The morality of abortion is entirely a matter of definition: is the fetus a person or not? The definition—whether derived from millennia of religious tradition or from individual analysis and subjective choice—both generates and justifies the intense emotions that are given free rein when fact is irrelevant.
There is no logical or empirical way to demonstrate the superiority of either definition, that of the fetus as a person or as a nonperson. Nor is there any way to demonstrate the superiority of either of the impulses that lie behind them—a tradition-rooted identification with the fetus; or an identification with the pregnant woman, defended analytically and pragmatically. These differences manifest themselves in different premises, each of which is both impervious to attack and incapable of logically or scientifically persuading the other side. Nonetheless, once you accept one of the definitions, you are committed to a specific position on the permissibility of abortion.
Let us assume that no one is willing to argue for the routine killing of normal, innocent persons. Then, if the fetus is defined as a person, there is nothing more to say. Abortion, so defined, is an extermination of persons, one that would (morally, though perhaps not legally) justify almost any response capable of stopping a massacre.
If, on the other hand, the fetus is defined as a nonperson, there is nothing more to...