Sydenham's reputation rests in part on his self-deceptive assertion that he allowed clinical observation to speak for itself. In fact, however, his mind was steeped in philosophical world-views that went back via the Scholastics and Galen to Plato and Aristotle. A brief survey of these views is presented to clarify Sydenham's message. The basic formula of logical set theory is then used to put in perspective Sydenham's disease postulates and later views on the nature of disease. The point is made that, in the case of cryptogenic diseases, we sometimes tend to revert unwittingly to ideas akin to those of Sydenham.