I can actually refute those two points you mentioned stormcrow, in that I am an atheist and I saw a ghost full on. I also saw it multiple times, at least five or six times. When I was a little kid, I remember laying in bed and waking up in the middle of the night and looking across my room and I saw a green hazy looking ghost that started off small but formed into a much larger shape as it walked across the room closer and closer to me. I saw it a few times like I said, though when it reached half way across the room it would disappear. Though often I hid under my covers and tried to act like it wasn't there because it scared me. There was never any stories of ghosts in my house and no one ever said anything about it and I never mentioned it to anyone else, so it isn't like I imagined it based off stories or anything.

The only problem is that I woke up in the middle of the night each time I saw it. Now I remember I was fully awake at the time I saw it, but I was a little kid and I didn't know about sleep paralysis or very much about dreams and stuff.

So it is possible it was some dream related hallucination. I wish I would see something like that now, because I believe I would be able to tell with a high degree of certainty if it was real or not. However, I have not seen anything like that since then. I really don't think my 6 year old self is a reliable source of information, since kids don't really have the skills to tell the difference between dream related hallucination, or even optical illusions, from real life.

So I do allow the possibly that ghost might be real, because I may have saw one. Though in all honesty, I think that it is unlikely that they really exist.