When the pro-life and pro-choice movements debate the morality of abortion, their arguments often center around the question of whether abortion is murder. Yet to resolve this controversy, both factions are faced with determining exactly when life begins—at fertilization, birth, or sometime in between. Members of the pro-life movement believe that abortion is murder because it intentionally kills a human being. As social services professor William Brennan explains, “When the abortionist invades the sanctuary of the womb, the passenger within is by all scientific criteria alive, growing, and developing. After the abortionist accomplishes his lethal task, the intrauterine victim is definitely no longer alive, and therefore dead. And it is the abortion procedure that brought about this death.” The Catholic Church supports this belief, teaching that life begins at fertilization and any attempt to destroy that life—whether by...