s.[492] He secured an intimate acquaintance with the lofty
speculations of Pythagoras, under Archytas of Tarentum, and in the
writings of Philolaus, whose books he is said to have purchased. He
studied the principles of Parmenides under Hermogenes,[493] and he more
than once speaks of Parmenides in terms of admiration, as one whom he
had early learned to reverence.[494] He studied mathematics under
Theodoras, the most eminent geometrician of his day. He travelled in
Southern Italy, in Sicily, and, in search of a deeper wisdom, he pursued
his course to Egypt.[495] Enriched by the fruits of all previous
speculations, he returned to Athens, and devoted the remainder of his
life to the development of a comprehensive system "which was to combine,
to conciliate, and to supersede them all."[496] The knowledge he had
derived from travel, from books, from oral instruction, he fused and
blended with his own speculations, whilst the Socratic spirit mellowed
the whole, and gave to it a unity and scientific completeness which has
excited the admiration and wonder of succeeding ages.[497]

[Footnote 491: Cousin's "Lectures on the History of Philosophy," vol. i.
p. 31.]

[Footnote 492: Aristotle's "Metaphysics," bk. i. ch. vi.]

[Footnote 493: Diogenes Laertius, "Lives of the Philosophers," bk. iii.
ch. viii. p. 115.]

[Footnote 494: See especially "Theaetetus," Sec. 101.]

[Footnote 495: Ritter's "History of Ancient Philosophy," vol. ii. p.
147.]

[Footnote 496: Butler's "Lectures on Ancient Philosophy," vol. ii. p.
22.]

[Footnote 497: Encyclopaedia 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_
aisthesis--_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