It exists only statistically - information seems to be more like entropy than like energy. That's quantum mechanics. If you measure an atom to have spin up, it COULD be because it was alway spin up, or it could be that it was spin anything-but-down and you just got lucky. You might say "but since the fact that it was spin-x didn't affect the result, how do we know it existed at all?" Well, that's what bell's inequality is for, basically. The data in your brain isn't a hidden variable, it's part of the quantum state, and so is subject to being messed with when measured.
This looks like a fascinating concept that I'll have to read up on.
If someone gets cremated or buried long enough for eir brain to fully decompose into dirt, it becomes extremely difficult to revive em. Nothing short of a vastly superhuman intelligence would have a chance of doing it. I suspect that it would be possible for a superintelligence to do it, but unless there's a more efficient way to do it, it would require recomputing the Earth's history from the time the AGI is activated back to the death of the last person it intends to save. Not only does this require immense computational resources that could be used to the benefit of people who are still alive, it also requires simulating people experiencing pain (backwards). On the other hand, this saves people's lives. Does anyone have any compelling arguments on why an FAI would or would not recreate me if I die, decompose, and then the singularity occurs a long time after my death?
Why do I want to know? Well, aside from the question being interesting in its own right, it is an important factor in deciding whether or not cryonics is worth-while.