The thesis of this essay is that most problems of psychiatric nosology, as currently formulated, are refractory to solution because of certain basic ambiguities in psychiatric concepts and operations. Scientific clarity and progress in this area depend upon clear agreement on the following issues: 1. The scope and subject matter that is to be designated as "psychiatry" (e.g., brain, mind, or behavior); 2. The scientific and technical methods that characterize this branch of knowledge (e.g., physics or psychology, physico-chemical techniques or psychotherapy); 3. The precise nature of the phenomena that we seek to classify (e.g., physical or chemical changes in the brain, social behavior, or behavior toward specific individuals). These are not three separate categories, but represent rather interlocking aspects of what must be, in the last analysis, operational descriptions of specific "psychiatric situations."It is suggested that we distinguish sharply between the following principal psychiatric situations on the current American scene: the mental hospital, private psychiatric practice (including the psychoanalytic situation), the child guidance clinic, the psychoanalytic training system, military service, the court of law and jail. Illustrative samples of an operational analysis of a few of these situations are presented. A similar scrutiny of the psychiatric situations that characterized the work of each of the principal figures in the history of psychiatry since Kraepelin is suggested and briefly sketched. This mode of approach prompts one to take a more "relativistic" view of psychiatry, by which is meant the appreciation that different observational methods imply differences in the very nature of the observed "material." Thus, global approaches to psychiatry may have to be abandoned in favor of more limited, and socially and methodologically better defined, plans of attack on specific problems. It is further inherent in this line of thought that a nosological system developed in, and appropriate to, one type of psychiatric situation cannot be validly transferred to another, radically different psychiatric situation. This is a principle familiar to us from other branches of science and technology and the various systems of classification that they employ.Considerations of nosology also prompt a scrutiny of the specific items that are classified. At present, probably the single most important diagnostic label in psychiatry is "schizophrenia." Some epistemological aspects of the problem of schizophrenia are briefly discussed, and it is suggested that this word may now function as a "panchreston" ( or "explain-all") which, instead of illuminating, obscures the essential problems that face psychiatry today.In conclusion, some observations are offered on the currently widespread disregard of nosological rules by psychiatrists and its inhibiting influence on progress in psychiatry.