Degérando, Histoire Comparée des Systèmes de Philosophie, vol. i. p. 48:— “Une multitude d’hypothèses, élevées en quelque sorte au hasard, et rapidement détruites; une diversité d’opinions, d’autant plus sensible que la philosophie a été plus developpée; des sectes, des partis même, des disputes interminables, des spéculations stériles, des erreurs maintenues et transmises par une imitation aveugle; quelques découvertes obtenues avec lenteur, et mélangées d’idees fausses; des réformes annoncées à chaque siècle et jamais accomplies; une succession de doctrines qui se renversent les unes les autres sans pouvoir obtenir plus de solidité: la raison humaine ainsi promenée dans un triste cercle de vicissitudes, et ne s’élevant à quelques époques fortunées que pour retomber bientôt dans de nouveaux écarts, &c.… les mêmes questions, enfin, qui partagèrent il y a plus de vingt siècles les premiers génies de la Grèce, agitées encore aujourd’hui après tant de volumineux écrits consacrés à les discuter”.
This state of mind in reference to belief is usual with most men, not less at the present day than in the time of Plato and Protagoras. It constitutes the natural intolerance prevalent among mankind; which each man (speaking generally), in the case of his own beliefs, commends and exults in, as a virtue. It flows as a natural corollary from the sentiment of belief, though it may be corrected by reflection and social sympathy. Hence the doctrine of Protagoras — equal right of private judgment to each man for himself — becomes inevitably unwelcome.
Aristotle failed in his attempts to refute the Protagorean formula — Every reader of Aristotle will claim the right of examining for himself Aristotle’s canons of truth.
We are told that Demokritus, as well as Plato and Aristotle, wrote against Protagoras. The treatise of Demokritus is lost: but we possess what the two latter said against the Protagorean formula. In my judgment both failed in refuting it. Each of them professed to lay down objective, infallible, criteria of truth and falsehood: Democritus on his side, and the other dogmatical philosophers, professed to do the same, each in his own way — and each in a different way.[79] Now the Protagorean formula neither allows nor disallows any one of these proposed objective criteria: but it enunciates the appeal to which all of them must be submitted — the subjective condition of satisfying the judgment of each hearer. Its protest is entered only when that condition is overleaped, and when the dogmatist enacts his canon of belief as imperative, peremptory, binding upon all (allgemeingültig) both assentient and dissentient. I am grateful to Aristotle for his efforts to lay down objective canons in the research of truth; but I claim the right of examining those canons for myself, and of judging whether that, which satisfied Aristotle, satisfies me also. The same right which I claim for myself, I am bound to allow to all others. The general expression of this compromise is, the Protagorean formula. No one demands more emphatically to be a measure for himself, even when all authority is opposed to him, than Sokrates in the Platonic Gorgias.[80]
[79] Plutarch, adv. Kolot. p. 1108.
According to Demokritus all sensible perceptions were conventional, or varied according to circumstances, or according to the diversity of the percipient Subject; but there was an objective reality — minute, solid, invisible atoms, differing in figure, position, and movement, and vacuum along with them. Such reality was intelligible only by Reason. Νόμῳ γλυκύ, νόμῳ πικρόν, νόμῳ θερμόν, νόμῳ ψυχρόν, νόμῳ χροιή· ἐτέῃ δὲ ἄτομα καὶ κενόν. Ἅπερ νομίζεται μὲν εἶναι καὶ δοξάζεται τὰ αἰσθητά, οὐκ ἔστι δὲ κατὰ ἀληθείαν ταῦτα· ἀλλὰ τὰ ἄτομα μόνον καὶ κένον.
Sextus Empiric. adv. Mathemat. vii. 135-139; Diog. Laert. ix. 72. See Mullach, Democriti Fragm. pp. 204-208.
The discourse of Protagoras Περὶ τοῦ ὄντος, was read by Porphyry, who apparently cited from it a passage verbatim, which citation Eusebius unfortunately has not preserved (Eusebius, Præpar. Evang. x. 3, 17). One of the speakers in Porphyry’s dialogue (describing a repast at the house of Longinus at Athens to celebrate Plato’s birthday) accused Plato of having copied largely from the arguments of Protagoras — πρὸς τοὺς ἓν τὸ ὂν εἰσάγοντας. Allusion is probably made to the Platonic dialogues Parmenides and Sophistes.
[80] Plato, Gorgias, p. 472.
Plato’s examination of the other doctrine — That knowledge is Sensible Perception. He adverts to sensible facts which are different with different Percipients.