Agonies and ecstasies

William James considered an hallucination to be ‘as good and true a sensation as if there were a real object there’, except that the ‘object happens to be not there, that is all’ — an admirable definition, and a favourite of Oliver Sacks, the eminent neurologist, who has written what he calls ‘a sort of natural history or anthology of hallucinations’, which he thinks are an essential part of the human condition.

He excludes schizophrenic hallucinations, on the grounds that they demand separate consideration, but includes every other kind. Some are induced by sensory deprivation, such as isolation and darkness — ‘the prisoner’s cinema’ — or visual monotony, as experienced by sailors gazing on a becalmed sea, or by travellers in deserts or polar regions. Mushers in the Iditarod dog-sled race, for example, who go for as long as a fortnight on minimum sleep, often hallucinate trains, orchestras and strange animals.