Why would someone wake up and claim they're Napoleon? Or believe they have been turned into a wolf and demand to be fed raw meat? For centuries, we've dismissed delusions as a problem for the shrinks to sort out in distant asylums. But delusions are more than just bizarre case studies - they tell stories of collective anxieties and traumas. In this groundbreaking history, Victoria Shepherd explores delusions from ancient times to present and implores us to identify reason in apparent madness. Isn't it perfectly understandable to believe you've got the wrong head when the guillotine takes the heads of hundreds every day? Who cannot sympathise with the man who believes he is already dead, when all his comrades died in the battlefields? We all have it in us to become delusional. In understanding delusions, we come closer to understanding ourselves.