Protagoras is a sophisticated one, the scientists are the enemies of Platon... The conclusion seems obvious. However, it calls for some nuances. More than an enemy, Protagoras appears in Platon as an exemplary figure, a valid interlocutor who, through his assertion that man is the measure of all things, embodies democracy, its conditions and its consequences. This book deals with the general, and classic, theme of the opposition between sophisticated and philosophy, but limiting it to a single issue with different facets: the idea of measurement. It reproduces a ‘protagorean’ thinking that places Platon in its intellectual context and shows how, through Protagoras’ figure, it is the debate with democracy that takes place not only as a political system, but also as a theoretical model with multiple implications (epistemological, moral, pedagogical, technical). What is the criterion on which the measure is based? How to design expertise and knowledge? What role should language play in education? What to do about the identity of the subject in the remembrance process? How to think about the unity of the City? All of the issues raised in the dialogue with the protagorean model. Understanding what Platon has to do with Protagoras is how this thought has challenged it and how it has developed its own theory of measurement, expertise, education and political unity.