Theories of symbolic logic are used in an attempt to clarify the medico-psychological concept of disease. It is argued that the class concept of diseased persons has as its extension the universal class of patients, and as its intension the attribute concept of patienthood (or disease in general). It is proposed that the diagnosis of patienthood has as its sufficient and necessary condition the experience of therapeutic concern by a person for himself and/or the arousal of therapeutic concern for him in his social environment. The experience of therapeutic concern has as its necessary condition, and also as its specific psychological cause, the observation of clinical symptoms and/or other pathological manifestations. The general domain of patients and its various subdivisions are empirical classes which are logically inexact—that is, contain neutral members in the form of dubious patients—and are polythetically characterized—that is, by a loose cluster of pathological attributes of which only a substantial proportion need be exhibited by a class member. The division of the domain of patients (and therefore the classification of patients) is commonly achieved by the addition to the intension of the domain of pathological attributes. These must, of course, be attributes which are not universally or predominantly shared by the domain members. Endopathological and exopathological abnormalities have proved themselves as the most useful classifying criteria. Their addition to the intension of the domain of patients separates particular classes from the rest of the domain. The intensions of these classes correspond to particular diseases, and list all the relevant clinical, endopathological, and exopathological data, if available. When a particular endopathological abnormality is by definition established as the specific cause of its clinical symptoms, then the intension in question corresponds to an aetiologically homogeneous disease entity.