Have you ever seen something that wasn’t really there or heard someone call your name in an empty house – or perhaps sensed someone was following you and turned around to find – nothing? Hallucinations are not the exclusive province of the insane. They are often linked to illness, sensory deprivation, injury or intoxication – or can even manifest with the simple act of falling asleep or waking up. Dr Sacks, as a young doctor in California in the 1960s, had a personal and professional interest in psychedelics and these, together with his early migraine experiences, launched a life long investigation into varieties of hallucinatory experiences. Here he recounts stories of his patients and his own mind-altering experiences in order to illuminate what hallucination can tell us about the organisation and structure of our brains, their influence on every culture’s folklore and art and why hallucination is a vital part of the human condition.