Don Marquis is professor of philosophy at the University of Kansas. He offers an argument against abortion that is not based on religious considerations or on assumptions about whether the fetus has a soul or is a person. According to Marquis, before we can answer the question "is abortion immoral" we first need to answer a more fundamental question, "Why is killing wrong in general?" In other words, in order to see whether or not it is wrong to kill a fetus, we need to understand why it is wrong to kill an adult human being according to Marquis.
Marquis suggests that the following is the reason why it is wrong to kill an adult human being. Killing an adult human being inflicts one of the greatest losses a being can suffer. But he also asks, "What is it that a human being loses when he is killed." Now the apparent answer to the question is, "Life." But this is not the totality of the loss according to Marquis. He proposes that to reduce the loss of the individual who is killed to "life" is an oversimplification. The "losses" are much more complicated than this. Interestingly he writes,
Killing an adult does not deprive him of his life up to the moment of the killing. In other words, it does not deprive him of his "past" life, the life he has already lived. It only deprives him of his "future" life, the part of his life that he has not lived yet. What is primarily important is not that killing him deprives him of his future biological life. Rather what matters is that it deprives him of the experiences for which biological life is necessary.
Some have argued Marquis' point in this fashion. Imagine how immoral it would be to put an adult into a permanent coma. You are not depriving him of his biological life but rather his future experiences. Putting someone in a coma so that he can no longer have experiences seems just as immoral as killing him outright. And this suggests that what is wrong with killing him is that it deprives him, not simply of his biological life, but of his future experiences.
Further, what is important is not every sort of future experience, but valuable future experiences such as "activities, projects, experiences and enjoyments" that are "valuable for their own sakes or are means to something else that is valuable for its own sake."
In addition, contrast this sort of future with someone who is about to be kidnapped and tortured perhaps in the hope that he will reveal political secrets that unbeknownst to the kidnappers he is in fact does not know. He is then killed when he does not divulge anything in which they are interested. This person has a future of experiences, but they are not valuable experiences.
In summary, what is wrong about killing an adult human being is that it deprives him of the value of his future. Marquis calls a valuable future (which is one characterized by activities, projects, experiences and enjoyments) a future like ours. So what makes killing an adult human being wrong is that it deprives him or her of a future like ours.
