Dr Taylor's book analyses the disease concept as it developed in medical history and seeks to clarify it with the help of concepts largely derived from logical class theories. A solution is proposed to the problem of how to distinguish between the class of 'patients' and the class of 'healthy persons' which corresponds to the actual diagnostic practices of doctors. The earliest theories of disease postulated concrete entities which exist independently of the body. The notion of disease entity has lost its original ontological connotations and instead its important feature has become the possession of a unitary and self-contained character. Dr Taylor describes the modern theories as essentially 'reactive' in character, that is the symptoms of a disease are the bodily reactions to the 'noxae'. After seeing the subject in its historical content, Dr Taylor goes on to discuss in detail the notion of the classification of diseases, making extensive use of modern views on the logic of classes.