The present study focuses on the thought of Protagoras and the context in which this thinker belongs, emphasizing the significance of his famous sentence of man as the measure of all things . Our overall objective is to make an overview of the philosophical context in which the sophist could be formed as an author. The specific goal in turn is to understand his doctrine from the sentence that is the heart of his thinking. Dependence mainly on the testimony of Plato in his dialogue Theaetetus, the reconstruction of the thought of Protagoras begins for us from the conception of the irrevocability of all perceptions/sensations experienced by each individual. The protagoric man-measure theme leads us from the formulation of sensible reality to the problem in terms of judgment and opinion, and so far, which is initially thought to each individual, shall be considered in addition to every man, to be thought of as each city. The boundary between what is measured and what is unmeasured, the problem of responsibility, the issue of education and the very emergence of philosophy are covered in our research, as Plato urges us to do in his exposition and critique of the sophist, as well as of the problems that affected Greece during that time.