[Footnote 493: ][ (return) ] Diogenes Laertius, "Lives of the Philosophers," bk. iii. ch. viii. p. 115.
[Footnote 494: ][ (return) ] See especially "Theætetus," § 101.
[Footnote 495: ][ (return) ] Ritter's "History of Ancient Philosophy," vol. ii. p. 147.
[Footnote 496: ][ (return) ] Butler's "Lectures on Ancient Philosophy," vol. ii. p. 22.
[Footnote 497: ][ (return) ] Encyclopædia Britannica, article "Plato."
The question as to the nature, the sources, and the validity of human knowledge had attracted general attention previous to the time of Socrates and Plato. As the results of this protracted controversy, the opinions of philosophers had finally crystallized in two well-defined and opposite theories of knowledge.
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: ][ (return) ] "Theætetus," § 23.
[Footnote 499: ][ (return) ] Ibid., §§ 25, 26.