“ In Truth, by way of justifying his rejection of philosophy or science, he maintained that “man is the measure of all things—of what is, that it is, and of what is not, that it is not.” Besides Truth, and the book Of the Gods which caused his condemnation at Athens, Diogenes Laërtius attributes to him treatises on political, ethical, educational and rhetorical subjects. Protagoras was the first to systematize grammar, distinguishing the parts of speech, the tenses and the moods. ”

“ According to Plato (Prot., 318 E) , he endeavoured to communicate “prudence” (εὐβουλία) to his pupils, “which should fit them to manage their households, and to take part by word and deed in civic affairs.” The education which he provided consisted of rhetoric, grammar, style and the interpretation of the poets. ”