A critique of "the best secular argument against abortion"

Don Marquis has put forward a non-religious argument against abortion based on what he claims is a morally relevant similarity between killing adult human beings and killing fetuses. He asserts that killing adults is wrong because it deprives them of their valuable futures. He points out that a fetus's future includes everything that is in an adult's future, given that fetuses naturally develop into adults. Thus, according to Marquis, killing a fetus deprives it of the same sort of valuable future that an adult is deprived of in being killed and this makes abortion seriously wrong. Commentators have raised a number of objections to Marquis's argument, to which he has satisfactorily responded. In this paper, difficulties with Marquis's argument that have not been considered by previous commentators are pointed out. A main thesis of this paper is that Marquis does not adequately defend his argument against several important objections that he himself has raised. These new considerations support the view that Marquis's argument is unsuccessful.