To begin with, Marquis claims that abortion, except perhaps in rare instances, is seriously wrong. His pro-life position is done without the appeal to personhood and done without appeal to religious premises. In spite of the fact that Marquis does not rely on the concept of personhood, he shares a key supposition with those who do: whether or not abortion is wrong, in his outlook, depends on something about the fetus; it depends on whether a fetus is the sort of being whose life it is seriously wrong to end. He believes the fetus is such a being, and he sets forth an analysis of why. He does this by first stating it is appropriate by discussing what it is about us that makes killing us, not only wrong, but seriously wrong.
To understand the wrongness in killing us must be done in terms of what killing us does to us. Marquis states that killing us imposes on us the misfortune of premature death. That misfortune underlies the wrongness. Further, that when one is dead, one has been deprived of life due to the fact that premature death is a misfortune. Also, that premature death deprives an individual of a future of value. Thus, killing someone is wrong, in general, when it deprives that person a future like ours, which he calls a FLO. In Marquis FLO theory are four arguments that support this account of the wrongness of killing.