The Sophists, who belonged to no particular school, laid hold on the elements of skepticism contained in both the pre-Socratic schools of philosophy, and they declared that "the σοφία" was not only unattainable, but that no relative degree of it was possible for the human faculties.[468] Protagoras of Abdera accepted the doctrine of Heraclitus, that thought is identical with sensation, and limited by it; he therefore declared that there is no criterion of truth, and _Man is the measure of all things_.[469] Sextus Empiricus gives the psychological opinions of Protagoras with remarkable explicitness. "Matter is in a perpetual flux, whilst it undergoes augmentations and losses; the senses also are modified according to the age and disposition of the body. He said, also, that the reason of all phenomena resides in matter as substrata, so that matter, in itself, might be whatever it appeared to each. But men have different perceptions at different periods, according to the changes in the things perceived.... Man is, therefore, the criterion of that which exists; all that is perceived by him exists; _that which is perceived by no man does not exist_."[470] These conclusions were rigidly and fearlessly applied to ethics and political science. If there is no Eternal Truth, there can be no Immutable Right. The distinction of right and wrong is solely a matter of human opinion and conventional usage.[471] "That which _appears_ just and honorable to each city, is so for _that city_, so long as the opinion prevails."[472] [Footnote 468: Encyclopædia Britannica, article "Sophist."] [Footnote 469: Plato's "Theætetus" (άνθρωπος--"the individual is the measure of all things"), vol. i. p. 381 (Bohn's edition).] [Footnote 470: Lewes's "Biographical History of Philosophy," p. 117.] [Footnote 471: "Gorgias," § 85-89.] [Footnote 472: Plato's "Theætetus," § 65-75.] There were others who laid hold on the weapons which Zeno had prepared to their hands. He had asserted that all the objects of sense were mere phantoms--delusive and transitory. By the subtilties of dialectic quibbling, he had attempted to prove that "change" meant "permanence," and "motion" meant "rest."[473] Words may, therefore, have the most opposite and contradictory meanings; and all language and all opinion may, by such a process, be rendered uncertain. One opinion is, consequently, for the individual, just as good as another; and all opinions are equally true and untrue. It was nevertheless desirable, for the good of society, that there should be some agreement, and that, for a time at least, certain opinions should prevail; and if philosophy had failed to secure this agreement, rhetoric, at least, was effectual; and, with the Sophist, rhetoric was "the art of making the worst appear the better reason." All wisdom was now confined to a species of "word jugglery," which in Athens was dignified as "the art of disputation."