The Origin of Ghosts?

Freud thought that ghosts are actually the visions of people who are afraid of death.  In this sense, ghosts would not be real at all but rather a projection of our subconcious mind. Jung looked for answers in his theory of the ciollective unconscious, the contents of which can in his view take on almost an autonomous life of their own, being projected or manifested as seemingly outer occurences. A somewhat plausible theory is that ghosts are telepathic images i.e. a sensitive person would pick up past vibrations from the area they were in and witness an event or person as it appeared many years ago. This would also explain instances where a person sees a loved one at or near the moment of the loved one's death, since the loved one could be unconsciously projecting their thoughts to the receptive person. William James produced an interesting theory based on the idea of spontaneous produced hallucinations caused by extra sensory perception, telepathy and clairvoyance.

Ghosts might also be the result of time slips, if time is nonlinear. An event that happened in the past might be seen briefly in our time because of a fluctuation in time/space. 

Courtesy of monstrous.com