This book explores the concept of psychophysics and details the development of the ideas which made the mathematisation of desire possible. The experience of desire accompanies us all throughout life, but dealing with it as psychologists and scientists is far from easy. Psychophysics was conceived to help map, mathematically, these unknowable feelings of desire. As such, this book will help to provide an accessible account of psychophysics while telling the story of its creation, which was, in essence, the birth of scientific psychology and contemporary cognitive neuroscience, alongside many of the technologies which characterize the contemporary world. It is a strange and intriguing story, which begins with the German physiologist Ernst Heinrich Weber in the first half of the nineteenth century, and its story will help the reader gain fresh insight into how scientists came to be able to map and quantify complex and private emotional states.