Good question.

I watch a lot of shows dealing with the paranormal including ghostly apparitions. I've actually seen a show where a psychiatrists proposed that many ghost sightings are little more than a hallucination brought on by the person's belief system, fear, and desire to see physical evidence of a ghostly presence. Now of course this leaves a lot of room for argument such as how do you explain the legit photos that have been captured of ghostly apparitions but it does offer food for thought.