# SLIDE 1: Title Slide – “Protagoras: The Measure of All Things”
Okay, here’s a question for you: Is there such a thing as truth with a capital T?
I mean, really think about it. When you say “this coffee is hot” or “that movie was terrible” or even “murder is wrong”—are you describing something objectively real about the universe? Or are you just… reporting on your own experience?
Because about 2,500 years ago, a Greek philosopher named Protagoras stood up and said something that absolutely detonated the intellectual world of ancient Athens. He claimed—and I want you to really hear this—he claimed that man is the measure of all things.
Not God. Not nature. Not some cosmic order written into the fabric of reality. You. The individual human being, standing right there in your sandals, with your particular history and your particular perspective.
Now, you might be thinking, “Okay, so what? That sounds kind of… obvious?” But here’s the thing nobody talks about: this idea was so radical, so fundamentally threatening to everything the ancient Greeks believed about truth and knowledge and morality, that it sparked a philosophical war that’s still going on today.
We’re talking about the birth of relativism. The first serious challenge to the idea that there’s a single, objective reality that we can all access. The opening shot in a debate about truth that runs straight through Plato, through the Enlightenment, through postmodernism, and right into your Twitter feed when people argue about whether facts are “just opinions.”
So buckle up. Because we’re about to meet the original Sophist—the first professional philosopher who had the audacity to charge money for teaching wisdom—and the man who asked a question that philosophy has never fully answered:
If each person is the measure of reality, is there any such thing as truth at all?
Let’s find out.
# SLIDE 2: Who Was Protagoras? – The Original Sophist
Alright, before we dive into the philosophy that made him famous—or infamous, depending on who you ask—we need to understand who this guy actually was.
Protagoras was born in Abdera, up in Thrace, sometime around 490 BCE. And here’s what’s remarkable: he basically invented a profession. He was the first person to explicitly call himself a “sophist”—a professional teacher of wisdom and rhetoric.
Now, today when we call someone a “sophist,” it’s not exactly a compliment, right? We mean they’re slippery, manipulative with words, more interested in winning arguments than finding truth. But that negative connotation? That came later, mostly thanks to Plato, who had… let’s say, some issues with the Sophists.
In Protagoras’ time, being a Sophist meant you were part of an intellectual revolution. These weren’t dusty academics sitting in ivory towers. The Sophists were traveling teachers who went from city to city throughout Greece, teaching practical skills—especially the art of argumentation and persuasion. And they got paid for it. Handsomely.
This was scandalous. Traditional Greek education was aristocratic, passed down through elite families. But Protagoras said, “No, wisdom can be taught. Excellence can be learned. And I’m going to teach it to anyone who can afford my fees.”
And people paid. Because in the democratic city-states of ancient Greece, especially Athens, the ability to argue persuasively in the assembly or in court wasn’t just useful—it was power. If you could master rhetoric, if you could make the weaker argument appear stronger, you could shape politics, win legal cases, influence the direction of your entire society.
Protagoras became famous. Wealthy. Influential. He hobnobbed with the political elite. Plato even wrote a dialogue named after him, where Socrates engages him in debate.
But here’s what made Protagoras different from just a skilled rhetorician or debate coach: he had a genuine philosophical vision. He wasn’t just teaching tricks for winning arguments. He was making a profound claim about the nature of reality itself.
And that claim—the one we’re about to examine—would change philosophy forever.
So what did he actually say? What was this revolutionary idea that sparked such controversy?
Let me show you…
# SLIDE 3: The Man-Measure Thesis – A Revolutionary Idea
Alright, here it is. The quote that launched a thousand philosophical debates. Protagoras opened his book Truth—and yes, the irony of that title is not lost on anyone—with these words:
“Man is the measure of all things: of the things that are, that they are, and of the things that are not, that they are not.”
Now, the first time you hear that, it might sound kind of… poetic but vague, right? Like something you’d see on a motivational poster. But here’s where it gets interesting—and here’s why this sent shockwaves through ancient Athens.
Protagoras is saying that you—the individual person standing there—are the ultimate authority on what’s real and what’s not. Not the gods. Not tradition. Not some philosopher-king who’s glimpsed eternal truths. You.
Think about what this means. If I say “this wine is sweet” and you say “this wine is bitter”—we’re not disagreeing about the wine. We’re both right. The wine is sweet for me. The wine is bitter for you. There’s no “real” taste of the wine that exists independently of our experience of it.
But wait—it gets more interesting. Because Protagoras isn’t just talking about taste in wine or whether it’s hot or cold outside. He’s talking about all things. Moral judgments. Political decisions. Questions of justice. What’s right. What’s wrong. What’s true. What’s false.
Now you can see why this was revolutionary—and dangerous. Because if every person is the measure of all things, then what happens to objective truth? What happens to moral absolutes? What happens to the idea that some things are just right regardless of what anyone thinks?
The traditional Greek worldview—the one Plato and Socrates desperately wanted to defend—said there’s a cosmic order, a natural law, eternal truths that exist whether we recognize them or not. Murder isn’t wrong because we decided it’s wrong. It’s wrong because it violates something fundamental about reality itself.
But Protagoras walks in and says, “Nope. Man is the measure.” And suddenly the entire foundation of objective morality, objective knowledge, objective anything is up for grabs.
You can imagine how well this went over with the philosophical establishment. It’s like showing up to a physics conference and announcing that gravity is just, like, your opinion, man.
But here’s the thing nobody talks about—and this is crucial: Protagoras wasn’t just being provocative for the sake of it. He wasn’t trolling the ancient Greeks. He had genuine philosophical reasons for this position, rooted in a sophisticated understanding of how human beings actually encounter reality.
So the question becomes: what did he actually mean by this? Because “man is the measure” is catchy, but it’s also ambiguous. What does “man” mean here? What does “measure” mean? And what exactly are we measuring?
Let’s break it down…
# SLIDE 4: Deconstructing the Meaning – “Man”
Okay, so when Protagoras says “man is the measure,” who exactly is he talking about? And this is where we need to be really careful, because the answer matters enormously for how we understand his philosophy.
Three crucial points about what “man” means here:
First: Individual, Not Collective
Protagoras is talking about each individual person, not humanity as some kind of collective abstraction. Not “mankind” in general. Not “the human species.” You. Specifically you, with your particular body, your particular history, standing in your particular moment.
This isn’t “humanity has collectively decided that murder is wrong.” It’s “I am the measure of whether this action, right here, right now, is wrong for me.”
You see how much more radical that is? We’re not talking about democratic consensus or cultural norms. We’re talking about individual perception and judgment as the ultimate arbiter of reality.
Second: Historically Situated
Each person brings their own unique history, experiences, and expectations to their understanding of the world. You’re not some abstract, universal subject—you’re a concrete human being shaped by your culture, your upbringing, your past experiences.
Think about it this way: when you taste that wine we talked about earlier, you’re not tasting it with some neutral, objective palate. You’re tasting it with your palate—shaped by every wine you’ve ever tasted before, by your cultural background, by whether you’re hungry or full, by your mood, by your memories.
And Protagoras is saying: that’s not a bug in how we know things. That’s the feature. That’s the only way we can know things—through our particular, situated, individual perspective.
Third: Concrete Reality
This is about real, flesh-and-blood people, not some philosophical abstraction. Protagoras isn’t interested in what some idealized “rational subject” would think. He’s interested in what you, the actual person, with all your quirks and limitations and biases, actually experience.
Which, when you think about it, is kind of refreshing. Most philosophers want to strip away all the messy, particular stuff about you to get to some pure, universal truth. Protagoras says, “No, the messy, particular stuff is the truth. That’s all we’ve got.”
But here’s where it gets philosophically sophisticated—and this is what people often miss about Protagoras. He’s not saying reality doesn’t exist outside of us. He’s not saying the external world is just an illusion.
What he’s saying is that we never have access to things as they are in themselves, independent of our experience of them. We only ever encounter things as they appear to us. And since different things appear differently to different people, based on their particular situations and perspectives…
Well, then each person really is the measure of how things are—for them.
But okay, that’s “man.” What about “measure”? What does it mean to say we’re measuring things? And what exactly are we measuring?
That’s where things get even more interesting…
# SLIDE 5: Deconstructing the Meaning – “Measure”
Alright, so we’ve figured out who “man” is—the individual, historically situated, concrete person. Now we need to tackle the verb: what does it mean to say that person is the measure of things?
And here’s where a lot of people get Protagoras wrong. They think he’s saying something like, “You get to decide what exists and what doesn’t exist. If you believe in unicorns, then unicorns are real for you!”
That’s not it. That’s not what he’s saying at all.
Protagoras isn’t claiming that man is the measure of things’ existence. He’s not saying you create reality with your mind. What he’s saying is much more subtle and, frankly, much more interesting:
Man is the measure of how things present themselves to us.
Let me say that again, because it’s crucial: We’re not measuring whether things exist. We’re measuring how they appear, how they show up in our experience, how we relate to them.
The wine exists. The wind exists. Justice exists as a concept. But the wine’s sweetness or bitterness, the wind’s warmth or coldness, the rightness or wrongness of an action—these aren’t properties that exist independently of someone experiencing them. They’re relational. They exist in the encounter between you and the thing.
What Are We Measuring?
Now, when Protagoras says “all things,” he means everything. And I mean everything. Facts, events, phenomena in their broadest possible sense—everything we encounter in our experience.
Sensory experiences? Yes. Moral judgments? Yes. Political decisions? Yes. Questions of beauty, truth, justice? Yes, yes, yes.
There’s no carve-out here. There’s no category of things that escape this principle. If you can experience it, think about it, make a judgment about it—then you are the measure of it.
But here’s what makes this genuinely profound: Protagoras is fundamentally rethinking what it means for something to be “true” or “real.”
Traditional philosophy—the kind Plato loved—says: “There’s the thing itself, existing independently, and then there’s your perception of it. Your job is to get your perception to match the thing as it really is.”
Protagoras says: “No. There is no ‘thing as it really is’ that you can access independently of your experience. All you ever have is the relationship between you and the thing. And that relationship—that’s what we’re measuring.”
Think about it this way: When you say “this room is cold,” you’re not making a claim about the room’s objective temperature. You’re describing your experience of being in that room. And if I say “this room is warm,” I’m not contradicting you—I’m describing my experience.
We’re both measuring the same thing—the room—but we’re measuring it from different perspectives, with different bodies, different tolerances, different contexts. And Protagoras says:
both measurements are valid.
Now you can see why this is so controversial. Because if both measurements are valid, if there’s no “correct” temperature of the room independent of our experiences…
Then what happens when we’re not talking about room temperature? What happens when we’re talking about justice? About right and wrong? About whether a law is fair or a war is justified?
That’s where the real implications start to hit home. And that’s where Protagoras’ philosophy becomes not just interesting, but genuinely threatening to the entire project of traditional ethics and politics…
# SLIDE 6: Implications – Relativism and Subjectivity
Okay, so let’s talk about what happens when you actually take Protagoras seriously. Because this isn’t just an abstract philosophical puzzle—this has real, concrete implications for how we think about truth, morality, and how we live together in society.
Protagoras’ man-measure thesis is widely interpreted—and I think correctly interpreted—as one of the first clear articulations of relativism in Western philosophy.
What’s relativism? It’s the view that truth, value, and reality vary from person to person, or from culture to culture. There’s no universal, objective standard that applies to everyone everywhere. Instead, what’s true or right or real is relative to the individual or group making the judgment.
Now, here’s where it gets uncomfortable. Because if Protagoras is right, then:
One: Your moral judgments aren’t objectively true—they’re just true for you. When you say “slavery is wrong,” you’re not describing a fact about the universe. You’re expressing your personal or cultural perspective.
Two: My moral judgments, even if they completely contradict yours, are equally valid. If I say “slavery is acceptable,” and I genuinely believe it, then it’s true for me.
Three: There’s no neutral standpoint from which to adjudicate between us. There’s no higher court of appeal. No objective moral law written into the fabric of reality.
You see the problem?
This is why Plato and Socrates fought so hard against the Sophists. Because if Protagoras is right, then the entire project of philosophy—the search for objective truth, for universal principles of justice, for knowledge that transcends individual opinion—that whole project is fundamentally misguided.
What Protagoras is doing is challenging the very possibility of objective knowledge. Not just in ethics, but in everything.
Think about what objectivity means: it means there’s a truth about something that exists independently of what anyone thinks or believes. The earth orbits the sun whether you believe it or not. Murder is wrong whether you agree or not.
Protagoras says: “Show me this objective truth. Show me how you access it without going through your own subjective experience, your own particular perspective.”
And here’s the thing—it’s actually really hard to answer that challenge. Because everything you know, you know through your own experience. Every argument you make, you make from your own perspective. Even when you appeal to logic or reason or scientific evidence—you’re still interpreting that evidence through your own cognitive framework.
So if truth and value become relative to the individual perceiver, what does that mean practically?
It means: If the wind feels warm to me, then it is warm for me. Not “seems warm” or “appears warm”—it IS warm, in the only sense that “warm” has any meaning.
If this action feels right to me, based on my experience and judgment, then it is right for me.
If this argument seems convincing to me, then it is convincing—for me.
Now, you might be thinking: “Wait, this can’t be right. This leads to chaos. If everyone’s truth is equally valid, then how do we have any shared reality? How do we make collective decisions? How do we condemn genuinely evil actions?”
And those are exactly the right questions to ask. Those are the questions that have driven philosophical debate for 2,500 years since Protagoras first made this claim.
Because here’s what’s fascinating: Protagoras himself didn’t think his relativism led to chaos. He thought it was actually the foundation for a more sophisticated understanding of politics and persuasion. He thought you could have a functioning society even without objective truth.
Plato disagreed. Violently. And the way Plato interpreted and attacked Protagoras’ position tells us a lot about what was really at stake in this debate.
So let’s look at how Plato tried to make sense of—and ultimately dismantle—the man-measure thesis…
# SLIDE 7: An Epistemological Interpretation – Plato’s Theaetetus
Alright, so we’ve seen what Protagoras claimed and why it was so radical. Now we need to talk about how his ideas were received and interpreted—and for that, we have to turn to his most famous critic: Plato.
Here’s the thing: we don’t have Protagoras’ original writings. They’re lost to history. What we have instead are Plato’s dialogues, where Protagoras appears as a character and where his ideas are discussed, analyzed, and—let’s be honest—systematically demolished.
So we’re in this weird situation where our main source for understanding Protagoras is someone who fundamentally disagreed with him. It’s like learning about Darwin exclusively from creationists. You’re going to get something, but you need to read carefully.
In the dialogue Theaetetus, Plato does something really interesting. He links Protagoras’ man-measure thesis to a specific theory of knowledge. He says: “Okay, if man is the measure of all things, then what Protagoras is really claiming is that knowledge is sensation.”
Think about what this means. If knowledge is just sensation—if knowing something is just perceiving it with your senses—then of course each person is the measure of what they know. Because each person has their own unique sensory experiences.
I taste the wine and experience sweetness. You taste the same wine and experience bitterness. If knowledge is sensation, then I know the wine is sweet, and you know the wine is bitter. We’re both right. We both have genuine knowledge—of our own sensory experiences.
And this is actually a pretty sophisticated interpretation of what Protagoras might have meant. Because it explains how each person can be the measure: we’re the measure of our own perceptions, our own sensory encounters with the world.
Here’s where it gets really interesting philosophically. Plato’s Protagoras—and we have to remember this is Plato’s version—is making a claim about the relationship between perception and reality.
The claim is: Your perceptions constitute your reality. Not “represent” reality. Not “give you access to” reality. They are your reality, in the only sense that “reality” has any meaning for you.
Because think about it: how do you know anything about the external world except through your perceptions? You can’t step outside your own experience to compare it with “reality as it really is.” All you ever have is your experience.
So when I say “the wind is warm,” I’m not making a claim about some objective property of the wind that exists independently of my experience. I’m describing my actual, lived encounter with the wind. And that encounter—that experience of warmth—that’s as real as it gets.
Now here’s the radical part—and this is what made Plato so uncomfortable: if this is true, then there’s no access to things beyond how they appear to us individually.
You can’t get behind your perceptions to some “true reality” lurking underneath. You can’t compare your experience with “the thing itself.” All you can do is compare your experience with other people’s experiences—and discover that they’re different.
The wind that feels warm to me feels cold to you. Not because one of us is wrong and the other is right. Not because one of us is perceiving correctly and the other incorrectly. But because we are different perceivers, with different bodies, different histories, different sensitivities.
Now, Plato thought this was absurd. He thought it led to contradictions and made knowledge impossible. And in the Theaetetus, Socrates spends a lot of time trying to show that the equation of knowledge with sensation doesn’t work.
But here’s what’s fascinating: even though Plato is trying to refute Protagoras, he takes him seriously enough to engage with his ideas in detail. He recognizes that this isn’t just sophistry—this is a genuine philosophical position that needs to be reckoned with.
What’s really at stake here is the nature of knowledge itself. Can we have knowledge of objective truths? Or is all knowledge necessarily subjective, relative to the knower?
Plato desperately wants to say: “Yes, we can have objective knowledge. We can know eternal truths. We can access the Forms—perfect, unchanging realities that exist beyond the flux of sensory experience.”
Protagoras says: “Show me. Show me how you access these eternal truths without going through your own subjective experience. Show me knowledge that isn’t filtered through your particular perspective.”
And that challenge—that fundamental epistemological challenge—has never been fully answered. It runs through the entire history of Western philosophy, from ancient Greece to modern debates about objectivity and truth.
But Protagoras wasn’t just an epistemological puzzle for later philosophers to solve. He was a real person, a real teacher, with real influence on the intellectual culture of ancient Greece. And that influence—for better or worse—shaped the entire trajectory of Western thought…
# SLIDE 8: Protagoras’ Influence and Legacy
So let’s step back and look at the bigger picture. What did Protagoras actually accomplish? What’s his legacy? And why should we care about a philosopher from 2,500 years ago whose original writings are lost?
Because here’s the thing: Protagoras didn’t just make an interesting philosophical claim. He fundamentally changed what it meant to be a philosopher and what philosophy could do.
First and most obviously: Protagoras was a master of argumentation. He developed sophisticated techniques for constructing and dismantling arguments, for making both sides of any question equally compelling.
There’s a famous story about Protagoras and one of his students. The student agreed to pay Protagoras’ fee after winning his first court case. But then the student never took any cases—he just avoided paying.
So Protagoras sued him. And here’s the beautiful paradox: If Protagoras wins the lawsuit, the student has to pay. But if the student wins, then he’s won his first case—which means he has to pay according to their original agreement!
Now, that’s probably apocryphal. But it captures something real about Protagoras’ approach: he was brilliant at finding the logical structure of arguments, at seeing how language and reasoning could be used—and manipulated—to achieve different ends.
And this wasn’t just intellectual gamesmanship. In the democratic city-states of ancient Greece, the ability to argue effectively was political power. Protagoras was teaching people how to participate in democracy, how to defend themselves in court, how to persuade their fellow citizens.
Second: Protagoras established a completely new profession. He was the first to explicitly present himself as a sophist—a professional teacher of wisdom who charged fees for his instruction.
And this was revolutionary. Before Protagoras, education was aristocratic, informal, passed down through elite social networks. Philosophy was something you did with your friends, not something you paid for.
Protagoras said: “No. Wisdom can be taught systematically. Excellence—arete—can be learned. And I’m going to teach it to anyone who can afford my fees.”
Now, this democratized education in one sense—it made philosophical training available beyond the traditional aristocracy. But it also commodified it. Wisdom became something you could buy.
And the traditional philosophers hated this. Socrates, who famously never charged for his teaching, saw the Sophists as corrupting philosophy by making it mercenary. Plato thought they were teaching people to win arguments rather than seek truth.
Which brings us to the third part of Protagoras’ legacy: the intense philosophical controversy he sparked.
Plato wrote multiple dialogues engaging with Protagoras’ ideas. Aristotle discussed and criticized him. The entire Platonic tradition defined itself partly in opposition to Sophistic relativism.
And here’s what’s remarkable: even though Plato and Aristotle rejected Protagoras’ conclusions, they took his arguments seriously. They recognized that he’d identified something genuinely difficult—a real philosophical problem that couldn’t just be dismissed.
Because here’s the thing: the questions Protagoras raised are still with us. We’re still arguing about them. Right now. Today.
Is truth objective or relative? Can we have knowledge that transcends our individual perspectives? Are moral values universal or culturally constructed? Is there a “view from nowhere,” or are we always trapped within our own subjective experience?
These aren’t just academic questions. They’re at the heart of contemporary debates about science, politics, ethics, religion—everything.
When someone says “that’s just your truth,” they’re channeling Protagoras. When someone insists on objective facts regardless of perspective, they’re channeling Plato’s response to Protagoras.
So why does Protagoras matter? Why spend time with a philosopher whose books are lost, whose ideas come to us filtered through his critics?
Because Protagoras asked a question that philosophy has never fully answered: If each person experiences reality from their own unique perspective, how can we claim to have knowledge of objective truth?
Plato thought he could answer this with his theory of Forms—eternal, perfect realities accessible through reason. Descartes tried with his method of radical doubt. Kant tried with his transcendental idealism. The logical positivists tried with their verification principle.
And every single attempt has run into problems. Every single attempt has had to grapple with the fundamental insight Protagoras identified: we can’t step outside our own experience to verify that our beliefs match reality.
So here’s what I want you to take away from this: Protagoras wasn’t just some clever rhetorician playing word games. He identified a genuine philosophical problem—maybe the fundamental problem of epistemology.
And that problem is still alive. It’s still challenging us. It’s still forcing us to think carefully about what we mean when we claim to know something, when we assert that something is true, when we make moral judgments.
You don’t have to agree with Protagoras. Plato didn’t. Aristotle didn’t. Most of the Western philosophical tradition didn’t.
But you do have to reckon with him. You have to take seriously the possibility that man really is the measure of all things—and think through what that would mean for everything else you believe.
Because that’s what philosophy is: taking seriously ideas that challenge everything you thought you knew, and seeing where they lead.
And Protagoras? He’s been leading us on that journey for 2,500 years.
# SLIDE 9: Protagoras in Plato’s Dialogues
Okay, so before we wrap up, I want to talk about something really fascinating—and kind of ironic.
Remember how I said we don’t have Protagoras’ original writings? That everything we know about him comes filtered through his critics, especially Plato?
Well, here’s the weird thing: Plato may have disagreed with Protagoras fundamentally, but he also immortalized him. By writing dialogues that feature Protagoras as a character, by taking his ideas seriously enough to engage with them in detail, Plato ensured that Protagoras would never be forgotten.
It’s like… imagine if the only way future generations knew about your ideas was through the writings of your biggest intellectual opponent. That’s the situation we’re in with Protagoras.
So Plato wrote two major dialogues that deal extensively with Protagoras and his ideas. The first is simply called Protagoras, and it’s a dramatic encounter—almost like a philosophical boxing match.
Picture this: Socrates meets Protagoras at the house of a wealthy Athenian. Protagoras is at the height of his fame, surrounded by admirers. And they get into this intense debate about whether virtue—arete, excellence—can be taught.
Protagoras says yes, absolutely, that’s literally his job—teaching people to be excellent citizens. He gives this long, elaborate speech, complete with a myth about Prometheus and the origins of human civilization, arguing that political virtue is something that can and must be taught for society to function.
Socrates, being Socrates, is skeptical. He starts asking his annoying questions, poking holes in the argument, making distinctions, until by the end of the dialogue they’ve somehow switched positions—Socrates is arguing that virtue might be teachable after all, and Protagoras is confused about his own position.
Classic Socratic move.
What’s remarkable about this dialogue is that Plato gives Protagoras a genuine voice. He’s not just a straw man to be knocked down. He’s portrayed as intelligent, sophisticated, worthy of respect—even if ultimately wrong.
Protagoras gets to make real arguments. He gets to defend his position with skill and eloquence. And the questions he raises—about education, about virtue, about whether excellence can be taught or is innate—these are genuine philosophical problems that Plato himself is wrestling with.
So even in a dialogue designed to challenge Sophistic thinking, Plato shows us why Protagoras was taken seriously. Why he was famous. Why people paid him enormous sums to study with him.
But the really deep philosophical engagement happens in the Theaetetus—the dialogue we touched on earlier when we talked about knowledge as sensation.
This dialogue is explicitly about the nature of knowledge. Young Theaetetus proposes that knowledge is perception—basically the epistemological interpretation of Protagoras’ man-measure thesis.
And Socrates says: “Ah, so you’re agreeing with Protagoras, who said man is the measure of all things.”
Then Plato does something fascinating: he gives the strongest possible version of Protagoras’ argument. He doesn’t just knock down a weak version—he constructs the most sophisticated, defensible interpretation of the man-measure thesis he can, and then tries to refute it.
This is intellectual honesty at its finest. Plato is saying: “I think Protagoras is wrong, but let me show you the strongest version of his position, because that’s the only version worth refuting.”
And here’s where it gets really interesting. In the Theaetetus, Plato has Socrates present what he calls Protagoras’ “secret doctrine”—a more sophisticated version of the man-measure thesis that involves a theory about the nature of perception and reality.
The idea is that nothing exists as a stable, independent thing. Instead, everything is constantly in flux, constantly changing. And perception happens when this flux of the perceiver encounters the flux of the perceived object.
So when I see something white, “whiteness” doesn’t exist as a property of the object, and “seeing” doesn’t exist as a property of me. Instead, “whiteness” and “seeing” come into being together, in the moment of perception, as a result of the interaction between me and the object.
Now, Plato probably made this up. There’s no evidence Protagoras actually taught a “secret doctrine.” But what’s fascinating is that Plato felt he needed to construct this sophisticated theory to make sense of Protagoras’ position.
So here’s what I want you to understand: Plato wrote these dialogues decades after Protagoras died. He could have just ignored him. He could have dismissed Sophistic relativism as obviously wrong and moved on.
But he didn’t. He kept coming back to Protagoras. He kept engaging with these ideas. He kept trying to refute them.
Why? Because Protagoras had identified something genuinely difficult. A real philosophical problem that couldn’t just be wished away.
If we can only know things through our own perceptions, and different people have different perceptions, then how can we claim to have objective knowledge? How can we say that some beliefs are true and others false, if truth is relative to the perceiver?
And here’s the beautiful irony: by taking Protagoras seriously, by engaging with his ideas in such detail, Plato ensured that those ideas would survive.
Protagoras’ books are gone. But his philosophy lives on in Plato’s dialogues. His arguments, his positions, his challenges to traditional philosophy—they’re all preserved, in detail, by the very philosopher who most wanted to refute them.
It’s like… Plato tried to kill Protagoras’ ideas by subjecting them to rigorous philosophical scrutiny. But in doing so, he made them immortal.
And I think there’s something profound here about what philosophy is supposed to be.
Philosophy isn’t about surrounding yourself with people who already agree with you. It’s not about dismissing opposing views without engaging them. It’s not about winning arguments through rhetorical tricks.
Philosophy is about taking seriously the strongest version of positions you disagree with. It’s about engaging with ideas that challenge your own. It’s about recognizing that even your opponents might have identified something true and important.
Plato and Protagoras disagreed fundamentally about the nature of truth, knowledge, and reality. But Plato respected Protagoras enough to preserve his arguments for posterity. To let future generations—including us, right now, 2,500 years later—wrestle with the same questions.
And that’s why we’re still reading both of them. Not because one of them “won” the debate and the other “lost.” But because the debate itself—the serious, rigorous engagement with difficult philosophical questions—that’s what matters.
Protagoras challenged the possibility of objective truth. Plato defended it. And the conversation between them?
That conversation is still going on.