Mental illness only accounts for some experiences, but it does not account for everything, therefor we cannot assume ghosts are purely the creation of someone's brain.
I am still undecided. I was told that as a kid I could be found in the basement playing with a ghost. I would even ask my parents if they could see the ghost i was playing with. You might attribute all that to an over active imagination, but this ghost was seen before I was born. Both my mother and grandmother had seen it previous to my conception. I don't know if they talked about the ghost in front of me, but even so I doubt at the age of 4 i would really understand what they were talking about, and eventually imagine the being they spoke of.
Supernatural
1.of, pertaining to, or being above or beyond what is natural; unexplainable by natural law or phenomena; abnormal.
If something is supernatural it is " being above" the natural, and therefore would not have to abide by natural laws. I most likely would consider any sort of being natural, but who knows |