39 pages 1 hour read



Nonfiction | Book | Adult | BCE

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Summary and Study Guide


Theaetetus is a philosophical work written by Ancient Greek philosopher Plato (427-347 BCEE). Written in 369 BCEE, it is an account of a dialogue between the Greek philosopher Socrates (470-399 BCEE) and a young geometry student, Theaetetus, about the nature of knowledge. Socrates asks Theaetetus questions that lead them to discuss, and assess, several theories and definitions of knowledge. These are, first, that knowledge is perception, that knowledge is true judgment, and that knowledge is judgment with an account. The dialogue ends with the conclusion that none of these definitions are adequate. However, Socrates also concludes that they are now aware of the general human ignorance on this topic.

This guide uses the Oxford University Press edition translated by John McDowell and published in 2014. The original text of Theaetetus is a continuous dialogue, containing no chapter headings. However, the 2014 Oxford University Press edition has 35 headings provided by the editor. The following guide uses these to organize the text into chapters.


In Chapters 1-7 Plato introduces the dialogue between Socrates and Theaetetus via Eucleides and Terpsion, two Greek citizens. The former recalls Socrates telling him about a discussion between himself and Theaetetus, which Eucleides wrote down, and which they examine. In the dialogue Socrates asks Theaetetus what he thinks knowledge is. Socrates explains that he sees his own role as a philosopher as akin to that of a midwife. That is, he helps others “give birth” to ideas but does not produce positive knowledge himself. Following this, Theaetetus attempts to define knowledge: knowledge is perception.

Chapters 8-15 look at the flux theory, which holds that everything changes, to provide a proper explanation of Protagoras’s definition of knowledge as perception. Perception is created when the motion of an object interacts with a sense organ. Socrates then explores some objections to Protagoras’s theory of knowledge. Chief among these is the problem of misperception. Namely, how does his theory account for the possibility of a perception that does not correspond to reality? There is also the issue of how any person can be wiser than another in Protagoras’s theory. Socrates then assesses the response to this criticism that wisdom lies in what is better or more useful instead of what is true.

In Chapters 16-18 Socrates criticizes the idea that wisdom is the promotion of healthier or more useful states rather than the truth. He argues that to justify the former claims about what is useful must be used. Such claims must either appeal to an independent standard of truth or further claims to usefulness, creating an infinite regress. There is also a digression about how philosophers differ from ordinary people. To fully refute Protagoras, the flux theory underpinning his definition of knowledge is criticized. Socrates does this by arguing that if everything is change, there can be no stable intelligible knowledge from perception.

Chapters 21-28 provide a final refutation of Protagoras. Socrates argues that knowledge exists in the operations of the mind and therefore cannot reside in perception. Consequently, he considers the idea that knowledge is true judgment. This raises the problem of false judgments. That is, how we can make an erroneous judgment about something that we know? Socrates tries to resolve this by comparing the mind to a piece of wax, but he finds that thinking of the mind in this way only resolves the problem of false judgment as it applies to the relation between the mind and perception, not the operations of the mind itself.

In Chapters 29-35 Socrates examines the thesis that knowledge is true judgment with an account. He recounts a dream to explain how an account can be given for complexes of elements but not for the elements themselves. Upon further investigation, Socrates realizes that complexes are also irreducible and that, as such, no account can be given for them. A final attempt to salvage this thesis via the idea that we can give an account of something in terms of its difference from other things also fails. This is because such an idea creates an infinite regress of accounts. Socrates concludes that true judgment with an account is not a good definition of knowledge either.

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