To many people, hallucinations imply madness, but in fact they are a common part of the human experience. These sensory distortions range from the shimmering zigzags of a visual migraine to powerful visions brought on by fever, injuries, drugs, sensory deprivation, exhaustion, or even grief. Hallucinations doubtless lie behind many mythological traditions, literary inventions, and  religious epiphanies. Drawing on his own experiences, a wealth of clinical cases from among his patients, and famous historical examples ranging from Dostoevsky to Lewis Carroll, the legendary neurologist Oliver Sacks investigates the mystery of these sensory deceptions: what they say about the working of our brains, how they have influenced our folklore and culture, and why the potential for hallucination is present in us all.