I’ve made a fairly large leap from hallucinations to ghosts.
People’s hallucinations seem sometimes to include the dead – certainly common in near-death experiences – experiencing dead loved-ones’ calls from the other side (see Nicky’s story). Perhaps they aren’t really comparable since the whole world of the hallucination is unreal, whereas ghosts are more often thought of as an unreal figure in a real context. But nonetheless both are lived experiences of unreal things – and both are associated with times of crisis.
Ghosts in literature are laden with moral meaning and, like Hamlet Snr, often signal wrongs stirring under the surface of life. Reports of ‘real’ ghost sightings, on the other hand, are often characterised by purposeless repetitive actions which ‘offer little response or adaptation to the actions of the persons who observe them’. FWH Myers, a founder of the Society for Psychical Research in 1882, went so far as to see this meaninglessness as the stuff of dreams, and to surmise that ghosts might be projections of the dreams of the dead: ‘The behaviour of phantasms of the dead suggests dreams dreamt by the deceased persons whose phantasms appear.’  Apart from the basic strangeness of this concept, Freud might take issue with this notion of the meaningless dream. But it is perhaps ghosts’ fixity – their lack of ‘adaptation to the actions of the persons who observe them’ that has led to their reputation as harbingers of justice. In imposing meaning on their repeated actions – in making them clues, we comfortingly suggest that humanity is powerless to cover up truths: ‘Foul deeds will rise/Though all the earth o’erwhelm them to men’s eyes’.
None of this has anything to do with ICU delirium, except that it made me wonder whether there was a connection between hallucination and (subjective) morality? Taboos crop up, like cannibalism, and old fears – burial, being trapped underground, being trapped under ice. Fears suggesting timelessness, the dark side of immortality – the same fears which created vampires and ghosts as lost souls. There are often fights, defending something, and hallucinations are peppered with images that sound like hell, or perhaps spiritual tests – the ‘valley of the shadow of death’, as Paul Robertson puts it. But this is imposing meaning after the fact. Taking a feeling – fear perhaps – and turning it into a narrative about guilt and judgment. The danger of any story is it’s manipulation of experience.
‘We spend our lives inventing stories, but story actually doesn’t exist. We exist and our apprehension of a story is how we explain the meanderings that we take. There’s no such thing as the empirical story, it’s just what happens to people.’
Bill Forsyth, in The Story of Film: An Odyssey, by Mark Cousins
 Hauntings & Apparitions, Andrew Mackenzie (first published by Paladin in 1983 on behalf of The Society for Psychical Research)