Plato's dialogue in which Socrates and the young mathematician Theaetetus try, and repeatedly fail, to define what knowledge is.
The Theaetetus is framed as a written record: Euclides tells Terpsion he transcribed a conversation Socrates once had, and had it read aloud rather than narrated, so what follows plays out as direct dialogue between Socrates, the mathematician Theodorus, and his gifted young student Theaetetus. The question on the table is simple to ask and, it turns out, brutally hard to answer: what is knowledge? Socrates refuses to accept lists of things known (geometry, shoemaking, carpentry) as an answer, insisting the group search for what knowledge itself is, not what it is of.
Three candidate definitions get tested in turn and each collapses. First, knowledge as perception, which Socrates links to Protagoras's claim that 'man is the measure of all things' and to a Heraclitean flux doctrine where nothing is stably one thing; this is undone by problems of memory, dreaming, and the fact that perception can't account for judgments about number or being. Second, knowledge as true judgment, which runs aground on the puzzle of how false judgment is even possible. Third, knowledge as true judgment plus an account (logos), explored through an image of learning to read where letters (elements) and syllables (wholes) may or may not each have their own account - and this too dissolves under questioning.
Throughout, Socrates casts himself as a midwife: barren of doctrines himself, but skilled at helping others' ideas come to birth and at testing whether those ideas are real or 'wind-eggs.' The dialogue famously ends without a positive answer - Socrates must leave for the King's Portico to face the indictment brought by Meletus, the very case that leads to his trial and death in the Apology. It matters because it is Plato's most sustained, focused treatment of epistemology, and its refusal to resolve the question has made it a touchstone for how philosophy handles definitional failure.
“So can we say what it is? What do you all say? Which of us will speak first?”
— Theaetetus · read this chapter →
“he means something like this: as each thing appears to me, so it is for me, and as it appears to you, so it is for you”
— Theaetetus · read this chapter →
“haven't you heard that I'm the son of a midwife, a fine, sturdy woman named Phaenarete?”
— Theaetetus · read this chapter →
“you'll soberly not think you know what you don't know”
— Theaetetus · read this chapter →
“For now I must go to the King's Portico, to face the indictment Meletus has brought against me.”
— Theaetetus · read this chapter →
It is a dialogue devoted entirely to the question 'what is knowledge?' Socrates questions the young mathematician Theaetetus and tests three successive definitions - knowledge as perception, as true judgment, and as true judgment with an account - and shows each one fails, ending without a positive answer.
He is a talented young Athenian mathematician, a student of Theodorus, who is praised early in the dialogue for his quickness and gentleness rather than his looks. Socrates uses the conversation to draw out and test Theaetetus's own developing ideas about knowledge.
Socrates says he inherited midwifery from his mother Phaenarete and practices a philosophical version of it: he is barren of doctrines himself but skilled at helping others give birth to their own ideas and at judging whether those ideas are genuine or 'wind-eggs' not worth keeping.
No. The dialogue tests and rejects knowledge as perception, as true judgment, and as true judgment plus an account (logos), and ends with Socrates leaving to face his indictment before any definition is accepted.
Socrates links Theaetetus's first definition, knowledge as perception, to Protagoras's claim that 'man is the measure of all things' - that things are, for each person, as they appear to that person - and then subjects this relativism to sustained criticism.
The dialogue's frame explains that Euclides wrote it down after learning Theaetetus had been wounded and was dying, and the conversation itself ends with Socrates departing for the King's Portico to answer the indictment brought by Meletus, the case that leads to the events of the Apology.
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