, namely, that newborns lack self-awareness and a conscious desire to go in living, yet it's still wrong to kill them. If time permitted, I would have included the problem of the indoctrinated slave.
Briefly, as Patrick Lee and Frank Beckwith point out, slaves can be conditioned to believe they have no interests and no desire for freedom or life, but they still have a right to these things even if they don't presently desire them. “Even if the slave is never killed,” writes Beckwith, “we would still think that he has been harmed precisely because his desires and interest have been obstructed from coming into fruition.” Why can’t the same be said for the human fetus?
True, abortion-choice philosopher David Boonin could reply that the slave’s ideal desire is for life and freedom, but as Beckwith point out, that judgment “seems to assume that the slave is a being of a certain sort" that ought to desire a right to life and freedom even when he does not actually desire them. However, if that is true, it is not desire that grounds the right to life, but the nature of the slave who would have this correct desire if he had not been indoctrinated. In short, the substance view of human persons can explain why the indoctrinated slave has rights even when he doesn’t desire them. Boonin’s own view can’t.
To make sure the point is not lost, Beckwith provides a final example. Suppose one of these indoctrinated slaves is pregnant. Because you agree with Boonin that having desires grounds a right to life, you hire a physician to alter the brain development of the slave’s fetus so that it never develops organized cortical activity and thus never desires life or freedom. Can Boonin reasonably say this is wrong? If rights presuppose desires and desires presuppose organized cortical brain activity, then Boonin’s criterion cannot account for the wrong done to the fetus in this case. Nor would it be wrong for that same scientist to purposefully create human clones who never develop organized cortical brain activity (and thus never experience desires) so that their body parts can be harvested for medical research.
Given the above examples, it’s clear rights cannot rest on desires but instead are grounded in our common human nature. As Lee points out, “it seems more reasonable to hold that the violation of someone’s rights is more closely connected with what truly harms the individual rather than with what he or she desires.” That is, what truly harms the slave is not that he consciously “desires” liberty but is deprived of it, but that rights he has by nature (in virtue of the kind of thing he is) are denied him whether he desires those rights or not.
For more, see my advanced pro-life apologetics notes here.