The ten people in one million who are synaesthetes are born into a world where one sensation (e.g. sound) conjures up one or more others (e.g. taste or colour). Although scientists have known about synaesthesia for two hundred years, until recently the condition has remained a mystery. Extensive experiments with more than forty synaesthetes led Richard Cytowic to an explanation of synaesthesia that emphasized the primacy of emotion over reason. In this medical detective adventure he reveals the brain to be an active explorer and offers a new view of what it means to be human that turns upside down conventional ideas about reason, emotion, and who we are.