Theaetetus

What is knowledge? Socrates puts the question to Theaetetus, a brilliant young mathematician who resembles him in his snub nose and bulging eyes, and they examine three answers: that knowledge is perception, that it is true belief, and that it is true belief with an account. Along the way Socrates describes himself as a midwife who helps others give birth to ideas, and examines Protagoras's claim that 'man is the measure of all things'. Each answer fails, and the dialogue ends in an honest puzzle.

Written around 369 BC in memory of the real Theaetetus, who had died after a battle, the dialogue is the founding work of the theory of knowledge. The English version is Benjamin Jowett's. Read Sophist, which continues it, and the Meno.

Plato (c. 428–348 BC) was born into an aristocratic family in Athens and became the devoted student of Socrates. After his teacher's trial and execution in 399 BC, he travelled, returned to Athens and founded the Academy, a school that lasted for centuries and counted Aristotle among its students.

Almost all his writings are dialogues in which Socrates questions his fellow citizens about justice, love, knowledge and the soul, from the Apology to the Symposium and the Republic. Known to Arab philosophers as Aflatun, he was read closely by al-Farabi and Ibn Rushd, and Western philosophy has been called a series of footnotes to him.