Added to these moral qualifications, Plato had the further qualification of a comprehensive knowledge of all that had been achieved by his predecessors. In this regard he had enjoyed advantages superior to those of Socrates. Socrates was deficient in erudition, properly so called. He had studied men rather than books. His wisdom consisted in an extensive _observation_, the results of which he had generalized with more or less accuracy. A complete philosophic method demands not only a knowledge of contemporaneous opinions and modes of thought, but also a knowledge of the succession and development of thought in past ages. Its instrument is not simply psychological analysis, but also historical analysis as a counterproof.[491] And this erudition Plato supplied. He studied carefully the doctrines of the Ionian, Italian, and Eleatic schools. Cratylus gave him special instruction in the theories of Heraclitus.[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 "Theætetus," § 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: 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.