Disease concepts and the logic of classes.

Four different disease concepts are examined. In pre-scientific times, diseases were regarded as independent entities outside and inside the body of patients. This view is still alive in such expressions as "disease carrier' and similar idioms. Sydenham's disease entity was akin to a Platonic "idea'; namely as "species', "substantial form' or "essence' which had a singular independent existence in some metaphysical realm. Therefore, the talk at the time was that a patient suffered from, say, "the' pox. Virchow's disease entities were semi-independent parasitic parts in the bodies of different patients. Therefore, patients were said to suffer from, say "a' pneumonia. In modern times diseases are no longer regarded as independent or semi-independent entities. They are merely attribute complexes in patients. As such, they require no grammatical article. Patients are said to suffer from, say, pneumonia. The different views of disease concepts are finally examined within a framework provided by the modern logic of classes. The pre-scientific and Virchowian disease entities are members in a class "extension'. Sydenham's disease entity corresponds to a class entity abstracted from class members. Modern diseases are attributes listed in the class "intension'. The different disease concepts thus have different logical implications.