Such are the objections urged by Sokrates against the Protagorean doctrine — Homo Mensura. There may have been perhaps in the treatise of Protagoras, which unfortunately we do not possess, some reasonings or phrases countenancing the opinions against which Plato here directs his objections. But so far as I can collect, even from the words of Plato himself when he professes to borrow the phraseology of his opponent, I cannot think that Protagoras ever delivered the opinion which Plato here refutes — That every opinion of every man is true. The opinion really delivered by Protagoras appears to have been[59] — That every opinion delivered by every man is true, to that man himself. But Plato, when he impugns it, leaves out the final qualification; falling unconsciously into the fallacy of passing (as logicians say) a dicto secundum quid ad dictum simpliciter.[60] The qualification thus omitted by Plato forms the characteristic feature of the Protagorean doctrine, and is essential to the phraseology founded upon it. Protagoras would not declare any proposition to be true absolutely, or false absolutely. The phraseology belonging to that doctrine is forced upon him by Plato. Truth Absolute there is none, according to Protagoras. All truth is and must be truth relative to some one or more persons, either actually accepting and believing in it, or conceived as potential believers under certain circumstances. Moreover since these believers are a multitude of individuals, each with his own peculiarities — so no truth can be believed in, except under the peculiar measure of the believing individual mind. What a man adopts as true, and what he rejects as false, are conditioned alike by this limit: a limit not merely different in different individuals, but variable and frequently varying in the same individual. You cannot determine a dog, or a horse, or a child to believe in the Newtonian astronomy: you could not determine the author of the Principia in 1687 to believe what the child Newton had believed in 1647.[61] To say that what is true to one man, is false to another — that what was true to an individual as a child or as a youth, becomes false to him in his advanced years, is no real contradiction: though Plato, by omitting the qualifying words, presents it as if it were such. In every man’s mind, the beliefs of the past have been modified or reversed, and the beliefs of the present are liable to be modified or reversed, by subsequent operative causes: by new supervening sensations, emotions, intellectual comparisons, authoritative teaching, or society, and so forth.
[59] Plato, Theætêt. p. 152 A. Οὐκοῦν οὕτω πως λέγει (Protagoras) ὡς οἷα μὲν ἕκαστα ἐμοὶ φαίνεται, τοιαῦτα μέν ἐστιν ἐμοί — οἷα δὲ σοί, τοιαῦτα δὲ αὖ σοί. 158 A. τὰ φαινόμενα ἑκάστῳ ταῦτα καὶ εἶναι τούτῳ ᾧ φαίνεται. 160 C. Ἀληθὴς ἄρα ἐμοὶ ἡ ἐμὴ αἴσθησις· τῆς γὰρ ἐμῆς οὐσίας ἀεί ἐστι· καὶ ἐγὼ κριτὴς κατὰ τὸν Πρωταγόραν τῶν τε ὄντων ἐμοί, ὡς ἔστι, καὶ τῶν μὴ ὄντων, ὡς οὐκ ἔστιν.
Comp. also pp. 166 D, 170 A, 177 C.
Instead of saying αἴσθησις (in the passage just cited, p. 160 D), we might with quite equal truth put Ἀληθὴς ἄρα ἐμοὶ ἡ ἐμὴ νόησις· τῆς γὰρ ἐμῆς οὐσίας ἀεί ἔστιν. In this respect αἴσθησις and νόησις are on a par. Νόησις is just as much relative to ὁ νοῶν as αἴσθησις to ὁ αἰσθανόμενος.
Sextus Empiricus adverts to the doctrines of Protagoras (mainly to point out how they are distinguished from those of the Sceptical school, to which he himself belongs) in Pyrrhon. Hypot. i. sects. 215-219; adv. Mathematicos, vii. s. 60-64-388-400. He too imputes to Protagoras both the two doctrines. 1. That man is the measure of all things: that what appears to each person is, to him: that all truth is thus relative. 2. That all phantasms, appearances, opinions, are true. Sextus reasons at some length (390 seq.) against this doctrine No. 2, and reasons very much as Protagoras himself would have reasoned, since he appeals to individual sentiment and movement of the individual mind (οὐκ ὡσαύτως γὰρ κινούμεθα, 391-400). It appears to me perfectly certain that Protagoras advanced the general thesis of Relativity: we see this as well from Plato as from Sextus — καὶ οὕτως εἰσάγει τὸ πρός τι — τῶν πρός τι εἶναι τὴν ἀληθείαν (Steinhart is of opinion that these words τῶν πρός τι εἶναι τὴν ἀληθείαν are an addition of Sextus himself, and do not describe the doctrine of Protagoras; an opinion from which I dissent, and which is contradicted by Plato himself: Steinhart, Einleitung, note 8). If Protagoras also advanced the doctrine — all opinions are true — this was not consistent with his cardinal principle of relativity. Either he himself did not take care always to enunciate the qualifications and limitations which his theory requires, and which in common parlance are omitted — Or his opponents left out the limitations which he annexed, and impugned the opinion as if it stood without any. This last supposition I think the most probable.
The doctrine of Protagoras is correctly given by Sextus in the Pyrrhon. Hypot.
[60] Aristotle, in commenting on the Protagorean formula, falls into a similar inaccuracy in slurring over the restrictive qualification annexed by Protagoras. Metaphysic. Γ. p. 1009, a. 6. Compare hereupon Bonitz’s note upon the passage, p. 199 of his edition.
This transition without warning, à dicto secundum quid ad dictum simpliciter, is among the artifices ascribed by Plato to the Sophists Euthydêmus and Dionysodôrus (Plat. Euthyd. p. 297 D).
[61] The argument produced by Plato to discredit the Protagorean theory — that it puts the dog or the horse on a level with man — furnishes in reality a forcible illustration of the truth of the theory.
Mr. James Harris, the learned Aristotelian of the last century, remarks, in his Dialogue on Happiness (Works, ed. 1772, pp. 143-168):—