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position of Heracleitus, that every thing is motion, and nothing besides or beyond it, and that out of it every thing comes into existence ; that nothing at any time cueists, but that everything is perpetually becoming (Plat. TJieaet. pp. 156, 152: Sextus Em-piricus inaccurately attributes to him matter in a perpetual state of flux, vAfj pev<rT^, Pyrrlion. Hyp. i. 217, 218). He then distinguished two principal kinds of the infinitely manifold motions, an active and a passive ; but premised that the motion which in one concurrence manifested itself actively, will in another appear as passive, so that the difference is as it were a fluctuating, not a permanent one (Theaet. pp. 156,157). From the concurrence of two such motions arise sensation 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 colour, hearing }ind sound (ib. p. 156), the definiteness of the colour and the seeing, of the perception and that which is perceived, is produced by the concurrence of corresponding motions (p. 156, d., comp. 159, c.). Consequently, we can never speak of Being and Becoming in themselves, but only for something (rivi), or of something (riz/os), or to something (irpos t<, p. 160, b., 156, c., 152, d. ; Arist. Metaph. ix. 3; Sext. Emp. Hyp. i. 216, 218). Consequently there is or exists for each only that of which he has a sensation, and only that which he perceives is true for him (Theaet. p. 152, a.,comp. Cratyl. p. 386 ; Aristocles, in Euseb. Praep. Evang. xiv. 20; Cic. Acad. ii. 46 ; Sext. Emp. I.e. and adv. Math. vii. 63, 369, 388, &c.) ; so that as sensation, like its objects, is engaged in a perpetual change of motion (Theaet. p. 152, b. ; Sext. Emp. Hyp. i. p. 217, f.), opposite assertions might exist, according to the difference of the perception respecting each several object (Arist. Metaph. iv. 5 ; Diog. Laert. ix. 5 ; Clem. Alex. Strain, v. p. 674, a. ; Senec. Epist. 88). The conclusions hitherto discussed, which he drew from the Heracleitean doctrine of eternal 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 (Theaet. p. 152, a., 160, d., Cratyl. p. 385, e. ; Arist. Metaph. x. 1, xi. 6 ; Sext. Emp. adv. Math. vii. 60, Pyrrlion. Hyp. i. p. 216 ; Aristocles, in Euseb. Praep. Evang. xiv. 20 ; Diog. Laert. ix. 51), and understood by the man, the perceiving or sensation-receiving subject. He was compelled, therefore, likewise to admit, that confutation was impossible, since every affirmation, if resting upon sensation or perception, is equally justifiable (Plat. Euthyd. p. 185, d. &c. ; Isocr. Helenae Enc. p. 231, Bekk. ; Diog. Laert. ix. 53) ; but, notwithstanding the equal truth and justifiableness of opposite affirmations, he endeavoured to establish a distinction 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 (Theaet. p. 167 ; comp. Sext. Emp. Hyp. i. p. 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 existence, knowledge, and all understanding (Plut. adv. Colot. p. 1109, a.; Sext. Emp. adv. Math. vii. 389). When Protagoras, in his book on the Gods,
maintained that we are not able to know whether and how they exist (Timon, in Sesct. Emp. adv. Math. ix. 56, comp. 58 ; Cic. de Nat. Deor. i. 1, 12, 23, 42 ; Diog. Laert. ix. 51, &c. To regard the expression, OTroioi rives elan, quales sint, as Frei does, I. c. p. 98, as a foreign addition, seems to me to involve difficulties), he probably could only have in mind the mutually opposed statements on the point, and must himself have been disposed to a denial as he could scarcely have been conscious of a corresponding sensation or perception.
It is not every pleasure, but only pleasure in the beautiful, to which Protagoras, in the dialogue which bears his name (p. 351, b.), allows moral worth ; and he refers virtue to a certain sense of shame (a!$ws) implanted in man by nature, and a certain conscious feeling of justice (Si'/o?), which are to serve the purpose of securing the bonds of connection in private and political life (ibid. p. 322, c. &c.) ; and, accordingly, explains how they are developed by means of education, instruction, and laws (p. 325, c. &c., comp. 340, c.). He is not able, however, to define more exactly the difference between the beautiful and the pleasant, and at last again contents himself with affirming that pleasure or enjoyment is the proper aim of tliegood (p. 354, &c.). In just as confused a manner does he express himself with respect to the virtues, of which he admits five (holiness, ocnorrjs^—and four others), and with regard to which he maintains that they are distinguished from each other in the same way as the parts of the countenance (ib. p. 349, b., 329, c., &c.). As in these ethical opinions of Protagoras we see a want of scientific perception, so do we perceive in his conception of the Heracleitean doctrine of the eternal flow of all things, and the way in which he carries it out, a sophistical endeavour to establish, freed from the fetters of science, his subjective notions, setting aside the Heracleitean assumption of a higher cognition, and a community of rational activity (£wos At/yos), by means of rhetorical art. That he was master of this in a high degree, the testimonies of the ancients leave indubitable. His endeavours, moreover, were mainly directed to the communication of this art by means of instruction (Plat. Prot. p. 312, c.), to render men capable of acting and speaking with readiness in domestic and political affairs (ib. p. 318, e.). He would teach how to make the weaker cause the stronger (rov ijttw \6yov Kpeirrao Troje?*', Arist. Rhet. ii. 24 ; A. Gellius, N. A. v. 3 ; Eu-doxus, in Steph. Byz. s. v. "AGb'ripa ; comp. Aris-toph. JVw&.113, &c. 245, &c. 873, 874, 879, &c.). By way of practice in the art he was accustomed to make his pupils discuss Theses (communes loci) on opposite sides (antinomically) (Diog. Laert. ix. 52, &c. ; comp. Suid. s. v. ; Dionys. Halic. Isocr. Timon in Diog. Laert. ix. 52 ; Sext. Emp. adv. Math. ix. 57 ; Cic. Brut. 12) ; an exercise which is also recommended by Cicero (ad Att. ix. 4), and Quintilian (x. 5. § 10). The method of doing so was probably unfolded in his Art of Dispute (rex^ epicr-tikwj/, see above). But he also directed his attention to language, endeavoured to explain difficult passages in the poets, though not always with the best success (Plat. Prot. p. 388, c. &c. ; comp. respecting his and the oppo'sed Platonic exposition of the well-known lines of Simonides, Frei, p. 122. &c.) ; entered at some length into the threefold gender of names (appew, &?fAea, and (T/ceur?, Arist. Rhei. iii. 5, EL Soph. c. 14 ; comp. Aristoph. Nub.