If a ghost is defined as a person's soul or spirit I think it would have to be considered supernatural.
If a ghost is weird crap happening that has no valid explination it could be considered natural even if you don't see a natural reason for whats happening. What I mean by this is maybe the common sets of circumstanes contributed to ghosts (cold spots, E.V.P., manifestations, disembodied voices, ect.) could be caused by a wholly natural reason that isn't understood. Lightning used to be thought of as pissed off gods until we figured out what it really was.
quote:Henry P. Stapp is a theoretical physicist at the University of California—Berkeley who worked with some of the founding fathers of quantum mechanics. He does not seek to prove that the soul exists, but he does say that the existence of the soul fits within the laws of physics.
It is not true to say belief in the soul is unscientific, according to Stapp. Here the word soul refers to a personality independent of the brain or the rest of the human body that can survive beyond death. In his paper, Compatibility of Contemporary Physical Theory With Personality Survival, he wrote: Strong doubts about personality survival based solely on the belief that postmortem survival is incompatible with the laws of physics are unfounded.