Once again, to see the force of this strategy, imagine two people on a sidewalk debating the issue. If someone told you that the two debaters had come to agree that the fetus is not a person, you’d probably form the belief that the “pro-choice” side had won; likely, you’d conclude that the “pro-lifer” had been convinced.
But Marquis’ argument doesn’t rely on the fetus being a person.
Here it is:
(1) What makes killing someone wrong, in most respects, is it deprives them of a future of value.
(2) When a fetus is killed, it suffers the same kind of loss.
(3) Abortion is immoral just as killing an adult or a child is immoral.
As Marquis puts it:
When I am killed…I am deprived of all the value of my future. Inflicting this loss on me is ultimately what makes killing me wrong. This being the case, it would seem that what makes killing any adult human being prima facie seriously wrong is the loss of his or her future. …
The future of a standard fetus includes a set of experiences, projects, activities, and such which are identical with the futures of adult human beings and are identical with the futures of young children. Since the reason that is sufficient to explain why it is wrong to kill human beings after the time of birth is a reason that also applies to fetuses, it follows that abortion is prima facie seriously morally wrong.
Notice that Marquis’ argument doesn’t rely on the fetus being a person. Marquis is in essence shoving the question of personhood aside and looking strictly at what it is that makes killing someone wrong.
If it turned out that what makes killing someone wrong crucially relies on personhood, then Marquis wouldn’t have an argument against abortion (using the assumptions he’s relied on). But the wrong-making feature of killing that he’s identified is something adults, children, and fetuses share when each of them are killed. So if the feature that makes killing someone from the first two groups wrong is that it deprives them of a future of value, then it also furnishes us with an argument against abortion, since the fetus, like the child and the adult, has a future of value.
(Interestingly, Marquis’ argument does not provide grounds for seeing euthanasia as wrong, given that in many cases the candidate for euthanasia does not have a future of value.)
Notice that Marquis’ argument is not vulnerable to the familiar “pro-choice” lament that anti-abortionists are “giving full rights to a potential person” or anything like that. The question of personhood is irrelevant.
Marquis’ argument relies on a theory about what makes killing someone wrong, and then noticing that the same effects occur when a fetus is killed — the fetus, like the adult, is wrongly stripped of a future of value. It’s not like, when an adult is killed, someone can plausibly respond “Yes, but, they only had this ‘future of value’ potentially — so there’s no wrongdoing here.” No one would accept this reasoning. That’s because, as Marquis notes, we see this future of value as something an adult possesses in the present. That’s precisely why we’re so scandalized when someone is killed — they are robbed of something — the most precious thing — they possess: their future of value.
That’s what makes abortion seriously immoral."