The Purpose of Galen's Treatise On Demonstration

This paper concerns the lost treatise On Demonstration (DD) written by Galen of Pergamum (129 - ca. 215 AD). Its aim is to reconstruct the purpose of this treatise, especially the question of how, in Galen's view, it was supposed to be useful for doctors. While showing that the methods described in DD were designed to settle disagreements among doctors, the paper argues that the choice of topics discussed there was partly determined by Galen's worry about a mode of reasoning, exemplified by scepticism, that leads people into believing that plain phenomena, such as those on which both medical practice and theory are based, do not exist.