SPEUSIPPUS (4th century B.c.), Greek philosopher, son of Eurymedon and Potone, sister of Plato, is supposed to have been born c. 407 B.C. He was bred in the school of Isocrates, but when Plato returned to Athens c. 387, Speusippus became a member of the Academy. In 361, when Plato undertook his third and last journey to Sicily, Speusippus accompanied him. In 347 the dying philosopher nominated his nephew to succeed him as scholarch, and the choice was ratified by the school. Speusippus held the office until his death in 339.
Of Speusippus's many philosophical writings nothing survives except a fragment of a treatise On Pythagorean Numbers. We gather (A) In regard to his theory of being: (I) whereas Plato postulated as the basis of his system a cause which should be at once Unity, Good, and Mind, Speusippus distinguished Unity, the origin of things, from Good, their end, and both Unity and Good from controlling Mind or Reason; (2) whereas Plato recognized three kinds of numbers—firstly, ideal numbers, i.e., the "determinants" or ideas ; secondly, mathematical numbers, the abstractions of mathematics; and thirdly, sensible numbers, numbers embodied in things—Speusippus rejected the ideal num bers, and consequently the ideas; (3) Speusippus traced number, magnitude and soul each to a distinct principle of its own. (B)
In regard to his theory of knowledge : he held that a thing cannot be known apart from the knowledge of other things; for, that we may know what a thing is, we must know how it differs from other things, which other things must therefore be known; (5) accordingly, in the ten books of a work called "0/Iota, he attempted a classification of plants and animals; (6) the results thus obtained he distinguished at once from "knowledge" and from "sensation" holding that "scientific observation" though it cannot attain to truth, may, nevertheless, in virtue of a certain acquired tact, frame "definitions." (c) In regard to his theory of ethics: (7) he denied that pleasure was a good, but seemingly was not prepared to account it an evil.
Speusippus and his contemporaries exercised a far-reaching influence upon Academic doctrine. When they rejected their master's ontology and proposed to themselves as ends mere classificatory sciences which with him had been means, they bartered their hope of philosophic certainty for the tentative results of scientific experience.