Have you ever seen something that wasn't really there? Sensed someone following you and then turned round to find nothing? In this fascinating new book, Dr Sacks weaves together stories of his patients and of his own mind-altering experiences, to illuminate what hallucinations can tell us about the structure of our brains, how they have influenced every culture's folklore and art, and why the potential for hallucination is not only present in us all, but is also a vital part of the human condition. *Also appeared in May Buyer's Notes*