There are good historical reasons for the classification of pathology into general, experimental and special pathology . The development of methods of investigation has led beyond that to the formation of specific lines of investigation such as histopathology, molecular pathology and immunopathology, etc. These empirically investigated areas are complemented by the wider field of theoretical pathology. This theoretical pathology is part of a theory of medicine. It is not empirical, not experimental and not descriptive, but considers the general phenomenon of “illness”. It is a purely intellectual, theoretical discipline in the framework of pathology; it reflects on the pathological, and concerns itself with the clarification of theoretical preliminary and basic questions of pathology which can be seen not only as the prolegomena but also as the postlegomena of this science. Considered as belonging to theoretical pathology are (1) the conceptualization of the field, (2) endeavor to interpret the pathological in its different manifestations, and (3) the question of the function of illness. From among the concepts of theoretical pathology are discussed here the notion of disease, the idea of disease and the concept of disease. Further suggestions will be made indicating a more precise and more uniform use of the terms “pathos”,“nosos”,“nosology”, “nosogram”, “pathogenesis”, “pathogenesis”, etc. Another subject of a theoretical pathology, which will not be further treated here, might be termed a search for an interpretation of illness against the background of broad general theories, e.g., illness as a biological phenomenon based on a theory of the organism, and similarly illness as a psychosomatic phenomenon and as a sociological and anthropological phenomenon. Finally there is the question of whether illness has a function, and if so, in which context. Theoretical pathology is necessarily concerned with many problems which touch on the philosophy of science, history of science, philosophy and theology.