Theaetetus
2. An Athenian, the son of Euphronius of Sunium, is introduced as one of the speakers in Plato's Theaeteius and Sophistes, in which dialogues he is spoken of as a noble, courageous, and well-disposed youth; in person somewhat like Socrates; and ardent in the pursuit of knowledge, especially in the study of geometry. (Plat. Theaet. pp. 143, 144. et alib.; Sophist. passim; Polit. pp. 257, 258, p. 266a.) Diogenes Laertius (2.29) mentions him as an example of the happy effects of the teaching of Socrates. Eusebius (Chron.) places "Theaetetus the mathematician" at Ol. 85, B. C. 440, a date which can only be accepted as referring, not to the time when he really flourished, but when, as a mere youth, he became the disciple of Socrates. (Comp. Fabric. Bibl. Graec. vol. iii. p. 78, note.)
From William Smith (ed.), Dictionary of Greek and Roman Biography and Mythology. Text provided under a CC BY-SA license by Perseus Digital Library, http://www.perseus.tufts.edu, with funding from the National Endowment for the Humanities and Tufts University. Data accessed from https://github.com/PerseusDL/canonical-pdlrefwk/ on 17 October 2026.