Don Marquis's argument against abortion (in "Why Abortion is Immoral") is pleasingly simple. He says the wrongness of killing one of us (folks like you and me who are obvious members of the "moral community") is due to the fact that killing deprives an individual of their entire future, a valuable "Future Like Ours" (FLO) in most cases. But abortion deprives a fetus of a FLO as well. So abortion is presumptively wrong, for the same reason any other killing is presumptively wrong.
What should we say? Since Marquis first made this argument in the 1980s, it's spawned a vast literature. I'm certainly not going to saying anything new and different here. I'm just going to think aloud a bit about how I would respond.
First, a response I would not make. Let's call this "Personal Identity Fancy Footwork" (PIFF!). What you might say is that an entity with my future didn't start to exist when I was conceived, but quite a bit later. Perhaps that entity (J, for short) came into being very late in my mother's pregnancy, or after she had given birth. That, afterall, is when a thing with my conscious mentality came into being. Nothing with my conscious mentality existed at conception, or a month into my gestation, or three months into it. Had my mother contemplated abortion when she was one month pregnant, she would have destroyed F ("my" fetus--so to speak) but not J. J didn't exist yet. F didn't have J's future, if F was not the same entity as J. Abortion certainly would have taken a future from F, but that was a non-conscious future with no special value. F was merely deprived of the next couple of months, months before F gave way to J.
PIFF advocates will have to answer someone who says, understandably enough, "If you take a future from F, then J's future is gone too, right?" The PIFF-er has a coherent response: F exists to be deprived of its future. But once F is gone, because of an abortion, J never comes into existence. So J never exists to be deprived of her future. After an abortion, J will be a merely possible entity. Doing things that eliminate merely possible entities and their futures can't be at all like killing. Otherwise, we're all murderers for not doing our utmost to conceive as many children as we possibly can. If F is non-identical to J, surely that does undermine Marquis's indictment of abortion.
I think not. I lean strongly to the view that F is identical to J. So destroying F does take a FLO away from F. Of course it's true that during the course of gestation and growth, F will come to have new properties (like being conscious, feeling pain, having preferences). And I think some of these properties are morally important. But I'm not prepared to say any of these new properties are "existentially" important--that they make for the end of one entity and the beginning of another. The idea that a new entity pops into being once consciousness emerges strikes me as a vestige of dualism. But that's a long story....
Suppose you agree that F is identical to J. You could avail yourself of identity considerations of another kind to construct a reductio ad absurdum of Marquis's argument. You could argue that F is identical to J, but F is also identical to an entity that exists prior to conception. F, you might say, is identical to the egg (E) that's fertilized at conception. If you can make a case for that, then you'll wind up saying that killing F is as bad as killing J; but also having to say (absurdly) that preventing E from being fertilized is as bad as killing F or J. Women who avoid intercourse during ovulation will have to be equated with murderers!
Is that the best available response to Marquis? I think the identity considerations underlying that response are also implausible. No, E is not identical to F. Again, long story.
A TALE OF FOUR BOOKSTORES
The reponse I'm inclined to give to Marquis grants that a new entity comes into existence at conception (or shortly thereafter--there are good reasons to date this just a little later than conception) and continues to exist throughout pregnancy, after birth, and into childhood and adulthood. Abortion therefore does deprive an entity of its whole future--a FLO, if it's going to be a typical future.
What I think we should balk at is the notion that killing is wrong purely because it deprives an entity of its future. No, it's wrong because it deprives an entity of a future to which it is entitled. I don't think you can speak of entitlement in the case of a one-month-old fetus. Just because a fetus is innocent or guiltless, it does not follow that it is entitled to anything.
It's easy to see that it's not always wrong to cause an entity to be deprived of something valuable, but rather wrong only when the entity is entitled to that thing. Take, for example, this tale of four bookstores. Once we had a store called Bookstop in Dallas, but Borders came to town, and people preferred Borders. Bookstop's owners were no doubt deprived of a huge amount of wealth and pleasure. Then Barnes and Noble came to town and all the Borders stores eventually closed. Poor Borders owners! They were also deprived of a huge amount of wealth and pleasure. Now Amazon is making the local Barnes and Noble stores look awfully empty. B&N's owners may be on the verge of being deprived. The winners in these business competitions did nothing wrong. They caused deprivations, but didn't cause weaker businesses to be deprived of anything they were entitled to.
Likewise, if Mary flirts with Margo's boyfriend Mark, and Mark starts to prefer Mary, Margo winds up deprived, but not deprived of anything she was entitled to. If two people are in a race, and the one behind gets in front, the early favorite winds up deprived of winning, but not deprived of anything she was entitled to. And on and on and on. Generally speaking, it's not causing a deprivation that's wrong, but causing someone to be deprived of something they're entitled to.
Is there a difference between what a one month old fetus is entitled to and what folks like us are entitled to? Yes. Why? Because though a fetus is the very same entity as the later infant, child, and adult it will become, it doesn't yet have the morally relevant properties of the later infant, child, or adult. It is not entitled to its future in the way that you or I are entitled to our futures. So abortion is nothing at all like murder. It has some of the elements (the individual killed is deprived) but not all of the elements (the individual killed is not entitled to its future). So Marquis's argument is unsound.
Unsound, but not completely devoid of insight. If a fetus does have a FLO (though one it's not entitled to) that explains why women are not confused at all when they take abortion seriously or have later mixed feelings. That makes very good sense, because abortion does cause a deprivation. It does take the future from an entity, and I think it's not incoherent at all to find that at least somewhat disturbing.