Theaetetus of Athens (/ˌθiːɪˈtiːtəs/; ‹See Tfd›Greek: Θεαίτητος Theaítētos; c. 417 – c. 369 BCE),[1] possibly the son of Euphronius of the Athenian deme Sunium, was a Greek mathematician. His principal contributions were on irrational lengths, which was included in Book X of Euclid's Elements and proving that there are precisely five regular convex polyhedra.[2] A friend of Socrates and Plato, he is the central character in Plato's eponymous Socratic dialogue.[3]
Theaetetus, like Plato, was a student of the Greek mathematician Theodorus of Cyrene. Cyrene was a prosperous Greek colony on the coast of North Africa, in what is now Libya, on the eastern end of the Gulf of Sidra. Theodorus had explored the theory of incommensurable quantities, and Theaetetus continued those studies with great enthusiasm; specifically, he classified various forms of irrational numbers according to the way they are expressed as square roots. This theory is presented in great detail in Book X of Euclid's Elements.
Theaetetus was one of the few Greek mathematicians who was actually a native of Athens. Most Greek mathematicians of antiquity came from the numerous Greek cities scattered around the Ionian coast, the Black Sea and the whole Mediterranean basin.
He evidently resembled Socrates in the snubness of his nose and bulging of his eyes. This and most of what is known of him comes from Plato, who named a dialogue after him, the Theaetetus. He apparently died from wounds and dysentery on his way home after fighting in an Athenian battle at Corinth, now presumed to have occurred in 369 BC; some scholars argue alternately for 391 BC as his date of death, the date of an earlier battle at Corinth.[4]
The crater Theaetetus on the Moon is named after him.
Thesleff, Holger (1989). "Platonic Chronology". Phronesis. 34 (1): 1–26. doi:10.1163/156852889X00017. p. 18, footnote 67: I find it essential to note that the historians of mathematics who take for granted that Theaitetos was still active in the 370s must be wrong. He made some important discoveries as a young man, and Plato and his friends were deeply impressed by this. But he is likely to have died in 390 B.C., and not likely to have written anything. When Plato wrote his first draft of the dialogue, presumably in the 370s, he saw this friend of his youth from an idealizing perspective, somewhat in the way he remembered his uncle Charmides.