Note 17 — Chapter XXVIII · THEÆTETUS.
Reading edition
Dr. Routh, in a note upon his edition of the Euthydêmus of Plato (p. 286 C) observes:— “Protagoras docebat, Πάντων χρημάτων μέτρον ἄνθρωπον εἶναι, τῶν μὲν ὄντων, ὡς ἔστι· τῶν δὲ μὴ ὄντων, ὡς οὐκ ἔστι. Quâ quidem opinione qualitatum sensilium sine animi perceptione existentiam sustulisse videtur.”
The definition here given by Routh is correct as far as it goes, though too narrow. But it is sufficient to exhibit the Protagorean doctrine as quite distinct from the other doctrine, ὅτι ἐπιστήμη οὐκ ἄλλο τί ἐστιν ἢ αἴσθησις.