In This Article Expand or collapse the "in this article" section Plato’s Theaetetus

  • Introduction
  • General Overviews
  • General Introduction to Plato’s Epistemology
  • Editions and Translations
  • Commentaries and Monographs on the Theaetetus

Classics Plato’s Theaetetus
Mi-Kyoung Lee
  • LAST MODIFIED: 15 January 2019
  • DOI: 10.1093/obo/9780195389661-0328


Plato’s Theaetetus is a dialogue devoted to the question of what knowledge (epistêmê) is; the narrators recount a conversation that Socrates had the day before his trial with Theodorus of Cyrene, who teaches geometry at Athens, and his young student, Theaetetus, who would later go on to make some important mathematical discoveries. After some introductory discussion about what Socrates is looking for, Theaetetus proposes three definitions of knowledge, each of which is subjected to critical examination, and then rejected in turn: knowledge is perception (151e–186e), it is true opinion (187a–200d), or it is true opinion accompanied by a logos (200d–210a). The dialogue ends in aporia. The Theaetetus along with the Meno and Republic form the three most important works for understanding Plato’s epistemology and theory of knowledge. Because of its elenctic or dialectical character—in which the candidate definitions are rejected on the basis of problems or refutations based on the cooperative discussion between Socrates and Theaetetus—it is difficult to pronounce confidently what conclusions the reader is meant to draw, and what views, if any, Plato as the author of the dialogue holds. The dialogue is chock full of original ideas and arguments which would go on to have an enormous influence on later philosophers, including, for example, Socrates’ comparing himself as a philosopher-teacher to a midwife, the idea that knowledge is perception, the thesis of relativism, the thesis of radical metaphysical flux, the comparison of the mind to a wax block, etc. It is also a masterpiece of philosophical writing and often thought of along with the Phaedo and Republic as one of Plato’s most brilliant dialogues. For more on Plato, see the separate Oxford Bibliographies in Classics article “Plato.”

General Overviews

Introductions to the main themes of the Theaetetus can be found in Chappell 2013, Thaler 2016a, and Thaler 2016b.

  • Chappell, Sophie Grace. 2013. “Plato on knowledge in the Theaetetus.” The Stanford encyclopedia of philosophy. Edited by Edward N. Zalta.

    Includes a good overview of the debate between the unitarian and revisionist interpretations of the Theaetetus, with references.

  • Thaler, Naly. 2016a. Perception and knowledge in Plato’s Theaetetus. Philosophy Compass 11.3: 160–167.

    DOI: 10.1111/phc3.12310

    Covers Theaetetus 142–186.

  • Thaler, Naly. 2016b. Judgment, logos, and knowledge in Plato’s Theaetetus. Philosophy Compass 11.5: 246–255.

    DOI: 10.1111/phc3.12311

    Covers Theaetetus 186–210d.

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