“To a biologist, birth is as arbitrary a milestone as any other. . . . No, the right to life must come, the moral philosophers say, from morally significant traits that we humans happen to possess. One such trait is . . . an ability to reflect upon ourselves as a continuous locus of consciousness, to form and savor plans for the future, to dread death and to express the choice not to die. And there’s the rub: our immature neonates don’t possess these traits any more than mice do.”