The Theaetetus (; Theaítētos, lat. Theaetetus) is a philosophical work written by Plato in the early-middle 4th century BCE that investigates the nature of knowledge and is considered one of the founding works of epistemology. Like many of Plato's works, the Theaetetus is written in the form of a dialogue, in this case between Socrates and the young mathematician Theaetetus and his teacher Theodorus of Cyrene. In the dialogue, Socrates and Theaetetus attempt to come up with a definition of episteme, or knowledge, and discuss three definitions of knowledge: knowledge as nothing but perception, knowledge as true judgment, and, finally, knowledge as a true judgment with an account. Each of these definitions is shown to be unsatisfactory as the dialogue ends in aporia as Socrates leaves to face a hearing for his trial for impiety.
As one of the major works of Plato's theory of knowledge, the Theaetetus was influential on Platonism from at least the time of the Skeptical Academy of the 3rd century BCE through the Neoplatonism of the 6th century CE. It has also been the subject of increased attention in modern times as a result of its influence on Edmund Gettier, who challenged the existing definitions of knowledge as a "justified true belief" in a paper that investigated Plato's theory of knowledge as outlined in this work.
Date and setting The Theaetetus is one of the few works of Plato that gives contextual clues on the timeline of its authorship: the dialogue is framed by a brief scene in which Euclid of Megara and his friend Terpsion witness a wounded Theataetus returning on his way home after from fighting in an Athenian battle at Corinth, from which he apparently died of his wounds. Euclid tells his friend that he has a written record of a dialogue between Socrates and Theaetetus, which occurred when Theaetetus was quite a young man. The dialogue is then read aloud to the two men by a slave owned by Euclid.
The exact timeline of which battle at Corinth has been a matter of some disagreement between scholars: it has generally been presumed that the battle occurred in 369 BC; when Theaetetus would have been in his late forties, but more recently, scholars including Debra Nails have argued that the battle referenced in the dialogue was a different battle in Corinth that occurred much earlier, in 391 BC, when Theatetus would have been in his late 20s.Debra Nails, The People of Plato. Indianapolis: Hackett Publishing, 2002; pp. 275–278
As far as internal characteristics, the dialogue is considered to be similar to the "earlier" dialogues of Plato, such as the Euthyphro or the Crito, in that Plato's Socrates discusses the nature of knowledge in the Theaetetus without giving any of his own views, and the dialogue ultimately ends in aporia without a satisfying answer. However, it also resembles many of the more philosophically complex "later" dialogues such as Parmenides, Phaedrus and the dialogues Sophist and Statesman which serve as a narrative sequel.
Characters of the dialogue Euclid of Megara - the founder of the Megarian school of philosophy, a student of Socrates and contemporary of Plato. Terpsion - a friend of Euclid who is unknown outside of this dialogue, although later fables about him, that were likely written on the basis of this dialogue are preserved by PlutarchPlutarch, de Gen. Socr. and in the Cynic epistles. Socrates - the famous teacher of Plato, who was executed by the people of Athens in 399 BC. In the dialogue, which takes place just before Socrates visits the Palace of the King Archon for his trial, Socrates is an old man of about 70. Theodorus - a Greek mathematician from Cyrene, 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 explored the theory of incommensurable quantities, and according to Diogenes Laertius, was said to have taught mathematics to Plato, although the historicity of this claim cannot be verified. Theaetetus - A Greek mathematician from Athens, who is credited in Book X of Euclid's Elements with developing a method for measuring irrational lengths in terms of square roots, as well as the proof that there are precisely five regular convex polyhedra.Greek Geometry from Thales to Euclid by George Johnston Allman (Hodges, Figgis, & Company, 1889, p. 206). According to the dialogue, he evidently resembled Socrates in the snubness of his nose and bulging of his eyes. He apparently died from wounds and dysentery after the battle in Corinth that occurs in the frame story of the dialogue.
Socrates, Theaetetus, and Theodorus reappear the following day in the Sophist an apparent continuation of the conversation contained within the book of Euclid, where they are also joined by an unnamed Eleatic stranger and a boy also named Socrates.
Outline The narrative of the dialogue is set in a wrestling school. Socrates asks Theodorus if he knows of any geometry students who show particular promise. Theodorus assures him that he does, but that he does not want to over-praise the boy, lest anyone suspect he is in love with him. He says that the boy, Theaetetus, is a young Socrates look-alike, rather homely, with a snub-nose and protruding eyes. The two older men spot Theaetetus rubbing himself down with oil, and Theodorus reviews the facts about him, that he is intelligent, virile, and an orphan whose inheritance has been squandered by trustees. Socrates declares Theaetetus will have benefited from discovering what he does not know, and that he may be better able to approach the topic in the future. The conversation ends with Socrates' announcement that he has to go to court to face a criminal indictment.
Socrates tells Theaetetus that he cannot make out what knowledge is, and is looking for a simple formula for it. Theaetetus says he really has no idea how to answer the question, and Socrates tells him that he is there to help. Socrates says he has modelled his career after his midwife mother. She delivered babies and for his part, Socrates can tell when a young man is in the throes of trying to give birth to a thought. Socrates considers his philosophical work as midwifery (Maieutics). This method, later also called Socratic method, consists in eliciting knowledge by a series of questions and answers.
Protagoras' theory of knowledge Socrates thinks that the idea that knowledge is perception must be identical in meaning, if not in actual words, to Protagoras' famous maxim "Man is the measure of all things."
Socrates wrestles to conflate the two ideas, and stirs in for good measure a claim about Homer being the captain of a team of Heraclitan flux theorists. Socrates dictates a complete textbook of logical fallacies to the bewildered Theaetetus. When Socrates tells the child that he (Socrates) will later be smaller without losing an inch because Theaetetus will have grown relative to him, the child complains of dizziness. In an often quoted line, Socrates says with delight that "wonder (thaumazein) belongs to the philosopher". He admonishes the boy to be patient and bear with his questions, so that his hidden beliefs may be yanked out into the bright light of day. When Socrates sums up what they have agreed on so far, it becomes problematic that knowledge is sense perception, for Socrates raises the question that "When the same wind blows, one of us feels cold and the other not?" As a result, he introduces the idea of Heraclitean flux to act as a defense to the wind objection. Heracliteanism shows that "Nothing is in itself just one thing... Everything is in a process of coming to be". Thus as there is no fixed meaning in things, but they draw their meaning in a referential difference to other things, the wind objection can be incorporated into Theaetetus's claim that "Knowledge is sense perception". As a result, they can then continue their inquiry as to the truth of this claim. It is important to note that the Heraclitean doctrine of Flux is not the same as the Protagorean doctrine. The Protagorean is radical truth relativism whereas the Heraclitean is radical reality relativism. It serves as a supporting theory to the Protagorean interpretation of Theaetetus's claim, in order that they might fully inquire as to the validity of this premise. Socrates admits that it is unfortunate that Protagoras is dead and cannot defend his idea against people such as himself. He says that the two of them are "trampling on his orphan" but the charge remains. Since Protagoras is dead, Socrates puts himself in the sophist's shoes and tries to do him the favor of defending his idea. Socrates concedes that if Protagoras were still alive, he would have more to say in his own defense, and that they are now essentially mistreating "his orphan child." Putting words in the dead sophist's mouth, Socrates declares that Protagoras asserts with his maxim that all things are in motion and whatever seems to be the case, is the case for the perceiver, whether the individual or the state. At the end of his speech, Socrates admits to Theodorus that Protagoras would have done a far better job of defending his own ideas. Theodorus tells Socrates that he must be kidding, that he has come to the task with boyish vigor. Theodorus does not claim to be a disciple of Protagoras, but states that he was a friend. Socrates invites Theodorus to put up a more vigorous defense of Protagoras, as he does not want it suggested that he has used the child's timidity (of Theaetetus) to aid him in his argument against the doctrine of Protagoras. Socrates, not at all certain that he has not misrepresented Protagoras in making each man the measure of his own wisdom, presses Theodorus on the question of whether any follower of Protagoras (himself included) would contend that nobody thinks anyone else is wrong. Theodorus proves to be helpless against Socrates' arguments. He agrees that Protagoras concedes that those who disagree with him are correct. In making Protagoras a complete epistemological relativist, where every person's individual perceptions are his reality and his truth, both Socrates and Theodorus paint Protagoras as maintaining an absurd position.
Socrates then proceeds to explain why philosophers seem clumsy and stupid to the common lot of humanity. Socrates explains that philosophers are open to mockery because they are not concerned about what interests most people: they could not care less about the scandals in their neighbor's house, the tracing of one's ancestry to Heracles, and so on. In contrast, the philosopher is concerned with things that are, such as beauty and knowledge, which are "truly higher up". It is here that Socrates draws the classic portrait of the absent-minded intellectual who cannot make his bed or cook a meal (175e). Socrates adds a big bifurcation to this speech, saying that there are only two kinds of lives to be lived: a divinely happy one, lived by righteous philosophers or a godless, miserable one, such as most people live. Socrates admits this was a digression that threatens to drown his original project, which was to define knowledge. Theodorus, the old geometer, tells Socrates that he finds this sort of thing easier to follow than his earlier arguments. Socrates says that the men of flux, like Homer and Heraclitus, are really hard to talk to because you can't pin them down. When you ask them a question, he says, they pluck from their quiver a little aphorism to let fly at you, and as you try to figure that one out, they wing another one at you. They leave nothing settled either in discourse, or in their own minds. Socrates adds that the opposite school of thought, that teaches of the "immovable whole" is just as hard to talk to. Socrates says he met the father of the idea, Parmenides, when he was quite young, but does not want to get into another digression over it.
Definitions of knowledge