Human life is sacred and must be protected wherever possible. But for some causes we may fight, die and kill. But while I support the state's right to conscript its young to fight against genocide and tyranny, I do not want the state to hang convicted murderers, and I question whether it should condemn an infant to death. As a Jew, I have a deep respect for the Roman Catholic position that regards human life as sacred. But Jewish tradition defines human life somewhat differently.
Jewish tradition depends heavily on scientific observation when deciding relevant issues.
Conjoined twins start from a single fertilized egg - in fact, such twins are probably not formed until at least three weeks after conception, when the developing embryo has failed to split completely. It seems illogical to suggest that one soul is formed at conception, but it becomes two separate souls three weeks later. So Jews place emphasis on the continuum of human life. Life has no precise beginning, but gains in importance during human development. Even human life is not absolute.