Theaetetus (dialogue)

• 1. Introduction

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  The Theaetetus is a philosophical dialogue written by Plato in the early-middle 4th century BCE, and it opens with a scene of aftermath: two men, Euclid of Megara and his friend Terpsion, watch a wounded soldier being carried home from battle at Corinth. That soldier is Theaetetus, a young Athenian mathematician, and he is dying. Euclid tells Terpsion that he has a written record of a conversation Theaetetus once had with Socrates, back when the mathematician was quite young. A slave reads that dialogue aloud to them. What follows is one of the founding works of epistemology: a sustained philosophical investigation into a question so basic it sounds almost childish. What is knowledge? Three answers are proposed, examined, and discarded. The dialogue ends without a resolution, as Socrates leaves to face the criminal charges that will soon cost him his life. The question of what it means to know something turns out to be far harder than anyone expected.

• 2. The Setting and Its Stakes

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  The frame story of the Theaetetus carries weight beyond mere scene-setting. Scholars have disagreed about which battle at Corinth the text refers to: one possibility is 369 BCE, which would place Theaetetus in his late forties at the time of the wound; another, argued by scholars including Debra Nails, points to an earlier battle in 391 BCE, when Theaetetus would have been in his late twenties. The difference matters because it changes how we understand the gap between the young man in the dialogue and the dying soldier in the frame. The dialogue itself is set on a very specific day: the same day as the Euthyphro, immediately before Socrates walks to the Porch of the King Archon to face a hearing for impiety. Socrates is described as an old man of about 70 at this point. The criminal charge against him is the one that will end in his execution in 399 BCE. By placing his investigation of knowledge on the last day of philosophical freedom Socrates would enjoy, Plato gives the abstract inquiry a concrete urgency.

• 3. The Characters

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  The dialogue takes place in a wrestling school, and Theodorus of Cyrene plays the role of introducer. 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. Theodorus was a mathematician who explored the theory of incommensurable quantities, and Diogenes Laertius records a claim that he taught mathematics to Plato himself, though the source notes the historicity of this cannot be verified. He describes the young Theaetetus to Socrates with deliberate understatement: the boy looks like Socrates, which in this dialogue is not a compliment. Theaetetus has a snub nose and protruding eyes, is intelligent and virile, and is an orphan whose inheritance has been squandered by his trustees. Euclid of Megara, who frames the entire narrative, was the founder of the Megarian school of philosophy and a student of Socrates. His companion Terpsion is unknown outside this dialogue, though later fables about him, probably written on the basis of this work, are preserved by Plutarch and in the Cynic epistles. The conversation between Socrates and Theodorus the following morning is continued in Plato's Sophist, where they are joined by an unnamed Eleatic stranger and a boy also named Socrates.

• 4. Knowledge as Perception

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  Socrates tells Theaetetus that he cannot make out what knowledge is and is looking for a simple formula. Theaetetus admits he has no idea how to answer. What follows is a demonstration of the Socratic method, which Socrates himself compares to midwifery: his mother delivered babies, and he delivers thoughts. The first definition Theaetetus offers is that knowledge is sense perception. Socrates immediately identifies this with Protagoras's maxim that man is the measure of all things, then drags in Homer and Heraclitus as fellow travelers in a philosophy of constant flux, where nothing is fixed and everything is in a process of coming to be. To handle the objection that when the same wind blows, one person feels cold and another does not, Socrates introduces Heraclitean flux as a supporting theory. He is careful to note that Protagoras's radical truth relativism and Heraclitus's radical reality relativism are different doctrines, though they can work together to prop up the perception-as-knowledge claim. Protagoras is dead by this point in the dialogue, and Socrates stages a mock defense of him, putting words in the sophist's mouth before conceding that Protagoras would have done a far better job himself. Theodorus, who describes himself as a friend but not a disciple of Protagoras, proves helpless against Socrates's arguments. Socrates draws the portrait of the philosopher here that has stayed famous: the absent-minded intellectual who cannot make his bed or cook a meal, unconcerned with neighborhood scandal or tracing ancestry back to Heracles, oriented entirely toward things that are truly higher up, like beauty and knowledge.

• 5. True Judgment and Its Limits

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  After the collapse of the perception argument, Theaetetus proposes that knowledge is true judgment, defined as belief that is free from mistakes. Socrates tests this with an image from the law courts: a lawyer who persuades a jury does not give the jurors knowledge of what happened; he produces conviction. Even if the persuasion happens to point toward the truth, the jurors have not thereby known anything, because they were not present at the events in question. True judgment and knowledge must be different things. To refine the definition, Theaetetus recalls being told that true judgment accompanied by an account, or logos, amounts to knowledge. To explore this, Socrates describes a dream in which primary elements can only be named; they cannot be known as existing or not. When combined, they form complexes that are knowable and expressible. He then works through the letters of his own name, taking S and O to ask whether the syllable formed from them can be known when the individual letters are not. The investigation leads to a reversal: if syllables cannot contain their letters, then the primary elements are in the same class as the complexes, and Socrates cites the example of a musician distinguishing individual notes to propose that elements are more clearly known than the combinations built from them, not less.

• 6. Three Meanings of Account

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  Socrates works through three different ways of understanding what an account might mean. The first is the most basic: making one's thought apparent vocally by means of words. But this definition collapses immediately, because everyone capable of speech can vocalize opinions, which would mean everyone capable of speaking has knowledge. The second definition is more demanding: giving an account means going through a thing element by element, identifying its parts. Socrates draws on Theaetetus's experience of learning to write, pointing out that misplacing individual letters of a name does not mean you know how to spell it. The third definition is that an account is the ability to state the mark by which an object differs from everything else: the Sun is distinct for its brightness, for instance. But this too fails. To know the distinguishing feature of something, you already need knowledge of it, which makes the definition circular. Socrates calls this silly and discards it. The dialogue ends not with a discovery but with a verdict: all the two of them have produced are what Socrates calls wind-eggs, and he must leave now to walk to the courthouse and face Meletus.

• 7. Wittgenstein and Later Reception

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  Edmund Gettier's 1963 paper, titled "Is Justified True Belief Knowledge," drew directly on Plato's Theaetetus and the Meno to develop what has become known as the Gettier problem: a class of scenarios in which a justified true belief is not sufficient for knowledge, precisely because justification and truth can coincide by accident. The paper revived philosophical attention to the Theaetetus in the 20th century. Ludwig Wittgenstein came to the dialogue from a different direction. He sent a copy to his friend Maurice O'Connor Drury with the observation that Plato in this dialogue is occupied with the same problems that he himself was writing about. He later quoted it in Philosophical Investigations. But Wittgenstein also offered a sharp criticism, telling O.K. Bouwsma that the young Theaetetus shows none of his supposed promise and has no fight in him at all, asking why he never makes a stand, and calling Socrates's interlocutors weaklings. The Theaetetus had already traveled a long way before Wittgenstein reached it. In the 1st century BCE, Eudorus of Alexandria quoted its definition of a philosopher as a likeness to a god to justify his synthesis of Platonism with Pythagorean teachings on virtue. Philo of Alexandria quoted from it frequently in the same period. An anonymous commentary, the earliest surviving Platonic commentary, dating from between the 1st century BCE and the 2nd century CE, compared the dialogue's treatment of knowledge to the definitions in the Meno. Iamblichus placed it in his Neoplatonic curriculum of twelve dialogues, where it followed the Cratylus and was classified as a theoretical work on logic. The dialogue also contains what is believed to be one of the very few extant references in the 4th century BCE to Epicharmus of Kos, whom Socrates calls the prince of Comedy, alongside Homer as the prince of Tragedy, and names both as great masters of either kind of poetry.