1. That which reduced all knowledge to the accidental and passively receptive quality of the organs of sense and which asserted, as its fundamental maxim, that "_Science consists in_ αἴσθησις--_sensation_."[498] This doctrine had its foundation in the physical philosophy of Heraclitus. He had taught that all things are in a perpetual flux and change. "Motion gives the appearance of existence and of generation." "Nothing _is_, but is always a _becoming"_[499] Material substances are perpetually losing their identity, and there is no permanent essence or being to be found. Hence Protagoras inferred that truth must vary with the ever-varying sensations of the individual. "Man (the individual) is the measure of all things." Knowledge is a purely relative thing, and every man's opinion is truth for him.[500] The law of right, as exemplified in the dominion of a party, is the law of the strongest; fluctuating with the accidents of power, and never attaining a permanent being. "Whatever a city enacts as appearing just to itself, this also is just to the city that enacts it, so long as it continues in force."[501] "The just, then, is nothing else but that which is expedient for the strongest."[502] [Footnote 498: "Theætetus," § 23.] [Footnote 499: Ibid., §§ 25, 26.] [Footnote 500: Ibid., §§ 39, 87.] [Footnote 501: Ibid., § 87.] [Footnote 502: "Republic," bk. i. ch. xii.] 2. The second theory is that which denies the existence (except as phantasms, images, or mere illusions of the mind) of the whole of sensible phenomena, and refers all knowledge to the _rational apperception of unity_ (τὸ ἔν) _or the One_. This was the doctrine of the later Eleatics. The world of sense was, to Parmenides and Zeno, a blank negation, the _non ens_. The identity of thought and existence was the fundamental principle of their philosophy. "Thought is the same thing as the cause of thought; For without the thing in which it is announced, You can not find the thought; for there is nothing, nor shall be, Except the existing."[503] [Footnote 503: Parmenides, quoted in Lewes's "Biog. History of Philosophy," p. 54.] This theory, therefore, denied to man any valid knowledge of the external world.