Another possibility is that an apparition whose description sounds suspiciously like the ghost librarian at the beginning of Ghostbusters might not actually be real. No offense to Shyanekh, but he wouldn't be the first person to feel very sure of something that wasn't rooted in anything external.
To be fair to you, that scene did scare the hell out of me as a kid
This was most certainly male though, the Ghostbuster's librarian was female wasn't it?
Ultimately I don't know precisely
what I saw*. I do know that a couple of people have described the same thing as me though. My uncle for example, he moved to Australia before I was born and as a child he lived in the house that I saw my ghost in. Years down the line he described the same thing as me.
I've honestly gone through this in my head for a long time and I'm fully aware of the potential for trickery our minds have. I've decided that I'm as certain about what I saw as I possibly can be. As such, I believe in ghosts but I don't expect my own story to convince anybody else.
I know that my word isn't worth a great deal over the internet, but I promise you this isn't a case of me going for the first thing that roughly makes sense. What I experienced impacted me deeply and has always been a factor in me questioning my beliefs and the beliefs of the general public.
*By this I mean I don't know if it was a person's soul, an afterimage or whatever. I only know that I saw something that could be thought of as a ghost.