Theaetetus: What Is Knowledge?

Introduction

Plato’s Theaetetus is a foundational dialogue on epistemology—the study of knowledge. Through a discussion between Socrates, the young mathematician Theaetetus, and Theodorus, the dialogue examines what it means to know something.

Rather than arriving at a firm definition, the dialogue functions as a philosophical investigation into false belief, perception, and the limits of human understanding.

This study guide outlines the key arguments in Theaetetus and explores Plato’s early approach to the question: What is knowledge?

1. The Three Definitions of Knowledge

Theaetetus offers three definitions of knowledge, each tested and ultimately refuted by Socrates.

A. Knowledge is Perception

“Man is the measure of all things.” —Protagoras

This idea, drawn from Protagoras, suggests that what each person perceives is true for them.

Socrates explores relativism: If perception is knowledge, then everyone is right in their own view.
Consequence: No objective truth exists.
Critique: Perception can be deceptive (e.g. dreams, illusions, illness), so it cannot be a reliable foundation for knowledge.

B. Knowledge is True Judgment

Socrates next questions whether true belief equals knowledge.

Example: A jury may reach the correct verdict, but if they do so without understanding, is it really knowledge?
Verdict: True judgment without explanation is still unstable and open to error.

C. Knowledge is True Judgment with an Account (Logos)

This final definition attempts to combine belief and reasoning:

To know is to have a true belief and be able to explain why.
Socrates tests this with analogies (e.g., knowing someone by description vs. recognizing them).
Problem: What counts as an “account”? Does naming or listing qualities suffice?
This approach fails to fully define knowledge and collapses under scrutiny.

2. Socratic Midwifery: Philosophy as Birth

Socrates describes his role in the dialogue using the metaphor of a midwife:

He does not “give birth” to knowledge, but helps others bring forth their ideas.
His goal is not to teach, but to test and refine thought through questioning.
Theaetetus is praised for his willingness to struggle and search—hallmarks of philosophical maturity.

3. The Problem of False Belief

Socrates explores whether one can truly hold a false belief:

Is error a result of misidentification, like confusing one person for another?
Plato introduces the “wax block” and “aviary” metaphors to explain memory and understanding:
Wax block: Impressions are stamped onto the soul like a seal—errors occur if impressions are faint or confused.
Aviary: Knowledge is like birds flying in the soul’s aviary; to know something is to “grab” the right bird.

Each model attempts to explain how people can mistake, forget, or misjudge, but none fully resolves the issue of false belief.

4. The Dialogue’s Open-Ended Conclusion

Theaetetus ends without a final answer to the question of what knowledge is. Socrates departs for his court trial (which we later learn leads to his death in the Apology).

Socrates: “The investigation must continue.”

This open ending reinforces a key theme of the dialogue: Philosophy is not about quick answers but enduring questions.

Key Philosophical Themes

1. Epistemological Relativism

Can truth be subjective?
If all perceptions are valid, can anything be false?

2. The Nature of Error

What does it mean to be wrong?
Is ignorance just the absence of knowledge, or something deeper?

3. The Limits of Language and Explanation

Can we ever fully explain what we know?
How do we account for intuition, memory, and tacit understanding?

4. The Philosophical Method

Socrates doesn’t offer dogma—he refines, tests, and purifies thought.
Philosophy is a process, not a product.

Wisdom and Takeaways

True knowledge requires more than belief—it demands clarity and justification.
Perception is unreliable; wisdom must look beyond appearances.
Philosophy is an endless questioning, not a fixed doctrine.
Failure to define knowledge is not failure—it is the beginning of wisdom.

Conclusion

Theaetetus offers a profound inquiry into what it means to know. Though it ends in aporia (no definitive answer), it sharpens the reader’s awareness of the complexity of knowledge. Socrates teaches that understanding begins with recognizing our ignorance—and with that, the soul is stirred into motion.