Speusippus (died 339/338 bc) was a Greek philosopher who became head, or scholarch, of the Greek Academy after the death in 347 bc of Plato, who had founded it in 387. A nephew and disciple of Plato, Speusippus accompanied him on his journey to Sicily in 361. He was also a partisan in his uncle’s relations with political rulers, including Dionysius II of Syracuse.
Little survives of Speusippus’ philosophical writings except a long excerpt from his work On Pythagorean Numbers, a few other fragments, and reports by other writers. Like his contemporaries and early successors in the beginning years of the Academy, he stressed the importance of numbers and numerical combinations and deemphasized ideas. The excerpt from Numbers, for example, explains the “perfections,” or special importance, of the number 10.
According to Aristotle’s report, which is frequently criticized for representing Plato inaccurately, Speusippus adopted the Platonic doctrine asserting the timeless derivation of all reality from two opposite principles, often called “the One” and “the indeterminate dyad,” terms meant to explain the presence of both unity and multiplicity in the universe. His colleagues, however, viewed “the One” and “the dyad” as principles of good and evil, respectively, but Speusippus denied the attachment of moral qualities. Using numerical labels, he also organized reality into successively more spiritual spheres. Between the spheres of pure numbers, or “mathematicals,” and of the body, or “the sensible,” he inserted the sphere of the soul, considered immortal in all of its parts. Though Speusippus is strongly criticized by Aristotle, his Homoia (“Similitudes”), a comparative study of plant and animal physiology, has been favourably compared with Aristotle’s own History of Animals and conceivably reflects Speusippus’ view that no single thing can be defined unless all are, because classification and definition are closely related.