This chapter presents a discussion on ghosts, for the overwhelmingly popular theory of ghosts is that each is in some way the animation of the soul of someone dead, which has come back to haunt the physical world as a visual apparition. However, the obvious counter-theory is that such effects are simply a form of hallucination. The first move in any discussion of optical effects has to be to differentiate between ghostly apparitions and simple hallucinations. Occasionally, however, hallucinations seem to be inspired by the unknown world. The neurologist Benjamin Libet has found that it takes about half a second for any new sensory impression to give rise to conscious sensation. A common social notion is that a ghost is in some way a relic of a person who has lived. Psychiatric interest in dreams has usually concentrated on the case in hand, which means concentrating on the present while disregarding the future.