Someone afflicted by mental illness is neurotic if he sees his symptoms as symptoms and psychotic if does not. A neurosis is therefore an 'ego-dystonic' mental illness, meaning that the viewpoint embodied in one's symptoms is not the viewpoint of the ego of the afflicted party. And a psychosis is therefore an 'ego-syntonic' illness, meaning that the viewpoint embodied in the symptoms coincides with that of the afflicted party's ego. Whereas ego-syntonic illnesses are unqualifiedly debilitating, ego-dystonic are sometimes adaptive and, within limits, may enhance the subject's ability to deploy his abilities. In this lecture, it is explained why this is so.