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.

By similar compensatory processes, the blind often ‘see’ things, while the deaf ‘hear’ them, especially music. Those suffering from anosmia, or loss of smell, sometimes hallucinate smells — as did the protagonist of Sacks’ book The Man Who Mistook His Wife for a Hat — and amputees experience phantom limbs. Nelson regarded his phantom arm as ‘a direct proof for the existence of the soul’.

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Sacks had his own experience of this in 1974, when he injured his left leg in a mountaineering accident, and also suffered neuromuscular damage, so for a while it disappeared from his body image; he wrote a book about it, A Leg to Stand On. Many bereaved people ‘see’ their loved ones, while other hallucinations are associated with violent death and guilt, such as Hamlet’s ghost (‘In my mind’s eye, Horatio’) and Macbeth’s dagger.

Hypnagogic hallucinations, which are distinct from dreams, tend to appear just before sleep, and probably happen to most people, though they can be so subtle as to be hardly noticeable.