I think that what Kira is saying has some interesting points to it, if there are the "Spirits" of dead people then is there a place they go after they stop? There must be something because there would be so many "spirits" all over the place because of the sheer number of the dead, I personally believe that something like a "spirit" doesn't exist and we are actually a spark of electricity in our head that fissions when you are born to almost jumpstart your body, and that electricity can run ~80 years or so, since thats seems to be the average life expectancy for places with the ability to prevent other diseases that would have killed us, after that it's usually a matter of your body shutting down, or your "electricity" going out