Theaetetus

Summary

The Theaetetus begins with a death in prospect. At Megara, Euclides tells Terpsion that he has just met Theaetetus being carried home to Athens from the army at Corinth, badly wounded and sick with dysentery. The meeting reminds him of a conversation Socrates once had with Theaetetus as a boy and later recounted to him; Euclides had written it down, and a slave now reads it aloud. Everything that follows is that written record, set in 399 BC, on the day before Socrates goes to answer the charges brought by Meletus. Theodorus, the mathematician from Cyrene, praises his pupil Theaetetus as quick, gentle and brave, and plain-featured like Socrates. Socrates puts to the boy a single question: what is knowledge? Theaetetus first offers examples, geometry and shoemaking, and Socrates explains that a list is not a definition; the boy shows he understands by describing how he and a friend classified the incommensurable square roots. When he despairs of answering, Socrates tells him that he is a midwife's son and practises midwifery himself, delivering young men of their ideas and testing whether each is a true birth or a phantom. The first offspring is that knowledge is perception. Socrates links it to Protagoras' saying that man is the measure of all things and to the Heraclitean view that everything is in motion, and builds out a theory of perception as the meeting of two motions. He then tests it from many sides: dreams and madness, the sick man to whom wine tastes bitter, the absurdity that a pig or a dog-faced baboon would be as much a measure as a god, and Protagoras' own defence that the wise man, like a physician with drugs, changes worse states into better rather than false beliefs into true. A long digression contrasts the philosopher, who falls into wells while looking at the stars, with the clever man of the law courts, and urges the soul to become like God, holy, just and wise, as far as possible. The definition finally falls because knowledge requires the mind's own reckoning about being, sameness and difference, which the senses alone cannot supply. Theaetetus next proposes that knowledge is true judgment. This forces the question how false judgment is possible, and the images of the soul as a wax tablet and as an aviary full of birds are tried and found wanting. The example of jurors rightly persuaded by an orator about events they did not witness shows that true opinion is not yet knowledge. The last proposal, true judgment with an account, is examined through Socrates' 'dream' of letters and syllables and three senses of 'account', and it too collapses. The dialogue ends without a definition, but Socrates says the boy will be gentler and humbler for it, and leaves to meet his accuser.

Plot or Argument

• Prologue at Megara

  Euclides and Terpsion speak of Theaetetus, carried wounded and sick with dysentery from the army at Corinth, and recall Socrates' prophecy about him. Euclides' slave reads out the conversation Euclides recorded, written in direct dialogue without narrative tags (c. 142a-143c).

• The question and the young geometer

  Theodorus praises Theaetetus; Socrates asks what knowledge is. Theaetetus offers a list of sciences and crafts, is told that examples are not a definition, and shows his talent by recounting his classification of square roots incommensurable with the unit (c. 143d-148e).

• Socrates' art of midwifery

  Socrates, son of the midwife Phaenarete, describes his own barren midwifery of souls: he delivers young men's ideas, tests whether they are real or phantoms, and is rightly accused of asking questions without answering them (c. 148e-151d).

• Perception, Protagoras and flux

  Theaetetus proposes that knowledge is perception. Socrates joins this to Protagoras' 'man is the measure' and to a secret doctrine of universal motion, develops a theory of perception as the offspring of two motions, and raises puzzles about dreams, madness and illness (c. 151d-160e).

• Protagoras answers back

  After mocking objections, Socrates speaks a defence on Protagoras' behalf: no belief is truer than another, but some are better, and the wise man changes worse states into better as a physician does with drugs. Socrates then turns the measure doctrine against itself, since Protagoras must grant that those who think him wrong are right (c. 161a-171e).

• Digression: the philosopher and the pleader

  Socrates contrasts the leisured philosopher with the courtroom speaker driven by the water-clock. The philosopher is laughed at like Thales in the well, yet only he knows what justice is; evil cannot vanish from mortal life, so one must flee to become like God as far as possible (c. 172b-177c).

• The end of the first definition

  Protagoras' doctrine fails about the future, where experts predict better than laymen, and strict flux would leave language no stable meaning. The soul, not the senses, reckons being, likeness and difference, so knowledge cannot be perception (c. 177c-186e).

• True judgment and the puzzle of falsehood

  Theaetetus proposes that knowledge is true judgment. To explain false judgment Socrates tries the wax tablet, a gift of Memory, and the aviary of birds caught and kept in the mind; neither accounts for errors in pure thought, such as confusing eleven with twelve (c. 187a-200d).

• Jurors, the dream and the account

  Jurors persuaded rightly by an orator have true opinion without knowledge, so the two differ. Theaetetus suggests true judgment with an account; Socrates' dream of elements and syllables, and three senses of 'account' (speech, enumeration of parts, a distinguishing mark), all fail (c. 200d-210b).

• Wind-eggs and the porch of the King Archon

  Socrates pronounces the boy's offspring wind, but says the inquiry will leave him humbler and gentler. He leaves to meet the charges brought by Meletus, promising to resume the next morning, which is the opening of the Sophist (c. 210b-d).

Characters

• Socrates, questioner

  Seventy years old and facing prosecution, he presents himself as a barren midwife of ideas. Playful, demanding and unusually tender with a promising young man.

• Theaetetus, young respondent

  An Athenian youth of rare mathematical gifts, later a celebrated geometer. Modest, persistent and honest about his perplexity; the prologue shows him years later, brave in battle and near death.

• Theodorus, mathematician of Cyrene

  Theaetetus' teacher and an old friend of Protagoras. He prefers geometry to argument and is drawn in reluctantly to defend his friend's doctrine.

• Euclides, narrator of the frame

  A follower of Socrates from Megara who recorded the conversation and checked it with Socrates; he has just seen the dying Theaetetus.

• Terpsion, listener in the frame

  Euclides' friend, who asks to hear the conversation and laments the loss Theaetetus will be.

• Protagoras, absent thinker

  The famous sophist, dead by the dramatic date, whose saying 'man is the measure of all things' Socrates both defends and refutes, at one point imagining him popping up from the ground to protest.

Lessons

• Examples are not yet a definition

  Asked what knowledge is, Theaetetus names geometry, astronomy and the crafts of the cobbler and carpenter. Socrates replies that he asked for one thing and was given many; to say 'knowledge of shoes' already uses the word being defined. Theaetetus then shows he grasps the point by recalling how he and a friend gathered an infinite number of square roots under one description. Students practise the difference between listing instances and stating what they share, a skill as useful in mathematics as in moral reasoning.

  • Why does Socrates object that 'knowledge of shoemaking' is a circular answer?
  • How does Theaetetus' work on incommensurable roots show that he can define after all?
  • Try to define 'courage' first by examples, then by a single account. What changes?

• The midwife who does not give birth

  Socrates claims no wisdom of his own: like a midwife past childbearing he helps others deliver and judges whether the child is real. The image explains his method of questioning and also its honesty, since a teacher who tests ideas must be willing to pronounce some of them phantoms, even when the young man is fond of them. Socrates says some quarrel with him for taking away a favourite notion, as mothers resist losing a child, and that he does it because he may not allow falsehood or hide the truth.

  • In what ways does Socrates say his art is like a midwife's, and in what ways different?
  • Why might a student resent having an idea tested, even if the testing is kind?
  • What do teachers and students owe each other when an answer turns out to be a 'wind-egg'?

• Is each person the measure?

  Protagoras' saying that man is the measure of all things fits the first definition well: if knowledge is perception and each perceives differently, then what seems to each is true for each. Socrates takes the view seriously enough to build its strongest form and to speak a speech in its defence. He then shows its self-defeating edge, since Protagoras must allow that those who think the measure doctrine false are right, and its failure about the future, where the physician's or vine-dresser's forecast beats a layman's. Students learn to treat a rival view fairly before rejecting it.

  • What does Socrates gain by speaking a defence on Protagoras' behalf before refuting him?
  • Explain the 'self-refutation' argument in your own words. Is it fair to Protagoras?
  • Where in ordinary speech do people say 'true for me'? Is that the same as Protagoras' claim?

• Becoming like God as far as possible

  In the digression Socrates contrasts the courtroom speaker, shrewd but small-souled, with the philosopher, clumsy in the market but free. Because evil always remains in mortal life, he says, we should flee from earth to heaven, and to flee is to become like God, holy, just and wise, so far as a human being can. God is never unrighteous, and the most righteous man is most like him. Here Plato's argument touches a theme Christian readers later took up through Scripture's call to holiness, but the passage should first be read on its own terms, as Socrates' account of why justice matters more than reputation.

  • Why does Socrates call the lawyer a 'servant' and the philosopher a freeman?
  • What does Socrates mean by saying one should seek virtue not merely in order to seem good?
  • Compare Socrates' 'become like God' with 1 Peter 1:15-16. Where do the two agree, and where does the source of holiness differ?

• True opinion is not yet knowledge

  Jurors persuaded by a skilled speaker about a theft they did not witness may judge correctly, yet they do not know. The example separates being right from knowing why one is right. Socrates' final attempts to say what must be added, an 'account', all fail, but the failure is instructive: knowledge seems to require grasping a thing's distinguishing reason, and that cannot be borrowed from someone else's persuasion.

  • Why does the jury example show that true opinion and knowledge are not the same?
  • Have you ever been right for the wrong reason? What would it have taken to know?
  • The dialogue ends without a definition. Does Socrates think the conversation was wasted? What does he say Theaetetus has gained?

Context

The frame refers to fighting near Corinth, almost certainly the campaign of 369 BC, so the historical Theaetetus, a real Athenian mathematician credited by later Greek sources with work on irrational magnitudes and the regular solids, appears to have died then. The inner conversation is set in 399 BC, just before Socrates' trial, and its closing line leads into the Euthyphro and, the next morning, the Sophist. The dialogue engages Protagoras (fifth-century sophist), the Heraclitean doctrine of flux, and contemporary mathematics associated with Theodorus of Cyrene.

Composition

Written after 369 BC, probably not long after, and generally placed after the Republic and before the Sophist and Statesman. Euclides explains that he has written the conversation as direct dialogue, dropping 'he said' and 'I said', which some read as Plato's comment on his own method. The library's text is Jowett's translation from The Dialogues of Plato (3rd ed., 1892), preceded in the Gutenberg file by his Introduction and Analysis.

Reception

Ancient commentators debated whether the dialogue was purely aporetic or concealed a doctrine; a partial ancient commentary survives on papyrus. The 'become like God' passage was widely cited by Middle Platonists and by early Christian writers such as Clement of Alexandria. Modern epistemology has returned repeatedly to its themes, from the wax tablet image of memory to the definition of knowledge as justified true belief, a formula often traced to the dialogue's last proposal. It is included in Great Books of the Western World (2nd ed., 1990), volume 6.

Comparisons

• Sophist

  The Sophist is set the next morning with the same respondent and solves the problem the Theaetetus leaves open: it explains false statement by distinguishing name and verb and by treating not-being as difference, where the wax tablet and aviary had failed.

• Meno

  The Meno also distinguishes true opinion from knowledge, and says true opinions are 'tied down' by reasoning about the cause. The Theaetetus tests a similar formula, true judgment with an account, and cannot make it work, which makes the two dialogues a natural pair for discussion.

• The Last Days of Socrates

  Socrates leaves the Theaetetus for the porch of the King Archon, where the Euthyphro begins. Read in sequence with the Apology, the midwife's humble account of himself becomes the defence speech of a man who says he knows only that he does not know.

• Posterior Analytics and Physics

  Aristotle's Posterior Analytics gives the positive account the Theaetetus withholds: scientific knowledge is demonstration from first principles that are grasped by understanding, not by sensation, and it explains why a thing is so.

• Six Metaphysical Meditations, with Hobbes's Objections and the Replies

  Descartes' First Meditation revives the dream argument Socrates raises, how one can tell waking from dreaming, and his wax example in the Second Meditation uses wax for a different purpose: not as memory's tablet but to show that the mind, not the senses, grasps what a body is.

Quotes

Medical Lens

The Theaetetus is an epistemological dialogue, but medicine supplies several of its frames and test cases. The prologue shows Theaetetus carried home wounded and suffering from dysentery in the army, a reminder of camp disease as a killer in Greek warfare. Socrates' self-portrait draws on the practice of Athenian midwives, including their use of drugs and incantations to ease or hasten labour. Illness, dreaming and madness are used to test whether every perception is true: the sick man tastes the same wine as bitter, and madmen think themselves gods. In Protagoras' defence the sophist's words are compared to the physician's drugs, an early statement of speech as treatment. The wax tablet makes memory a physical impression that can be too hard, too soft, muddy or small, a model of individual differences in learning and recall. These are philosophical uses of medical material; they should not be read as clinical doctrine.

• Prologue, c. 142a-b

  Euclides reports that Theaetetus, wounded at Corinth, is suffering more from the sickness in the army; Terpsion names it dysentery.

  Fever Infection WAR Wounds
• Midwife speech, c. 149a-151d

  Socrates describes Athenian midwives: past childbearing themselves, able to bring on or relieve pains with drugs and incantations, and skilled at telling who is pregnant.

  Childbirth Maternal Pharmacology Poisons
• Objections to perception, c. 157e-160c

  Dreams, disease and madness are raised against the theory, including madmen who think they are gods and the wine that tastes sweet to Socrates well and bitter to Socrates sick.

  Dreams Nightmares Madness Psychosis Mind Body
• Defence of Protagoras, c. 166d-167c

  The wise man is compared to a physician who changes the sick man's state with drugs; the sophist does by words what the physician does by medicine.

  Physicians Doctors Pharmacology Poisons Philosophy AS Therapy
• End of Protagoras' doctrine, c. 178b-179a

  Expert prediction about the future, such as a physician's forecast of whether a patient will develop a fever, shows that not every person is equally a measure.

  Physicians Doctors Fever Infection
• Wax tablet, c. 191c-195a

  Memory is modelled as impressions in wax of differing size, hardness and purity; Socrates explains quick learners, forgetful ones and confused ones by the quality of their wax.

  Dementia Memory Mind Body

Creative Prompts

• Dialogue

  Write a one-page conversation in which Theaetetus, recovering briefly on the road from Corinth, tells Euclides what he still remembers of Socrates' questions and whether he ever found a definition of knowledge.

• Classroom

  Stage a mock trial in which jurors hear only an advocate's account of an event and then a witness's. Afterward, discuss which jurors had true opinion and which had knowledge, using Socrates' jury example.

• Essay

  Socrates says the inquiry left Theaetetus 'humbler and gentler'. In 500 words, describe a time when learning that you did not know something changed how you treated other people.

Further Reading

• Sophist, Plato

  The conversation of the next morning, in which an Eleatic visitor explains how false statement is possible.

• Meno, Plato

  An earlier treatment of true opinion and knowledge, with the image of Daedalus' statues that run away unless tied down.

• Euthyphro and Apology, Plato

  The scene at the porch of the King Archon and the trial that follows.

• Introduction and Analysis to the Theaetetus, Benjamin Jowett

  Printed before the dialogue in the library's Gutenberg text.

• Plato's Theory of Knowledge, F. M. Cornford

  Translation of the Theaetetus and Sophist with running commentary (1935).