FreedomFrog:
why would you feel it's not probable for spirits or ghosts?
Firstly there's a difference between "possible" and "probable". I think it's possible that there is organic life and other planets and I also think it's possible that some form of non-corporeal entities exist which could be described as ghosts. So both are possible (or at least I cannot prove they are impossible). The question is how probable are they?
Well, unfortunately there's no convincing evidence of the existence of either type of entity so we're going to have to give it some thought.
As people have pointed out, we evolved on this planet and quite quickly too, so it seems to me that given the right conditions life of some sort will almost inevitably form. We can use something like the Drake equation to estimate how abundant life is likely to be, but unfortunately we only have one known example so estimates vary wildly. However even if only one star in a trillion (1,000,000,000,000) provides a suitable environment for life to begin, then there would still be millions of examples in the known universe, albeit too far away for us to ever know about. My personal opinion - and it's not really much more than a best guess - is that life will be found to be abundant in the universe or at least to have originated many times, most of it never evolving past the single-celled stage.
Now ghosts, spirits and demons are a different matter. Here, we don't even have the luxury of one example. Lots of anecdotes but no evidence. Initially this appears similar to the situation with extraterrestrial life, and while I would certainly put UFO and ghost encounters in the same category, it seems much less plausible to me that spirit creatures have left no evidence of their existence. Aliens, after all ,almost certainly live very far away and may not have the intelligence, technology or inclination to communicate with us. Ghosts, on the other hand, are generally alleged to share this planet with us and to be actively trying to communicate with us. The fact that there is no record of them being successful, in itself weighs against the likelihood of such entities existing.
But perhaps there's some reason for their apparent absence other than non-existence. I gave several reasons for the apparent absence of aliens, and it would be churlish of me not to allow that there may be similar reasons for the non-appearance of ghosts, even if I can't think of any.
For life on other planets, I can imagine how they got there. If they exist, then most likely they evolved the same as we did. The process is well-known and understood and it seems virtually certain that any form of self-replicating entity that happened to begin existing would immediately be subject to the same Darwinian forces of natural selection that have shaped life on earth. For spirit creatures, I can't do that. They are generally considered not to reproduce so there is no path for evolution to take. Further, they are not even made from anything known to exist in our universe. How can they interact with normal matter if they are made of something else?
Certainly, the common view of ghosts as the conscious personalities of dead people is inherently absurd. Everything we know about consciousness indicates it is a function of the neural connections in the brain, and that without a brain there can be no consciousness. The idea that, essentially, there is a second identical person who lives undetectably alongside the flesh-and-blood person throughout their life, only separating at death and maintaining the personality of the deceased (and often their clothes in spirit form) is the product of primitive superstitious minds who knew a lot less about the universe than we did.
That's essentially it, then. I think aliens may exist because there's probably nothing unique about the process that led us to exist and the universe is a very big place. And I think ghosts almost certainly don't exist because there's no evidence and no known mechanism to explain their existence.