A summary of Myles Burnyeat's article 'Knowledge is Perception: Theaetetus 151d-184a'


First, a quick introduction to the issues:

Three theses are in play:

  1. Theaetetus' definition of knowledge as perception.
  2. Protagoras' doctrine that man is the measure of all things. However things appear to a person is the way they are for that person. Things may appear different to different people, in which case they really are different for those different people.
  3. Heraclitus' theory of flux. Everything is changing all the time: nothing just is, but rather everything is always becoming.

152c says that a candidate for knowledge must 1) be always of what is and 2) be unerring.

One prevalent reading (called "A") of these theses has it that Plato accepts a form of Protagoreanism and a form of Heracliteanism, but they are restricted to the realm of sensibles and perception. Sensible things are in flux, and each person's perception of them is incorrigible. So perception is unerring, but it is not of what is. This reading usually holds that Plato is consistent between Theaetetus and Republic in that he holds that knowledge is restricted to forms (compare Fine's thesis about the Republic which says that the Two Worlds view is wrong and that there is knowledge of sensibles as well as forms). This reading holds that knowledge cannot be perception and that Protagoreanism and Heracliteanism are true of the sensible world.

Another reading (called "B") holds that Theaetetus' proposal that knowledge is perception requires him to commit to Protagoreanism, and that will in turn require him to commit to Heracliteanism. And Protagoreanism requires the thesis that knowledge is perception. In other words, the three theses stand and fall together. The arguments up to 183 amount to absurdities which result from these three theses and culminate in the impossibility of language.
The conclusion of the dialogue is that all three are impossible, whether in the sensible world or in the world of the forms, or in a world that includes both.

Burnyeat apparently thinks that it is still possible that protagoreanism and heracleiteanism and knowledge is perception are right if properly formulated and defended (i.e. reading A?)

Exposition of the Three Theses (151d-160e)

Empiricism is the doctrine that all knowledge has sense experience as its source. Empiricism often finds that perceptions of things like hot or red are paradigmatic instances of knowledge.

"Perception is knowledge" breaks down into two theses:

Socrates' Protagoreanism at 151e-152a gives a clearer meaning to the claim that knowledge is perception. 152b-c argues that Protagoreanism leads at least to 1) all perceiving is knowing. In generalized form, x appears F to a if and only if a perceives that x is F, and perceiving that x is F is an instance of knowing that x is F. To get there, Plato needs to say that appearing is identical to perceiving, because Protagoreanism only mentions appearing. Does Plato himself believe (A), or does he just think that Theaetetus needs it (B)?

152c-e introduces Heracliteanism. To say that "x is F for a" is just to say "x becomes F for a." Reading A thinks that it is something Plato believes in. Reading B thinks Theaetetus needs it, but Plato is not committed to it.

153dff argue that when one thing has a compresence of opposites (for example, a mouse can be both large and small at the same time and so have opposites co-present in it), that compresence can be explained in terms of relatives: "x is large" and "x is small" are compatible if you mention that in relation to which x is small or large. Protagoreanism says that seeing white is a relative thing: one person sees white, another may see something else. The relation between the person who sees white and the white object is one of perceived whiteness, but another person has a different relationship (because no two people are the same person). Thus the conflicting appearances at 153d-154b are analyzed as follows:

Theaetetus' definition holds that perception and perception alone provides certainty and knowledge. Protagoras provides the reliability of every perception, which is accomplished through relativization. There is no question of one person being right about a color (or any other appearance): there is no perceiver-independent fact of the matter about the color. The whole physical world and knowledge of it becomes a private affair.

Heraclitus provides the thesis that there are no things, only processes, no properties of things, only motions, a claim which can be interpreted in physical or metaphysical terms.

In physical terms, it would mean that whiteness and sight are the 'quick motions' that go on in between the eye and the object. Objects and eyes emit streams of particles according to the theory presented in the Theaetetus.
A metaphysical interpretation would say that Heracliteanism is not literally about physical particle streams, because that sort of story involves things that ARE, and are stably. The metaphysical interpretation holds that this is about "seeing is seeing something OVER THERE FROM HERE, and whiteness is whiteness manifesting itself THERE TO ME HERE." (P 330). The point is that seeing and whiteness require reference to two locations. Thus "the Heraclitean story is a metaphysical projection of a world in which the Protagorean epistemology holds good." (ibid.) I have to admit that I am mystified by how we can plausibly explain Burnyeat's purported 'metaphysical' version of this account of perception: that there might be such an explanation seems possible, but the discussion in the Theaetetus seems so clearly to be talking about physical things that I am sceptical.

FYI: 157e-158e has the first extant version of the idea that it is impossible to determine whether we are sane or insane, dreaming or awake. The conclusion of the dialogue is that any experience has as much claim to be called true as any other.

158e-160c has a difficult argument that concludes universally that there is no identity over time. In the end it is not clear whether we can refer to anything: people may be a collection of perceptions and objects may be a series of ideas, but then what sense can we give to people and objects?

Now that Plato has laid out the three theses in general, he proceeds to critique them.

Critique of the three theses (160e-184a)

Plato first considers whether wisdom is possible in the epistemology created by these three theses. The idea is that wisdom is specialist knowledge that one person knows BETTER than another person. It is impossible to call any person wise if every perception of every person is true, or so it seems. Consider the following propositions, which are all considered equivalent:

  1. each person is the measure of his or her own wisdom (161e, 169d)
  2. everyone is equally wise (162c)
  3. there is no such thing as wisdom/no one is wise (166d)
  4. no one is wiser than anyone else (161d)
So how can Protagoras maintain that he teaches others or makes them better? Common sense tells us that Protagoras is trapped: he has no way out.

At 162d-e, Protagoras is made to protest against such common sense interpretations of what he says: it's not clear whether he can possibly find a way out.

Reading A says that Plato gets out of the problems (how to deal with the physical world and communication) by restricting Protagoreanism to sensibles.
Reading B thinks that the problems are just some among the many absurdities of the triple thesis (Theaetetus, Protagoras, and Heraclitus) which stand and fall together, and Plato is not concerned to get out of them, but rather is simply doing away with all 3 theses.

The next step is to expand the scope of the Protagorean formula "x appears F to a":

What this step does is to include all knowledge and all appearances, not just sensory ones, within the scope of the triple thesis. On reading A, Plato is acknowledging the difference between sensibles and knowables. On reading B, Plato is exploring whether the triple thesis can stand if it is applied to other things besides sensibles.

In 164-168, an idea is explored: you can see dimly, but you cannot know dimly (Bailly aside: why can't one know dimly? what is learning and forgetting?), and so knowing cannot be seeing, but seeing is perceiving, and so perceiving cannot be knowing. The argument relies on the idea that you should be able to substitute a definition for the thing that is defined. Socrates says that objections to the perception=knowing thesis such as the fact that you cannot know dimly, but you can see dimly, or other ones that rely on some dissymetry between knowing and perceiving are misguided. Burnyeat is not so sure: he does not see what is the matter with testing a proposed definition by substituting the definition for the thing defined. Plato, however, insists that such tests address the form, not the substance of the thesis (164c-165e, 166b-c, 168b-c). So far, I'm with Burnyeat on this issue, but Plato is so insistent that I would like to figure out more about this.

The Defense of Protagoras

166aff attempts to defend the Protagorean thesis agains the objection that it makes expertise impossible by claiming that expersise is a pragmatist thing: an expert can effect an improvement in our perceptions, feelings, and thought. "The true or truer state of mind is the one which has the most satisfactory consequences." (335) Aside from the claims about being truer, that pragmatist interpretation is the prevailing interpretation of 166aff. Results are important: true or not. But the meaning of "most satisfactory" and "improvement" is problematic, because whatever sense they have seems to involve an objective non-relative matter: there has to be a fact of the matter independent of perception to decide what improvement is. Otherwise we are simply left with two alternatives and no objective way to decide (could the decision be subjective? and leave the argument intact). What is more, the answer to the question "are there experts" also becomes objective. If any objectivity creeps in, then the theory expounded is not a relativist one. But 167a-b insists that both the 'better' and the 'worse' mental states are equally true. So the pragmatist view of truth is not present there.

What if Protagoras claims that people obviously think things like "I am better off because of this change in my life" and "I am worse off because of that change in my life." The "experts" are the ones who effect changes in people and about whom people say "I am BETTER off because of the change caused by so-and-so in my life"? Isn't that a fine pragmatist defense of Protagoras?

Burnyeat suggests that Protagoras can perhaps cling to: "(D1) State of mind S1 is better for person a than state of mind S2 if and only if it seems to a that S1 is better than S2." He suggests it is a short step to "(D2) x is an expert for a if and only if it seems to a that a is better off thanks to x." (336).

D1 and D2 will save Protagoras' theory (167d and 168b have Protagoras claiming that he can save his theory: Burnyeat provides the only way he sees).

But "better" in D1 and D2 is still problematic. At 166d, Protagoras says "I call wise the man who in any case where bad things both appear and are for one of us, works a change and makes good things appear and be for him."
There are two ways to take that, one compromising the generality of the claim that everything is relative, the other maintaining it: "makes good things appear and be for him" could mean that actually better things also appear better for him OR that when things appear better to him, that makes it the case that they are better (for him). Later, at 171d-172b and 177c-d, we get a clearly compromising Protagoras.

What is at stake here is whether the relativist can be a thoroughgoing relativist and hold onto the idea that there is expertise or not. If the relativist can, then objective truth may have no place in human life. If Protagoras need not compromise, then he can abolish objective truth. Theodorus is a mathematician, and his presence ought to remind us of the problematic nature of mathematics and mathematical expertise vis-à-vis objective truth.

Protagoras attacked: false judgements

169d-171d is a further attack on Protagorean relativism. Protagoreanism holds all of the following:

  1. Most people believe that false judgements occur (and they believe that they themselves have some false judgments or occasionally make false judgements).
  2. Whatever someone believes is true (for him or her).
  3. Therefore, false judgements are true (for the people who make the judgements).
  4. There is false judgement (this results from 1 (for anyone who thinks there is false judgement) via Protagoreanism, or maybe it's just an empirical claim).
  5. False judgement is being wrong about something.(just a definition of false judgement)

The little qualifier "for him or her" in the above argument helps Protagoras. Protagoras can say that many people believe false judgements occur, and so it is true for them. That does not make it true for Protagoras. So those people may be inconsistent, but Protagoras need not be. Socrates mounts an argument which purports to refute Protagoras. The key to this argument is at 171a, where the qualifiers are dropped, but most people think they cannot be, and so Plato has made a mistake (or has been dishonest, depending on your opinion of Plato's argumentative tactics). The argument goes as follows:

Most scholars think that Socrates is not entitled to drop the qualifiers at "a" above, and so Socrates is not entitled to his conclusion.

Burnyeat points out that Socrates' attack was flawed, but that he was right that there is something deeply suspicious about a general claim that "it is true that all truth is relative," because that proposition itself cannot be relative if it is to be true. Even if we say that it really means "it is true for me that all truth is relative," that is still an assertion that is not itself relative. Can the relativist then say that "it is true for me that all truth is relative for me" is not problematic ? No, that too appears non-relative. So perhaps the relativist can say "it is true for me that it is true for me that all truth is relative for me," etc. What are we to make of this infinite regress? Is there anything the matter with it?

The critique resumed: 1) refutation of Protagoras (177c-179b)

From 178b-179b, Socrates argues that any judgement about the future must also be excluded from the list of relative things.

The argument is something like the following:

The relativist might claim that what I think now about the future MAKES it true that it will happen (178d-e). Such self-fulfilling prophecies do occur sometimes, but it is not clear that they can be generalized to include all cases.

Perhaps the relativist will claim that I am not the same person on Tues as I was on Mon. He or she might claim (163d-164b) that at any given time my past just is whatever it now appears to me to have been. So Monday's self has a different past from Tuesday's self. Just so, the future just is whatever it seems to me now. Future selves have different pasts and futures from today's self. (In that case, however, the relativist will have a hard time saying why the two selves are the same person: if they are not, then there is no continuity of persons, which is hard to swallow, but not inconceivable).

If the relativist cannot tell a good story about such things, then he or she will have trouble with all things that have anything to do with the future. But action has to do with the future, so the relativist will have problems with action (doing one thing for the sake of another usually involves doing something now for something in the future, and it also seems to involve a belief that I am the same person now as I will be in the future).

Next, Plato moves on to another argument (179c-183c), which goes something like the following:

Because things are in flux, the Heraclitean theory holds that: "a perceptual judgement like 'I see something white' identifies and records a momentary subject's momentary perception of a momentary object's momentarily occurring whiteness, a whiteness which exists privately for that perceiving subject alone" (348). Empiricism requires that our sense-data are in every respect incorrigible. But Heracliteanism makes every perception as true as it is false, because each is private and momentary: there is no way to compare them to each other across time or from subject to subject (remember, if the perceiver too is in flux, there is no way for a perceiver to compare one momentary perception to another momentary perception). Language has no meaning, because there is no common experience. Making sense judgements totally incorrigible paradoxically means that they have no content that makes sense over time or between people.

Reading A will claim that confining the scope of flux to sensible objects solves a problem by leaving knowledge possible outside of the sensible realm. Reading B holds that these claims are simply the culmination of the absurdities resulting from the triple thesis, whether we confine them to sensibles or not.

The argument starts with the question of whether Heracliteanism is confined to spatial movement or also applies to qualitative change (181c-d). If it allows only spatial change, then things will be stable qualitatively, and so Heraclitean flux will not hold. If a thing is stable, there is a fact of the matter about it. If there is a fact of the matter, then the Protagorean thesis won't hold: even if there is only momentary stability and no identity over time, still, it allows the possibility of being wrong. The mere possibility of being wrong means that perception can be mistaken, perception is not knowledge, and Protagorean relativism was wrong that human is the measure.

At 179c, Socrates distinguished between perception itself and perceptual judgements. On this distinction, the relativism doctrine becomes split as well: people's perceptions AND their judgements of things accurately reflect how things are.

It is taken for granted that perceivers will be able to accurately judge their perceptions: the idea is that perceiving hot is not a matter of judging. But there is nothing in the triple thesis that prevents incorrect perceptual judgements. "x is/becomes F for a at time t" is absolute, but a could somehow mistakenly judge that perception. If there is any stability in there, once again, then there is a possibility of being wrong, because there is a fact of the matter (we all know that perceptions differ from person to person, even at the same moment, but if there is a fact of the matter, then at least one of two opposite perceptions about the same aspect of the thing at the same time has to be wrong).

In the end, Socrates says, Sure, every perception is true, but there is nothing left for perception to be right about, no way to communicate it, and all statements are equally right and wrong.

I wish this had been good-bye to relativism, but Burnyeat was not able to honestly claim that Plato (or he himself) has completely proven that relativism is wrong.  One thing he has done is to show that it is deeply problematic: it is not a clear and simple solution, nor is it intellectually as respectable as it seems.