The existence of "Ghosts" means...

[CristW]

OP, this can turn into a slippery slope real quick.
If ghosts, then lingering soul?
If soul, then maybe heaven and hell?

And since we know the bible is be, then all is bs.
Pretty sure it's not aliens (and I'm crazy star trek).
It's almost always those under duress, etc, who see these things.
You don't need to see something from your optic nerve to see something.
The brain is a very powerful tool and sometimes if you want to see something badly enough, you can and will. Also, I will hazard a guess and say that the vast majority who claim to have seen or felt a ghost are religious to start with. Therefore are biased to start with.

1. What is "OP"? (Ad Hominem?)
2. "Ghosts" could simply be ETs operating in another dimension to scare off people (Not a slippery slope example...rather "theory").
3. People under duress do see things but this was never part of the discussion (just one example why "ghosts" are not real but rather the behaviour of the brain - Hippocampus - which leads to the Agent of Sociolization in Sociology). I could agree with you on this.
4. The case of demonic possession along with the association of "ghosts" is an example of "religious' indoctrinization which affects the psychology of the individual into misinterpreting mental illness and other phenomena.

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What do you mean by ghost? Depending on what was discovered would determine what we could infer from it.

There is no concrete data therefore a "theory" has to be developed. Again, if "ghosts" were past people who are dead then the whole planet would be infested with "Ghosts".

"Theory":

1. The omission of a "ghost" infested planet means...
2. The addition of another phenomena being observed (UFOs/ETs) could be the origin of LIMITED apparitions or "ghosts".
3. There are also no concrete evidence that humans have been visited by ETs because we would be continously interacting with them(general public). Unless, of course, they do not want to be seen.