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Theaetetus of Athens (; grc-gre, Θεαίτητος; c. 417 – c. 369 BC), possibly the son of Euphronius of the
Athenian Athens ( ; el, Αθήνα, Athína ; grc, Ἀθῆναι, Athênai (pl.) ) is both the capital and largest city of Greece. With a population close to four million, it is also the seventh largest city in the European Union. Athens dominates ...
deme In Ancient Greece, a deme or ( grc, δῆμος, plural: demoi, δημοι) was a suburb or a subdivision of Athens and other city-states. Demes as simple subdivisions of land in the countryside seem to have existed in the 6th century BC and ear ...
Sunium Cape Sounion (Modern Greek: Aκρωτήριο Σούνιο ''Akrotírio Soúnio'' ; grc, Ἄκρον Σούνιον ''Άkron Soúnion'', latinized ''Sunium''; Venetian: ''Capo Colonne'' "Cape of Columns") is the promontory at the southernmost ...
, was a
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
mathematician. His principal contributions were on
irrational Irrationality is cognition, thinking, talking, or acting without inclusion of rationality. It is more specifically described as an action or opinion given through inadequate use of reason, or through emotional distress or cognitive deficiency. T ...
lengths, which was included in Book X of Euclid's ''Elements'' and proving that there are precisely five regular convex polyhedra. A friend of
Socrates Socrates (; ; –399 BC) was a Greek philosopher from Athens who is credited as the founder of Western philosophy and among the first moral philosophers of the ethical tradition of thought. An enigmatic figure, Socrates authored no ...
and
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was a Greek philosopher born in Athens during the Classical period in Ancient Greece. He founded the Platonist school of thought and the Academy, the first institution ...
, he is the central character in Plato's eponymous Socratic dialogue. Theaetetus, like Plato, was a student of the Greek mathematician
Theodorus of Cyrene Theodorus of Cyrene ( el, Θεόδωρος ὁ Κυρηναῖος) was an ancient Greek mathematician who lived during the 5th century BC. The only first-hand accounts of him that survive are in three of Plato's dialogues: the '' Theaetetus'', th ...
. 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 The Gulf of Sidra ( ar, خليج السدرة, Khalij as-Sidra, also known as the Gulf of Sirte ( ar, خليج سرت, Khalij Surt, is a body of water in the Mediterranean Sea on the northern coast of Libya, named after the oil port of Sidra or ...
. 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 The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Ro ...
and the whole
Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...
basin. He evidently resembled
Socrates Socrates (; ; –399 BC) was a Greek philosopher from Athens who is credited as the founder of Western philosophy and among the first moral philosophers of the ethical tradition of thought. An enigmatic figure, Socrates authored no ...
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 Dysentery (UK pronunciation: , US: ), historically known as the bloody flux, is a type of gastroenteritis that results in bloody diarrhea. Other symptoms may include fever, abdominal pain, and a feeling of incomplete defecation. Complications ...
on his way home after fighting in an Athenian battle at
Corinth Corinth ( ; el, Κόρινθος, Kórinthos, ) is the successor to an ancient city, and is a former municipality in Corinthia, Peloponnese, which is located in south-central Greece. Since the 2011 local government reform, it has been part ...
, 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.
Debra Nails Debra Nails (born November 15, 1950) is an American philosophy professor who taught at Michigan State University. Nails earned her M.A. in philosophy and classical Greek from Louisiana State University before going on to earn a Ph.D. in philosophy ...
, ''The People of Plato''. Indianapolis: Hackett Publishing, 2002; pp. 275–278
The crater Theaetetus on the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
is named after him.


See also

*
List of speakers in Plato's dialogues following is a list of the speakers found in the dialogues traditionally ascribed to Plato, including extensively quoted, indirect and conjured speakers. Dialogues, as well as Platonic ''Epistles'' and '' Epigrams'', in which these individuals ap ...


References


External links


A Discussion of Theaetetus' Contributions to Euclid's ''Elements''
* Ancient Greek geometers 4th-century BC Athenians 410s BC births 369 BC deaths 4th-century BC mathematicians {{greece-mathematician-stub