Do you love the warmth of the sun on your face? Or perhaps, in the immortal words of Garbage, you’re only happy when it rains. Maybe mild salsa burns your tender tongue, or instead cayenne doesn’t even register. Whatever your responses to various sensations, you don’t really worry too much about what’s “really” hot, cold, spicy, or mild. It’s what it is to you, and no one can tell you-you’re wrong. So why does anyone think they can tell you your opinion on any matter—abortion, drug legalisation, globalisation, etc.—is wrong? Don’t you have a right to your opinion?

It depends on what you mean by “right.”[1] If you think of a right in terms of that which you are owed—you have a right to vote—then, no. No one is obliged to accept your opinion; you don’t have a right to demand they do. Of course, when we think of a right in terms of a freedom from interference—you have a right to privacy—then, yes. You can think whatever you want, even if it’s outrageously false. Want to think the earth is flat? Sure! Want to think the universe was created about 6,000 years ago? No problem!

Protagoras: ‘Man is the Measure of All Things’

The Ancient Greek sophist, Protagoras, is credited with asserting, ‘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.’[2] Protagoras’s view seems obviously reasonable as a way to justify the claim that each of us has a right to our opinion. After all, who else but each individual can measure or judge her own sense perceptions—what we see, hear, touch, taste, and smell? How, for example, could we deny that we feel pain upon stubbing our toe or taste sweetness when we put a drop of honey on our tongue?

Plato, however, sees difficulties with this view. As he has it, Protagoras equates (sense) perception with knowledge. That means there is no distinction between what appears to be, and what is—no distinction between seeming and reality. A further implication is that one can never be wrong. After all, if my judgment merely reflects what I perceive, and my perceptions can’t be erroneous, then I cannot have any false judgments. But my judgments change: I have a cold and don’t taste the sweetness of the honey. My cold goes away, and I taste the sweetness of the honey. The honey is not sweet and then is sweet again. On this view, there is no objective fact of the matter, only my perception. “It’s true for me,” or “This is my truth,” as one might be inclined to say.


Inconsistencies and outright contradictions abound not only within the scope of one’s own perceptions but also between individuals. How can the same food be spicy to me but not to you? These perceptual disagreements are not overly worrying. We know that when someone says irritably, “It’s too hot out!” while their friend exclaims happily, “The weather is perfect!” these claims are true for them, and not meant to be so for anyone else. Indeed, each individual is the barometer of their respective feelings; no one but me can experience my feelings.

But now consider emotions, which we often evaluate. We may say that someone is too quick to anger, or that we tend to overreact. On Protagoras’s view, however, no one is too angry or reactionary. Indeed, the tension between subjective (and so relative) experience and objective evaluation can still be resolved in terms of social norms. For example, in some cultures, keening is the response to death. Other cultures would view it as an almost vulgar display of emotion. Neither is objectively wrong.

There are, nevertheless, real difficulties with the relativistic view articulated by claiming ‘man is the measure of all things.’ These come to the fore when we make judgments about conduct. Recall on Plato’s interpretation of Protagoras, knowledge and truth is relative to the perceiver. There is no objective standard one can use to determine error, for none exists. Consequently, our moral judgments are all equally correct, even if they are utterly incompatible.

With no way to resolve the disagreement, one is left to the infallibility of one’s own opinion. This does not sit well with anyone who believes knowledge is not relative to the perceiver. We might shrug off differences of opinion about all sorts of things that don’t impact our lives. We do not, however, countenance opinions we think are dangerously wrong. Consider white supremacists, people who harbour antipathy toward non-whites. The implications of racism have been lived out for centuries around the world, so there’s no need here to rehearse the litany of violence, bloodshed, death, and destruction. Is the racist’s view correct? Unless you are one, you’d respond emphatically, “No!”

The fact is, people do disagree; people think they know thus-and-such, independently of how they feel about it, and so also hold that anyone who denies thus-and-such is mistaken. In some cases, people would go so far as to blame those whose actions are guided by that denial. So, for example, anyone who thinks a white supremacist is mistaken about the view that whites are superior to non-whites is also going to blame that white supremacist for acting according to their opinions, where praise and blame express our moral evaluations.

Plato: Knowledge is Justified True Opinion

While we may be comfortable with relative judgments about sensations, emotions, and even aesthetic experiences like responses to artworks as beautiful, things get tricky when we think about things in objective terms. A follower of Protagoras is going to have a hard time explaining concepts like justice, virtue, and duty. According to Plato, that’s because opinion alone is not equivalent to knowledge.


Suppose I take a guess at the route to Larissa, or I have an opinion about it, but cannot explain why. Now suppose that opinion is incorrect. We can easily see that it is inferior to actually knowing the way. But what if my opinion is correct? How could that be less valuable (or at least something different in kind) than knowledge? Both correctly guide action, which is what we expect and generally why they’re preferred over ignorance. Yet Plato both differentiates knowledge from opinion and clearly values the former over the latter.

For Plato, opinions reflect ignorance. They are qualitatively different from knowledge, which, as such, is to be preferred. By themselves, opinions cannot withstand scrutiny. Knowledge, however, is tethered by reasons. Opinions effectively stop where they are asserted; they are not grounded in anything apart from how things seem to the person holding them. On the other hand, a true opinion buttressed by a rational account is knowledge.

Now a question arises. How can we be assured we have the right reasons? According to Plato, what stabilises true opinion is a rational account, an unchanging justification. There are two possible sources for any justification: reason and sensation. Since sensation is fleeting, and its objects are continually changing, we can’t use it as the source of our knowledge. Reason, on the other hand, does the work we need to claim knowledge. In turn, the object of knowledge is also fixed. What allows us, for example, to know that both Chihuahuas and Great Danes are dogs? There must be something that unites all disparate instances, which allows me to make the declaration that ‘X is a dog’ in the first place: the Form of Dog.

“Form” is the name given to the original transliteration of eidos, which is “idea.” Plato does not think that Forms are mental entities, i.e., ideas. He thinks they are really existent essences—blueprints if you will—of what there is. If we are committed to the claim that knowledge is permanent, concerns what is, and can only be grasped by reason, the Forms fit the bill. These are the only real objects of knowledge.

The Gettier Problems


For hundreds of generations, Plato’s view of knowledge held sway. Although Plato himself criticised the theory, it was considered the best definition around. Even those who rejected Plato’s theory of Forms could still think he was right about the contours of knowledge—that is, that knowledge is constituted by an opinion that is both true and justified. What justifies the true opinion would be an issue, in other words, not the fact that it was justified.

Then, in 1963, Edmund Gettier upended this tradition. In an astonishingly short paper—a mere three pages—Gettier presented two cases in which a justified true belief (opinion) fails as knowledge. The first case asks us to suppose Smith has concluded, “The man who will get the job has ten coins in his pocket.” He draws this conclusion from the belief that Jones is going to get the job—for which both men have applied—and that Jones has ten coins in his pocket. Let’s suppose that Smith has empirical evidence for the proposition. Nevertheless, the true belief, “The man who will get the job has ten coins in his pocket,” is justified, but it’s not knowledge. That’s because it turns out that Smith is the one who gets the job, and he happens to have ten coins in his pocket.

In the second case, which is edited here for brevity, Smith correctly claims, “Either Jones owns a Ford or Brown is in Barcelona.” He has evidence to believe that Jones owns a Ford, and he randomly chooses Barcelona for the whereabouts of his friend, Brown. It turns out, however, that Jones does not own a Ford, but “by the sheerest coincidence,” Brown is in Barcelona. The belief, “Either Jones owns a Ford or Brown is in Barcelona,” is a justified true belief, but it surely isn’t knowledge. After all, the one belief for which Smith actually had evidence turned out to be false, while the other, though true, was so accidentally.

In both cases, there are important factors that undermine the view that Smith’s justified true belief is necessary and sufficient for knowledge. These factors include coincidence, chance, and elements for which one wouldn’t be expected to account.

So, are you entitled to your opinion? Sure, but that doesn’t mean you’re right, or that anyone has to respect it. What we should all try to do is investigate whether or not we’re correct, rather than cling—desperately and stubbornly—to our familiar views.

[1] Jamie Whyte has a brief introduction to errors of reasoning entitled Crimes Against Logic. He discusses the errors involved in claiming “I have a right to my opinion!”

[2] This line is thought to be the first line of Protagoras’s work, Truth. Like his other work, On the Gods, it has not survived, and what we know of him comes to us from Plato, Aristotle, Diogenes Laertius, and Sextus Empiricus. Both Plato (Theaetetus, 151d-60e; 177c-179d) and Sextus Empiricus (Outlines of Pyrrhonism, I, 216), for example, attribute the claim to him.

The opinions in The Freethink Tank’s Opinion category are those of the author and are no reflection of the views of the website or its owners.