It is difficult to make out what Diogenes Laërtius states about other tenets of Protagoras, and to reconcile them with the doctrine of “man being the measure of all things,” as explained by Plato (Diog. Laërt. ix, 51, 57).
[586] Aristotle (in one of the passages of his Metaphysica, wherein he discusses the Protagorean doctrine, x, i, p. 1053, B.) says that this doctrine comes to nothing more than saying, that man, so far as cognizant, or so far as percipient, is the measure of all things; in other words, that knowledge, or perception, is the measure of all things. This, Aristotle says, is trivial, and of no value, though it sounds like something of importance: Πρωταγόρας δ᾽ ἄνθρωπόν φησι πάντων εἶναι μέτρον, ὥσπερ ἂν εἰ τὸν ἐπιστήμονα εἰπὼν ἢ τὸν αἰσθανόμενον· τούτους δ᾽ ὅτι ἔχουσιν ὁ μὲν αἴσθησιν ὁ δὲ ἐπιστήμην· ἅ φαμεν εἶναι μέτρα τῶν ὑποκειμένων. Οὐθὲν δὴ λέγων περιττὸν φαίνεταί τι λέγειν.
It appears to me, that to insist upon the essentially relative nature of cognizable truth, was by no means a trivial or unimportant doctrine, as Aristotle pronounces it to be; especially when we compare it with the unmeasured conceptions of the objects and methods of scientific research which were so common in the days of Protagoras.
Compare Metaphysic. iii, 5, pp. 1008, 1009, where it will be seen how many other thinkers of that day carried the same doctrine, seemingly, further than Protagoras.
Protagoras remarked that the observed movements of the heavenly bodies did not coincide with that which the astronomers represented them to be, and to which they applied their mathematical reasonings. This remark was a criticism on the mathematical astronomers of his day—ἐλέγχων τοὺς γεωμέτρας (Aristot. Metaph. iii, 2, p. 998, A). We know too little how far his criticism may have been deserved, to assent to the general strictures of Ritter, Gesch. der Phil. vol. i, p. 633.
[587] See the treatise entitled De Melisso, Xenophane et Gorgiâ in Bekker’s edition of Aristotle’s Works, vol. i, p. 979, seq.; also the same treatise, with a good preface and comments, by Mullach, p. 62 seq.: compare Sextus Emp. adv. Mathemat. vii, 65, 87.
[588] See the note of Mullach, on the treatise mentioned in the preceding note, p. 72. He shows that Gorgias followed in the steps of Zeno and Melissus.
[589] Isokratês De Permutatione, Or. xv, s. 287; Xenoph. Memorab. i, 1, 14.
[590] Aristophan. Equit. 1316-1321.
[591] Isokratês, Or. xv, De Permutation. s. 170.