Writings. — From the list of the writings of Protagoras, which Diogenes Laertius (ix, 55) doubtless borrowed from one of his Alexandrine authorities (he describes them as still extant, tori to. awZi'ifiti'a ai<Tov fiijiXla Tavra : comp. Welcker's account of Prodicus, in his Kleine Schriflen, ii, 447, 405), and which he gives prob- ably with his accustomed negligence, one may sec that they comprised very different subjects: ethics (Deoi aptruJv and Uepi rwv ovk opSwe role. ar^pojiroiQ Tvpaa- aofisviov, Htpi <pi\oTi[iiac,) ; politics (Tltpi 7ro\iTtiac, Uepi r//e iv (>PX\l Karaaraotwc : comp. Frei, p. 182, etc.); rhetoric (' AvrCKoyvSiv cro, Tf-.yyn\ ipurrix&v), and other subjects of different kinds (UponroKTiKoe, TIepi /iaBi]fJidro)V, Uepi 7r«X»/c. Utpi rwv iv A'icov). The works which, in all probability, were the most impor- tant of those which Protagoras composed — Truth ('A\>)- Sein), and On the Gods (Tlfpi QftDv) — are omitted in that list, although in another passage (ix, 51) Diogenes Laertius refers to them. The first contained the the- ory refuted by Plato in the Tkecetefus (p. 101, c; 102, a; 100, c; 170, e), and was probably identical with the work on the Existent (Tltpi too ovtoq ), attributed to Protagoras by Porphyry (in Euseb. Prep. Ennui. x. 3, p. 468, Yiger). This work was directed against the Eleatics (Ilpoc. tovq iv rb bv XiyovTcte,), and was still extant in the time of Porphyry, who describes the argumentation of the book as similar to that of Plato, though without adding any more exact statements.
Doctrines. — With the peculiar philosophical opinions of Protagoras we obtain the most complete acquaintance from the Thewtetns of Plato, which was designed to re- fute it, and the fidelity of the quotations in which is confirmed by the much more scanty notices of Sextus Empiricus and others. The sophist started from the fundamental presupposition of Heraclitus that every- thing is motion, ami nothing besides or bevond it. and
that out of it everything comes into existence; that nothing at any time exists, but that everything is per- petually becoming (Plato, Thecet. p. 150, 152: Sextus Empiricus inaccurately attributes to him matter in a perpetual state of flux, iiXjj ptvori], Pyrrhon, Hyp. i, 217, 218). He then distinguished two principal kinds of the infinitely manifold motions, an active and a pas- sive; but premised that the motion which in one con- currence manifested itself actively will in another ap- pear as passive, so that the difference is. as it were, a fluctuating, not a permanent one (Theeet. p. 156, 157). From the concurrence of two such motions arise sensa- tion or perception, and that which is felt or perceived, according to the different velocity of the motion; and that in such a way that where there is homogeneity in what thus meets, as between seeing and color, hearing and sound (ibid. p. 150), the definiteness of the color and the seeing, of the perception and that which is per- ceived, is produced by the concurrence of corresponding motions (ibid. 150, d ; comp. 159, c). Consequently, we can never speak of Being and Becoming in themselves, but only for something (rivi), or of something (tivvq), or to something (irpoc n, p. 160, b; 150, c; 152, d; Arist. Metaph. ix, 3; Sext. Emp. Hyp. i, 210, 218). Therefore there is or exists for each only that of which he has a sensation, and only that which he per- ceives is true for him ( limit, p. 152. a: comp. Cratyl. p. 386; Aristocles, in Euseb. Prcep. Evang. xiv, 20; Cicero, Acad, ii, 40; Sext. Emp. I. c. and A dr. Math. vii, 03. 309, 388, etc.); so that as sensation, like its ob- jects, is engaged in a perpetual change of motion (Thecet. p. 152, b ; Sext. Emp. Hyp. i, p. 217, fol.), opposite as- sertions might exist, according to the difference of the perception respecting each several object (Arist. Metaph. iv, 5; Diog. Laert. ix, 5; Clem. Alex. Strom. v, 074, a; Senec. Epist. 88). The conclusions hitherto discussed, which he drew from the Heraclitean doctrine of eter- nal becoming, Protagoras summed up in the well-known proposition: the man is the measure of all things; of the existent, that they exist; of the non-existent, that they do not exist (Thecet. p. 152, a; 100, d; Cratyl. p. 385. e; Arist. Metaph. x, 1 ; xi, 0 ; Sext. Emp. Adv. Math, vii, 00 ; Pyrrhon. Hyp. i. 216 ; Aristocles, in Evseb. Prcep. Evang. xiv, 20 ; Diog. Laert. ix, 51) ; and under- stood by the man, the perceiving or sensation-receiving subject. He was compelled, therefore, likewise to ad- mit that confutation was impossible, since every affir- mation, if resting upon sensation or perception, is equal- ly justifiable (Plato, Eitthyd. p. 185, d, etc.; Isocr. He- lena, Enc. p. 231, Bekk.; Diog. Laert. ix, 53); but, not- withstanding the equal truth and justifiableness of op- posite affirmations, he endeavored to establish a distinc- tion of better and worse, referring them to the better or worse condition of the percipient subject, and promised to give directions for improving this condition, i. e. for attaining to higher activity (Thecet. p. 107 ; comp. Sext. Emp. Hyp. i, 218). Already, before Plato and Aristotle (Metaph. iv, 4; comp. the previously quoted passages), Democritus had applied himself to the confutation of this sensualism of Protagoras, which annihilated exist- tence, knowledge, and all understanding (Plutarch, Adv. Colot. p. 1109, a; Sext. Emp. Adv. Math, vii, 389).