c. 417-c. 369 B.C.

Greek Mathematician

Believed to have influenced Euclid's work in books X and XI of the Elements, Theaetetus studied what Pappus described as "The commensurable and the incommensurable, the rational and irrational continuous quantities"—that is, rational and irrational numbers. Student of Theodorus of Cyrene, he was a friend and associate of both Socrates and the latter's pupil Plato.

In fact Plato (427-347 B.C.), who clearly admired him, is the principal source regarding Theaetetus, who became a central figure in two Platonic dialogues, Theaetetus and the Sophist. In the first of these, Plato recorded a discussion between Socrates (c. 470-390 B.C.), Theodorus (465-398 B.C.), and Theaetetus that apparently occurred in 399 B.C.

Plato noted that Theaetetus's father was a wealthy man named Euphronius of Sunium who had left his son a large fortune. Trustees of the will had...