Theaetetus (1999) Informative Summary


The dialogue opens with Socrates, Theodorus, and the young Theaetetus, a student of geometry, discussing the nature of knowledge. Socrates, acting as a midwife, guides Theaetetus through a series of definitions, progressively refining the concept. They first consider the Protagorean view that “knowledge is perception,” and examine its connection to the Heraclitean theory of flux, where everything is in constant motion and change. This leads them to question the validity of knowledge based solely on perception, as it can be subjective and prone to error.

They then explore the idea that “knowledge is true opinion,” but quickly encounter the problem of how false opinion can exist. Socrates delves into the nature of thought, comparing the mind to a block of wax receiving and retaining impressions, highlighting the role of memory and association in forming beliefs. However, they ultimately realize that this analogy does not fully account for the possibility of error, especially in abstract concepts like mathematics. The dialogue concludes with Socrates leaving the question of knowledge unanswered, recognizing the complexity and elusiveness of true understanding.

Key Findings:

  • The complexity of defining knowledge: The dialogue demonstrates the difficulty in arriving at a definitive and universally accepted definition of knowledge.
  • The limitations of perception: The Protagorean and Heraclitean theories are shown to be inadequate as a foundation for knowledge, as perception is subjective, relative, and prone to error.
  • The importance of reasoning and abstraction: The dialogue highlights the role of reasoning, analysis, and abstraction in moving beyond perception to a more profound understanding.
  • The nature of thought and memory: Socrates explores the mechanics of thought, using metaphors like the block of wax to illustrate how the mind forms impressions, remembers, and associates.
  • The challenge of false opinion: The dialogue raises the persistent philosophical question of how false opinion can exist if knowledge is either present or absent, exploring various potential explanations.


  • Theaetetus is a student of Theodorus, a famous geometrician. This establishes Theaetetus’ background and his connection to the field of mathematics.
  • Theaetetus is a brave and intelligent young man. Theodorus praises Theaetetus for his courage in battle and his intellectual prowess.
  • Protagoras believed that “Man is the measure of all things.” This is a key principle in the dialogue, and its implications are explored throughout the conversation.
  • Protagoras wrote a book titled “The Truth,” but he allegedly revealed his true philosophy to his disciples in private. Socrates suggests that Protagoras’ actual views were more radical than what he publicly expressed.
  • Heracleitus believed that everything is in constant motion and change. This philosophy is used to support the idea of knowledge as perception, but also ultimately leads to its downfall.
  • Theaetetus initially defines knowledge as perception. This aligns with the Protagorean perspective, but is later challenged by Socrates.
  • Socrates is a “man-midwife,” who helps others bring forth their own thoughts. This metaphorical image underscores Socrates’ role as a guide and facilitator of learning.
  • Theodorus was a former student of Protagoras, but he later abandoned philosophy for mathematics. This illustrates the different approaches to knowledge, and Theodorus’ aversion to the complexities of philosophical debate.
  • The philosophers of Ephesus are “mad about the flux,” and cannot engage in reasoned discourse. This depicts the extreme and fanatical nature of some who embraced the Heraclitean philosophy.
  • Parmenides believed in the permanence of “Being,” and opposed the theory of flux. This introduces the opposing philosophical perspective to the Heraclitean view.
  • Socrates argues that Protagoras’ theory contradicts the common human experience of judging others’ wisdom. If “man is the measure of all things,” then how can we acknowledge the existence of wiser individuals?
  • Socrates compares philosophers and lawyers, highlighting their contrasting approaches to knowledge and life. This digression provides a humorous and insightful comparison between the detached pursuit of truth and the pragmatic world of legal practice.
  • The philosopher is often seen as “unacquainted with the world,” focused on abstract ideas and theoretical questions. This highlights the philosopher’s potential detachment from everyday life and their tendency to be seen as impractical.
  • The penalty of injustice is not physical suffering, but the gradual descent into further injustice. This emphasizes the moral consequences of wrongdoing, and the cyclical nature of wickedness.
  • Protagoras argued that true wisdom is a practical power of improving one’s life, not about possessing greater truth. He emphasizes the practical application of knowledge and the ability to transform oneself for the better.
  • Socrates uses the example of the vine-grower and the cook to illustrate that not everyone is equally capable of judging the future. This undermines the Protagorean idea that all individuals are equally qualified to make judgments.
  • Theodorus is reluctant to engage in a long debate with Socrates. He feels too old and uninterested in the complexities of the philosophical discourse.
  • Socrates compares the senses to “Trojan warriors in the horse,” suggesting that they are not independent entities but instruments of the mind. This highlights the mind’s role in interpreting and integrating sensory information.
  • The mind is capable of perceiving abstractions such as Being, sameness, and difference, independent of physical sense organs. This recognizes the mind’s capacity for abstract thought and its ability to form universal concepts.
  • False opinion is not simply a misinterpretation of sense data, but also involves a confusion of thought and memory. This suggests that errors in thinking can occur even without incorrect sensory input.
  • The mind can “conversed with herself” through a process of questioning and answering, leading to a conclusion or opinion. This underscores the importance of internal dialogue and critical thinking in forming beliefs.
  • Socrates uses the metaphor of an aviary to illustrate the difference between “having” and “possessing” knowledge. This introduces the concept of knowledge as a potential resource, not just a state of being.
  • Theaetetus proposes that “knowledge is true opinion accompanied by definition or explanation.” This definition is further explored and analyzed by Socrates.
  • Socrates argues that the first elements of language, like letters, cannot be defined, but their combination into syllables or words creates meaning. This explores the relationship between words, concepts, and the acquisition of knowledge.
  • The concept of knowledge is ultimately left undefined. The dialogue concludes by acknowledging the elusiveness of true knowledge and the difficulty in achieving a complete understanding.


  • Theaetetus was wounded in the battle of Corinth. This provides historical context for the dialogue.
  • Theaetetus was a youth when he had his first conversation with Socrates. This emphasizes his youthful naiveté and openness to philosophical inquiry.
  • The dialogue took place shortly before the death of Socrates. This underscores the historical significance of the conversation.
  • Theodorus visited Athens on several occasions. This suggests that there was a regular flow of philosophical exchange between Athens and other Greek cities.
  • Protagoras was a well-known and successful sophist who charged a high fee for his teachings. This highlights the social and economic status of Sophists in ancient Greece.
  • Protagoras was dead at the time of the dialogue. This signifies the passing of an influential figure and the ongoing debate about his philosophical legacy.
  • Socrates was expecting to meet Meletus at the porch of the King Archon. This alludes to Socrates’ impending trial and execution, highlighting the dramatic tension of the dialogue.


  • Dialectic: A method of philosophical inquiry involving question and answer, aiming to arrive at truth through reasoned debate.
  • Sophists: Teachers of rhetoric and philosophy who were often criticized for their focus on persuasion and their skepticism about traditional values.
  • Flux: A state of constant change and motion, as espoused by Heracleitus, where nothing remains fixed.
  • Perception: The process of receiving sensory information and experiencing the world through the five senses.
  • Opinion: A belief or judgment formed without complete knowledge or certainty.
  • True opinion: A belief that happens to be correct, but is not necessarily based on knowledge or understanding.
  • Definition: A precise and comprehensive statement of the meaning of a concept.
  • Eristic: A form of argumentation that focuses on winning a debate rather than seeking truth.
  • Abstraction: The process of forming general concepts from specific instances, moving beyond the concrete to the abstract.
  • Relativity: The idea that knowledge is dependent on the perspective of the individual or the context in which it is considered.


  • The wind blowing on different people, one feeling hot, the other feeling cold. This example is used to illustrate the Protagorean principle that perception is relative.
  • The six dice, which are both more than four and less than twelve. This demonstrates the potential for contradictions in perception based on relative comparisons.
  • The wine which is pleasant to Socrates when he is well, but unpleasant when he is ill. This further illustrates the subjective nature of perception and its dependence on individual circumstances.
  • The Athenian dicasts (jurors) having true opinions about a crime, but not knowledge. This is used to argue that true opinion is not the same as knowledge, as it can be based on persuasion rather than complete understanding.
  • The story of Thales falling into the well while gazing at the stars. This humorous anecdote illustrates the philosopher’s potential detachment from the practicalities of everyday life.
  • The example of a lawyer in court, restricted by time, legal procedures, and the opposing counsel. This contrasts the lawyer’s pragmatic approach to knowledge with the philosopher’s more leisurely pursuit of truth.
  • The story of the man who boasts of his lineage but fails to comprehend the vastness of time and generations. This illustrates the philosopher’s perspective on the insignificance of worldly matters compared to the vastness of history and existence.
  • The comparison of the mind to a block of wax. This metaphor helps to visualize how the mind receives and stores impressions, forming memories and beliefs.
  • The comparison of the mind to an aviary. This metaphor illustrates the distinction between “having” and “possessing” knowledge, as if the mind were a container of various thoughts and ideas.
  • The analogy of letters and syllables to explain the nature of definition. This example demonstrates how meaning is created through the combination of smaller elements.

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