Seeing things? Hearing things?

What does it mean if you hear things that no one else hears?  See things that no one else sees?  Does it mean you are having a spiritual experience?  Or, do you have some form of psychotic disorder?

I submit for your reading pleasure a NY Times essay by Oliver Sacks.  Dr. Sacks suggests there may be some other possible reasons why you might hear or see or feel something that isn't heard, seen, or felt by others.  In fact, he points to research that a large portion of those who do have these experiences never tell others or doctors about them for fear of being labeled falsely with schizophrenia.