Plato's Theaetetus: The Relentless Pursuit of What Knowledge Really Is

You are reading these words. Do you know you are reading them? Most people would say yes without hesitation. But press harder. How do you know you are not dreaming? How do you know your eyes are not deceiving you? How do you know the meaning you take from these sentences matches what was intended? When we start asking what knowledge actually is, we discover that one of our most basic concepts dissolves into puzzles.

This is where Plato begins in the Theaetetus, a dialogue written around 369 BCE that remains the starting point for Western epistemology. The question seems simple: what is knowledge? By the end of the dialogue, after examining and rejecting three sophisticated answers, Plato leaves us without a definition. The conversation ends in aporia, philosophical puzzlement, the recognition that we do not know what we thought we knew. This apparent failure is actually the dialogue's achievement. It reveals the depth of a question we usually take for granted and establishes the problems that epistemologists have wrestled with ever since.

The Theaetetus opens with a frame narrative that gives the dialogue unusual emotional weight. Euclides of Megara tells a companion that Theaetetus, the brilliant mathematician, has just been carried past on a stretcher, wounded in battle and dying of dysentery. Euclides recalls a conversation Socrates had with Theaetetus years earlier, when Theaetetus was a young man of exceptional promise. Socrates himself, we learn, was about to face trial for impiety, the trial that would lead to his execution.

This frame reminds us that philosophical inquiry happens within mortal lives. Theaetetus, who displays such intellectual brilliance in the dialogue, will die young. Socrates, who guides the inquiry with such patience, will be executed by his fellow citizens. The search for knowledge is not an abstract exercise. It is undertaken by finite beings with limited time, who stake their lives on what they believe to be true.

The young Theaetetus has been studying with the geometer Theodorus, who praises him to Socrates as the most gifted student he has ever encountered. Socrates, intrigued, engages Theaetetus in conversation. What follows is one of the most rigorous examinations of a concept in the history of philosophy.

Before the inquiry begins, Socrates describes his philosophical method through a striking image. His mother, Phaenarete, was a midwife. Socrates claims to practice a similar art, but for ideas rather than babies. He helps others give birth to their thoughts, examines whether those thoughts are genuine or merely "wind-eggs" (unfertilized, empty), and assists in the labor of intellectual discovery.

I am so far like the midwife that I cannot myself give birth to wisdom, and the common reproach is true, that though I question others, I can myself bring nothing to light because there is no wisdom in me.

This image captures something essential about the Socratic method. Socrates does not lecture or transmit information. He asks questions that force his conversation partners to articulate and examine their own beliefs. The knowledge, if it comes, must come from within. But Socrates also emphasizes that most intellectual offspring turn out to be stillborn. The midwife's job includes recognizing when a birth has failed.

When Socrates asks Theaetetus what knowledge is, Theaetetus initially offers examples: geometry is knowledge, cobbling is knowledge, and so on. Socrates gently redirects him. The question is not what things we can know, but what knowledge itself is. What do all instances of knowledge have in common that makes them knowledge?

Theaetetus ventures his first definition: knowledge is perception (aisthesis). When I see something, I know it. When I hear or touch or taste something, I know it. Perception puts us in direct contact with reality.

Socrates immediately connects this view to Protagoras's famous doctrine that "man is the measure of all things." If knowledge is perception, and perception varies from person to person, then there is no objective truth, only what appears to each individual. The wind that feels cold to you and warm to me is neither cold nor warm in itself. It is cold for you and warm for me. Each person's perception is true for them.

Socrates also links perception-based knowledge to the metaphysics of Heraclitus, who held that everything is in constant flux. If reality is perpetually changing, then perception (which captures momentary states) is the only possible form of knowledge. There is nothing stable to know.

The combination is powerful. Protagorean relativism plus Heraclitean flux yields a coherent worldview in which perception is knowledge. But Socrates proceeds to dismantle it from multiple angles.

First, there are practical difficulties. If all perceptions are equally valid, then Protagoras cannot claim to be wiser than anyone else. Why should we pay him to teach us if his beliefs are no more true than ours? More pointedly, if Protagoras's doctrine is true, and most people believe it is false, then by his own measure it must be false for them, and therefore false for the majority. The doctrine seems to refute itself, though some scholars have defended more subtle readings of Protagoras.

Second, perception cannot capture what we most want to know. We perceive with our eyes and ears, but we think about what we perceive with our minds. When I see something and hear something, and judge that they are both real, or that they are similar, or that they exist, these judgments are not themselves perceptions. They involve concepts like "being," "sameness," "difference," and "unity" that no sense organ can detect. The mind grasps these through its own activity.

Third, perception is always of the present moment, but knowledge includes the past and future. I can know that the Battle of Marathon occurred, but I cannot perceive it. I can know that the sun will rise tomorrow, but I cannot perceive tomorrow. If knowledge were perception, we could know nothing beyond our immediate sensory experience.

The refutation of the first definition establishes a point that echoes through subsequent epistemology: knowledge requires more than passive reception of sensory data. The mind must actively process, compare, and judge. Whatever knowledge turns out to be, it involves the soul's own activity.

Having abandoned perception, Theaetetus offers a second definition: knowledge is true belief (alēthēs doxa). This seems more promising. Knowledge is not mere perception but a judgment that gets things right. When I believe that Athens is in Greece and Athens actually is in Greece, I know it.

This definition captures something important. Knowledge does seem to require getting things right. A false belief is not knowledge, no matter how confident we feel. But Socrates raises a telling objection through what has become known as the "jury argument."

Consider a jury in a law court. A skilled orator persuades them that the defendant committed a crime. Suppose the defendant actually did commit the crime. The jury now has a true belief about the defendant's guilt. But do they have knowledge?

Socrates argues they do not. The jury was not present at the crime. They did not see what happened. They have been persuaded by rhetoric, not by direct acquaintance with the facts. Even though their belief happens to be true, it seems wrong to say they know the defendant is guilty. They have been convinced, not informed.

This argument introduces a distinction that would later become central to epistemology: the difference between merely true belief and justified true belief. The jury's belief is true but not justified in the right way. They arrived at it through persuasion rather than through evidence or reasoning that connects them to the facts.

The failure of the second definition points toward what seems missing: some account of how the belief is formed, some connection between the believer and the truth that makes the belief more than accidentally correct.

Theaetetus, prompted by something he once heard, offers a third definition: knowledge is true belief accompanied by an account or explanation (logos). This is remarkably close to the "justified true belief" definition that dominated epistemology until the twentieth century. To know something, you must believe it, it must be true, and you must be able to give reasons for your belief.

But what exactly is an account? Socrates considers three interpretations, and finds problems with each.

The first interpretation: an account is putting one's thought into words. But this cannot distinguish knowledge from mere true belief. Anyone with true belief can express it in words. The person who truly believes that Theaetetus is sitting can say "Theaetetus is sitting." Adding words does not transform belief into knowledge.

The second interpretation: an account is an enumeration of the elements that compose something. To know a wagon, you list its parts (wheels, axle, body, and so on). To know a word, you spell out its letters. This seems more promising, since analysis reveals structure, but Socrates finds a problem.

Consider learning to spell. When you learn the word "Theaetetus," you learn that it is composed of certain letters in a certain order. But do you know each letter? If knowing something requires knowing its elements, and letters are elements, then you must know the letters. But letters have no further elements to enumerate. Either you know them without an account (which contradicts the definition), or you cannot know them at all (which makes knowledge of words impossible).

This is the problem of first elements. Any analysis must bottom out somewhere in things that cannot themselves be analyzed. If knowledge requires an account, and an account requires breaking things into parts, then the ultimate parts (the elements) cannot be known. But if the elements cannot be known, how can the wholes composed of them be known?

The third interpretation: an account is stating what distinguishes the thing from everything else. To know Theaetetus is to identify what makes him different from all other people (his snub nose, his prominent eyes, and so on). This seems to capture what we want: genuine understanding of the individual thing.

But Socrates argues this interpretation is circular. To have true belief about Theaetetus, you must already be thinking about him rather than someone else. Your belief must already pick him out. If it does not, you do not have true belief about Theaetetus at all; you have true belief about someone else. So the distinguishing features are already implicit in the true belief. Adding them explicitly as an "account" adds nothing new.

With all three interpretations of "account" failing, the third definition collapses. The dialogue ends without a positive answer to its central question.

The Theaetetus is often called an "aporetic" dialogue, one that ends in puzzlement rather than resolution. Some readers find this frustrating. After all that work, we still do not know what knowledge is. But the apparent failure is philosophically productive in several ways.

First, it clears away inadequate answers. We now know that knowledge is not simply perception, not simply true belief, and not simply true belief with an account (at least not on the interpretations considered). Negative progress is genuine progress. Knowing what knowledge is not helps us search more effectively for what it is.

Second, the dialogue reveals the depth of the problem. The question "what is knowledge?" seems simple until you try to answer it. The Theaetetus demonstrates that our ordinary concept of knowledge contains tensions and complexities we do not normally notice. This is itself a form of wisdom, Socratic wisdom, the recognition of one's own ignorance.

Third, the dialogue models philosophical method. Socrates does not impose doctrines. He helps Theaetetus articulate his own views, then tests them rigorously. When they fail, he does not mock or condemn. He encourages Theaetetus to try again. The process of inquiry matters as much as the conclusion.

Finally, the Theaetetus points toward Plato's positive epistemology developed elsewhere, particularly in the Republic and Meno. Knowledge, for Plato, ultimately involves grasping the Forms, the eternal, unchanging essences that underlie the changing world of perception. The Theaetetus does not introduce the Forms (and scholars debate whether their absence is significant), but its negative conclusions about perception and belief create space for them. If knowledge is not perception and not mere true belief, perhaps it is something altogether different: rational insight into a reality more fundamental than the sensory world.

One of the most memorable passages in the Theaetetus is the allegory of the aviary, which Socrates introduces while examining false belief. How is it possible to believe something false? If I believe that Theaetetus is Theodorus, I must confuse two things I know. But how can I mistake what I know for something else I know?

Socrates compares the mind to an aviary full of birds. Acquiring knowledge is like capturing birds and putting them in the aviary. Having knowledge is like having the birds in your possession. But using knowledge is like reaching into the aviary and catching a specific bird. Sometimes you reach for a pigeon and accidentally grab a dove.

The allegory suggests that knowledge can be possessed without being actively used, and that errors occur in the retrieval process. This anticipates modern distinctions between stored knowledge and working memory, between competence and performance. We know many things that we are not currently thinking about. Mistakes happen when we try to access what we know.

But Socrates finds problems with this model too. If the birds in the aviary are pieces of knowledge, then grabbing the wrong bird means grabbing knowledge of the wrong thing. But knowledge of something is not ignorance of it. The allegory does not fully explain how we can believe what is false.

The puzzle of false belief (how error is possible) runs through the dialogue and connects to the central question. Understanding knowledge requires understanding its opposite, and ignorance turns out to be as puzzling as knowledge itself.

The questions raised in the Theaetetus are not merely historical curiosities. They speak to contemporary concerns across multiple domains.

In the age of information, we face an unusual situation: more data is available than ever before, yet many people feel less certain about what they know. The Theaetetus helps explain why. Information is not knowledge. Perception (or its digital equivalent, consuming content) does not automatically produce understanding. True belief acquired through algorithmic feeds may be no better than the jury's belief acquired through rhetoric. What is missing is the active engagement of the mind, the critical evaluation, the connection to reasons and evidence.

The dialogue's treatment of Protagorean relativism is equally relevant. The claim that "my truth" is as valid as "your truth" echoes through contemporary discourse. Plato's response remains forceful: if all beliefs are equally true, then the belief that not all beliefs are equally true is also true for those who hold it. Radical relativism undermines itself. There must be some standard beyond individual perception by which beliefs can be evaluated.

In science and expertise, the Theaetetus illuminates ongoing debates about the nature of scientific knowledge. Is scientific knowledge simply true belief that happens to be well-confirmed? Or does it require something more, perhaps understanding of underlying mechanisms, or integration into a coherent theoretical framework? The dialogue's rejection of simple "true belief plus account" suggests that genuine knowledge involves more than meeting formal criteria.

The problem of first elements appears in contemporary physics and mathematics. What are the fundamental constituents of reality? Can they be known, or only postulated? If our knowledge of complex things depends on knowledge of their elements, and the elements resist analysis, what does this imply about the foundations of science?

In artificial intelligence, the Theaetetus raises questions that have become pressing. Can a machine have knowledge? A large language model can produce true statements and provide explanations. Does it therefore know things? Plato's dialogue suggests caution. The model has no perception in the relevant sense, no direct contact with reality. Its "beliefs" are pattern-matches, not judgments. Its "accounts" are recombinations of training data, not genuine understanding. Whatever knowledge is, it may require something current AI systems lack, though this remains an open question as the technology evolves.

In everyday life, the dialogue invites reflection on what we claim to know. How much of what we "know" is actually perception (fleeting impressions), or true belief (correct opinions we cannot justify), or true belief with an account (justified but perhaps accidentally so)? The Theaetetus encourages intellectual humility. Recognizing the difficulty of genuine knowledge is the first step toward achieving it.

In education, the Theaetetus challenges certain dominant models. If knowledge is not simply information transfer, then education cannot be simply information delivery. The midwife image suggests that genuine learning requires the student's active participation: the sometimes difficult labor of working through problems, the recognition that many intellectual offspring are stillborn. Teaching is not filling empty vessels but helping minds give birth to their own understanding.

The Theaetetus established epistemology as a central philosophical discipline. Every subsequent theory of knowledge has had to address the problems Plato raised: the relationship between perception and knowledge, the inadequacy of true belief, the difficulty of specifying what must be added to belief to yield knowledge.

The dialogue's influence on ancient philosophy was immediate. Aristotle developed his own epistemology partly in response to Platonic puzzles. The Stoics and Epicureans offered competing accounts of how perception relates to knowledge. The Skeptics took the Theaetetus's aporetic ending as evidence that knowledge is impossible.

In medieval philosophy, the dialogue shaped debates about the relationship between faith and reason. If knowledge requires more than true belief, what is the epistemic status of religious faith? Aquinas and others wrestled with how divine revelation relates to human understanding.

Modern epistemology, from Descartes through Kant to the present, continues to engage with Platonic themes. The rationalist tradition emphasizes the mind's active contribution to knowledge, echoing Plato's insistence that perception alone is insufficient. The empiricist tradition tries to ground knowledge in sensory experience while acknowledging the problems Plato raised. Kant's synthesis (that knowledge requires both sensory input and conceptual processing) can be seen as an attempt to resolve tensions the Theaetetus first identified.

The twentieth century saw renewed attention to the dialogue. The "justified true belief" definition that Plato examined became standard in analytic epistemology until Edmund Gettier's famous 1963 counterexamples showed it was still inadequate. Gettier's puzzles are structurally similar to Plato's jury argument: cases where true belief is justified but still falls short of knowledge. The search for what must be added continues.

Contemporary virtue epistemology, which focuses on intellectual character traits rather than propositional justification, echoes Platonic themes. Perhaps knowledge requires not just true belief with reasons, but true belief formed through the exercise of intellectual virtues. This approach shifts attention from beliefs to believers, from propositions to persons.

The Theaetetus also connects to other philosophical traditions. Buddhist epistemology distinguishes between conventional knowledge (useful for navigating the world) and ultimate knowledge (insight into the nature of reality). This parallels Plato's distinction between opinion about the changing sensory world and knowledge of eternal Forms. Both traditions suggest that ordinary cognition, however practically useful, falls short of genuine understanding.

The Theaetetus ends with Socrates departing for the court where he will face the charges that lead to his death. The inquiry into knowledge remains unfinished. But perhaps that is the point. The search for knowledge is not a puzzle to be solved and set aside. It is an ongoing activity, one that defines the philosophical life.

Socrates tells Theaetetus that if he later gives birth to genuine insights, they will be better for having been examined. If he remains empty, he will at least be less annoying to his companions because he will not think he knows what he does not know. Either way, the inquiry has value.

This is the Theaetetus's deepest lesson. The question "what is knowledge?" matters not because we need a definition to put in a dictionary, but because asking it transforms how we relate to our own beliefs. The unexamined belief is not worth holding. The examined belief, even if it survives scrutiny, is held differently: more tentatively, more responsibly, with greater awareness of what would count against it.

In an age of information overload and confident assertion, the Theaetetus offers a countervailing wisdom. Not knowing what knowledge is turns out to be compatible with, perhaps even necessary for, the genuine pursuit of understanding. The dialogue that fails to define knowledge succeeds in showing us what it means to seek it.

• Plato, Theaetetus (translated by M.J. Levett, revised by Myles Burnyeat). An accessible translation with an excellent introduction.
• Myles Burnyeat, The Theaetetus of Plato. The definitive modern commentary, combining philosophical depth with historical sensitivity.
• Timothy Williamson, Knowledge and Its Limits. A contemporary epistemology that argues knowledge is more fundamental than belief, engaging with problems Plato raised.
• Linda Zagzebski, Virtues of the Mind. An influential work in virtue epistemology that connects intellectual excellence to the Platonic tradition.
• Gail Fine, Plato on Knowledge and Forms. A rigorous examination of Plato's epistemology across the dialogues, situating the Theaetetus within his broader project.