A medical diagnosis is a definition. The term ‘diagnosis’ is derived from the Greek and implies the distinguishing or discernment of a disease. But it implies other things, too. Without a proper diagnosis in medicine it is impossible to arrive at a rational prognosis and treatment of the disease (Warner 1952). Similarly, in psychiatry the diagnostic classification of data is usually expected to go beyond the description of behaviour and to make a statement about the past, present and future states of the patient (Frank 1975; Slater and Roth 1974). By medical criteria, this emphasis upon the information that is to be conveyed by the diagnosis is crucial, and the evaluation of the current diagnostic system is largely dependent upon its success in this function.