This discussion has to centre 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. A hallucination is a malfunction of the sensory system, leading it to report something which is not there. It can disrupt any sensory system. (I myself once answered a telephone, convinced that it had been ringing. It hadn’t: I had had an acoustic hallucination.) A typical medical hallucination is the illusion of feeling the creep of small creatures on the skin, a common sensory delusion is delirium tremens. It is a private feeling and cannot be 118tested by an external carer, though that carer may note that no small creatures can be seen on the skin. There are, of course, many ways to distinguish a hallucination from a sensory reality. A simple one is permanence, or even just persistence. Instrumental tests are often useful. Some of the ‘phantasms’ recorded by Gurney, Myers and Podmore (Chapter 8) even showed a reflection in a mirror, or cast a visible shadow.