Protagoras, from Diogenes Laertius, 9.51
“[Protagoras] was also the first to say that there are two arguments in opposition to each other concerning every matter. He argued with these and was the first to do this. He also began his work in this way: “A person is the measure of all things, that they are what they are and that they are not what they are not.”
And he used to say that the soul was nothing besides the senses, as Plato claims in the Theaetetus and that everything is true. Elsewhere, he began in this way: “Concerning the gods, I am not able to know that they exist or that they don’t exist. For many things impede knowledge—including both a lack of clarity and the brevity of human life.” Protagoras was expelled by the Athenians because of this introduction. They then burned his books in the marketplace once they had sent a herald around to collect them from those who owned them.”
Καὶ πρῶτος ἔφη (DK 80 B 6a) δύο λόγους εἶναι περὶ παντὸς πράγματος ἀντικειμένους ἀλλήλοις· οἷς καὶ συνηρώτα, πρῶτος τοῦτο πράξας. ἀλλὰ καὶ ἤρξατό που τοῦτον τὸν τρόπον (DK 80 B 1)· “πάντων χρημάτων μέτρον ἄνθρωπος, τῶν μὲν ὄντων ὡς ἔστιν, τῶν δὲ οὐκ ὄντων ὡς οὐκ ἔστιν.” ἔλεγέ τε μηδὲν εἶναι ψυχὴν παρὰ τὰς αἰσθήσεις, καθὰ καὶ Πλάτων φησὶν ἐν Θεαιτήτῳ, καὶ πάντα εἶναι ἀληθῆ. καὶ ἀλλαχοῦ δὲ τοῦτον ἤρξατο τὸν τρόπον (DK 80 B 4)· “περὶ μὲν θεῶν οὐκ ἔχω
εἰδέναι οὔθ’ ὡς εἰσίν, οὔθ’ ὡς οὐκ εἰσίν· πολλὰ γὰρ τὰ κωλύοντα εἰδέναι, ἥ τ’ ἀδηλότης καὶ βραχὺς ὢν ὁ βίος τοῦ ἀνθρώπου.” διὰ ταύτην δὲ τὴν ἀρχὴν τοῦ συγγράμματος ἐξεβλήθη πρὸς ᾿Αθηναίων· καὶ τὰ βιβλία αὐτοῦ κατέκαυσαν ἐν τῇ ἀγορᾷ, ὑπὸ κήρυκι ἀναλεξάμενοι παρ’ ἑκάστου τῶν κεκτημένων.