Assessment of psychiatric practice has been difficult largely because of the apparent diversity of psychiatrists' theories, procedures, and goals. Searching for a unifying principle, we propose system identification failure (SIF), a defect in data-processing, as the unit of psychopathology underlying all psychiatric disorder (and other behavioral error). Definitive treatment must not only identify and correct the data-processing defect, but also eliminate the secondary symptoms. Emphasis is not on method of treatment but on goal or intent. Preparatory treatment includes supportive psychological and environmental measures and somatic therapies (regardless of type, duration, or intensity) which make definitive treatment possible. Palliative treatment must be frequently reviewed. Preventive treatment should prevent SIF-formation. We consider our proposal a distillate of the aims and implicit intent of experienced psychiatrists regardless of their training or theoretical orientation. The search for a functional definition of the practice of psychiatry was perhaps at one time an academic or lexicographic exercise, but, with the advent of peer review, it has become a pragmatic matter deserving of earnest attention. What is psychiatric practice? In a universe criss-crossed by psychiatrists "riding madly off in all directions." not only are the descendants of Anna O. and Dora analyzed in vivo and those of Little Hans at one remove, but also are distant figures and cultures subjected, often iv vitro, to critical scrutiny, while the "talking treatment" is applied to couples, families, groups, "networks" and communities. When one adds to this the diversity of biological therapies and institutional, environmental, and behavioral manipulations, the task of finding a common theme, much less procedure, becomes a worthy problem in hermeneutics, general semantics, or system analysis. Thus we must ask not only the operational question, "What do psychiatrists do?" BUt also, "What did you to to treat the specific psychopathology of this patient?" "Did it work?" AND, "Hos do you know?" Failure to deal with such questions as these will mark psychiatry's inability to cope with a reality that includes Professional Standards Review Organizations and an imminent National Health Insurance...