But the endless abortion debate shows how hard it is to locate the boundary. Those in favor of abortion draw the line at viability, but viability is a fuzzy gradient that depends on how great a risk of an impaired child the parents are willing to tolerate. The only thing both sides agree on is that the line must be drawn at some point before birth. Neonaticide forces us to examine even that boundary. To a biologist, birth is as arbitrary a milestone as any other.
Many mammals bear offspring that see and walk as soon as they hit the ground.
The usual primate assembly process spills into the first years in the world. And that complicates our definition of personhood. 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 having a unique sequence of experiences that defines us as individuals and connects us to other people. Other traits include 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.