Colin wrote:In JKR's world, what is alive and what is dead?
People in portraits
Ghosts
Dementors
Poltergeists
Wizarding Chess pieces
Phoenixes
Sphinxes
The beings inside prophacies
These are a few of the items in her world that seem to have "Life" but would be "dead" in our world.
I cannot answer for sure but I can offer my opinion.
People in Portraits:
They are not dead because they were never alive in the first place. They are an illusion, the wizard equivilent of a computer program. Wizard photographs are "programed" to move and I imagine that portraits are just an extention of this. They are not only "programed" to move, but also speak and mimic human emotions.
Ghosts:
These guys are dead. They just happen to be still around to enjoy it.
Dementors:
See above post.
Poltergiests:
Dead. A poltergiest is a mean spirit who is not techniclly a ghost. I suppose the difference would be that ghosts are non-corporeal (they can't touch or move things) whereas a poltergeist can (Peeves throws things around all the time). Nonetheless, he seems to be a supernatural spirit of some sort -perhaps from someone who died, perhaps from another dimension, but both lead me to the conclusion that he is dead in the same way that ghosts are.
Wizarding Chess Peices
I imagine that these are complex "programes" in the same way that portraits are. They are clearly not dead, however I don't believe that they are "alive" either, because I cannot imagine the wizarding commnunity allowing children to play a game where living, breathing creatures go about killing one another. The are probably programed to interact and argue with the player for comical value.
Pheonixes:
Alive. They pass into death for a few moments when they burn up, but when they return, they seem to be fully functioning birds. Unlike ghosts or poltergeists, when pheonixes return from the grave, they are once again
living.
Sphinxes:
Alive. They are just another sort of magical creature like centours, house elves or hippogriphs.
Beings inside prophercies:
I can not even pretend to know what these things are. Maybe they are forces from another reality, sent to this one in order to keep it flowing as it should. We will never get a look at this other reality, so I guess we will never know what the heck they are (That is if they even come from another reality, this last one is just speculation on my part).
What else seems to defy death?
Vampires and zombies:
Well, vampires have only been referred to and zombies have not even been mentioned, but just about every other author and/or filmaker has established that these creatures are dead.
Possesed people:
Quirrel for example. I think he's still alive... just really deformed and messed up.
Harry:
He defied death due to a love protection spell his mother cast over him. So he is alive (duh).
Voldermort:
When the Adava Kedavra curse bounced off Harry and landed back on Voldermort, why didn't Voldermort die? Anybody certainly else would have. Voldermort was so badly injured that his body was no longer corporial, yet he was not truly dead. In GoF he mentions self experimentation, and this probably explains his less-than-human apperance. My theory on how he survived the curse is that by that stage. he had become a being similar to the dementors -neither alive nor dead. Clearly he is no longer what we would call "human".
And who is in Tom Riddle's grave
Tom Riddle is in Tom Riddle's grave. Tom Riddle senior that is

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Sorry 'bout the long post, hope my opinion helps. BTW, I'd be interested to hear if other people agree/disagree with anythin I've said, its always a good thing to hear more opinions.