John R. Paul, Professor of Preventive Medicine at the Yale University School of Medicine, wrote numerous monographs and papers defining and delimiting the field of preventive medicine and its fundamental science, clinical epidemiology. For Dr. Paul, preventive medicine was part of the continuum of clinical medicine; others believed that it was a separate entity deserving of departmental status. This paper discusses Dr. Paul's definition and philosophy of preventive medicine in contrast to other disciplines, such as social medicine and public health.