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.

Top 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.

• 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.

• 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.

• 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.

• 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.

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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.

• 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.

• 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.

• 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.

• 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.

• 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.