In 1917, working alone in a remote Swiss asylum, psychiatrist Hermann Rorschach devised an experiment to probe the human mind: a set of ten carefully designed inkblots. After Rorschach's early death, his "inkblot test" made its way to America, where it took on a life of its own. Co-opted by the military after Pearl Harbor, it later became an advertising staple, a cliché in Hollywood and journalism, and an inspiration to everyone from Andy Warhol to Jay Z. In this first-ever biography, Damion Searls draws on unpublished letters and diaries to tell the unlikely story of the test's creation, its controversial reinvention, and its remarkable endurance.