The book is a scientific biography of American neurophysiologist and cybernetician Warren S. McCulloch, one that places his life and work in historical context. By focusing on the various identities that he assumed throughout his life's major work--the study of the brain and mind--the book examines the intermingling of McCulloch's professional and personal worlds, and by doing so provides a much-needed contribution to the history of American brain research in the twentieth century. The book complicates standard accounts of McCulloch by examining his activities outside the scope of cybernetics, demonstrating that McCulloch performed several other identities in addition to his role as a cybernetician: student, neurophysiologist, neuropsychiatrist, mentor, and engineer. The book argues that one of McCulloch's lasting achievements was to free the study of the brain from the purview of medicine--both institutionally and in terms of scientific practice. Overall, McCulloch's work facilitated the emergence of the brain as a new kind of scientific object, one with stronger ties to philosophy, biology, physics, psychology, and engineering.