This book deals with the theory that mental illness is an illness of the brain. With the advent of psychosurgery the theory entered the experimental stage; now the era of direct intervention in the brain for the purpose of alleviating mental troubles could begin. The introduction of psychosurgery brought about a rearrangement in epistemological rules and a new structure of authority within the psychiatric profession. Egas Moniz's conviction that mental illness could be treated through experimenting directly on the brain has been extended to include electrical, laser and deep brain stimulation, and today pharmacology is presented as a precisely targeted chemical intervention in the brain. These are backed by the very same argument that Moniz advanced: such experimentation will increase our knowledge of how the brain functions and this, in turn, will lead to more precise, rapid and effective interventions. A programme of psychiatry understood as experimentation with the brain follows rules that are different from the way psychiatry was practised before the advent of psychosurgery. In this sense, it is a new paradigm. The conviction that mental illness is a sign of a malfunctioning brain leads to a call for merging psychiatry and neurology. The author argues that this is conceptually incoherent, and he does so by demonstrating that the reason psychiatry and neurology are distinct is because, from the medical point of view, their terrains are different and because they pose different sets of methodological, epistemological and philosophical questions. The author also argues that the new vision of a unified neurology and psychiatry is clinically damaging.