Current trends in the methodology of classification

Classification is a fundamental cognitive activity of ordering the objects of the external and the internal (phenomenological) world into sets on the basis of their relationships. The strategies employed vary from the search of "focal essences" identifying natural classes to the purely statistical approach of numerical taxonomy. Medical classifications are eclectic and pragmatic, without clear predominance of any single strategy. In addition to having its share of the general problems of medical taxonomies, classification in psychiatry has to face the dilemma of classifying diseases or people and their problems. The implications of this for the 10th revision of ICD are examined, and criteria for a good classification are tentatively laid down.