Plato’s Theaetetus: How Do We Know What We Know?

In this dialogue, Socrates discusses various theories of knowledge. This article focuses on the most famous: the idea that knowledge is perception.

Mar 26, 2023By Luke Dunne, BA Philosophy & Theology

plato theaetetus how do we know


How do we know what we know, or claim to know? No simple answer seems adequate to that question, yet in this dialogue, Plato offers an extensive consideration of one of the most famous, and simplest, attempted definitions of knowledge: that it simply is perception. This article begins by explaining who Theaetetus was (or, at least, is presented as in the course of the dialogue). It then moves to consider some implications Socrates draws out from Theaetetus’ definition of knowledge as perception. Socrates’ criticisms are then set out, as are some of the methodological commitments underpinning them. Lastly, one potential response to Socrates’ criticisms is considered, the significance of which is left to the judgment of the reader.


How Does Plato’s Theaetetus Begin?

diogenes plato painting
Diogenes and Plato, Mattia Preti, 1649, via Google Arts & Culture


At the outset of this dialogue, Theaetetus’ credentials as a great intellectual (a mathematician, specifically) and a great man (he died on military service, defending Athens) are thoroughly vouched for. In the dialogue’s initial exchanges, he is presented as equally nervous and tentative. This prompts Socrates to offer a famous conception of his own role in the production of philosophical truth and knowledge. He describes his art as the ‘art of midwifery’, and explains it as follows:


“The most important thing about my art is the ability to apply all possible tests to the offspring, to determine whether the young mind is being delivered of a phantom, that is, an error, or a fertile truth. For one thing which I have in common with the ordinary midwives is that I myself am barren of wisdom.”


This last remark refers to the practice, common in Ancient Greece, that the duty of midwifery was reserved for those who could not themselves bear children.


Defining Knowledge

plato seneca aristotle
Plato, Seneca, Aristotle, Medieval period, via Wikimedia Commons.

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Theaetetus is then pressed to offer a definition of knowledge. He will offer several over the course of this dialogue, but it is the first of these which is the most famous and which concerns us here:


“It seems to me that a man who knows something perceives what he knows, and the way it appears at present, at any rate, is that knowledge is simply perception.”


Socrates is initially pleased with this answer for its frankness, but at once claims that it is not new. In fact, he relates it to a theory held by one of Socrates’ main philosophical rivals, namely Protagoras, the Sophist:


“This is no ordinary account of knowledge you’ve come out with: it’s what Protagoras used to maintain. He said the very same thing, only he put it in rather a different way. For he says, you know, that ‘Man is the measure of all things: of the things which are, that they are, and of the things which are not, that they are not.”


Socrates proceeds to ask some clarificatory questions. His first line of questioning concludes when he asks whether we are bound to accept that, if knowledge is perception, then the same wind which one person feels to be cool and warm must be said to be both warm and cool at one and the same time.


On Protagoras’ Theory

athenian acropolis photogrpah
Photograph of the Acropolis from Philopappos Hill, 2013, via Wikimedia Commons.


Having gained Theaetetus’s acceptance that this must indeed be so, Socrates returns to Protagoras’ thought. Socrates suggests that this theory relates to another Protagorean theory:


“The theory that there is nothing which in itself is just one thing: nothing which you could rightly call anything or any kind of thing. If you call a thing large, it will reveal itself as small, and if you call it heavy, it is liable to appear as light, and so on with everything, because nothing is one or anything or any kind of thing”.


The point of such a theory is to show that everything there is, exists because it is in a state of motion, a state of becoming. However, although Socrates ascribes this theory to Protagoras, he doesn’t ascribe it to him alone. Instead, he claims that all of the major philosophers before him (with the exception of Parmenides) and the major poets hold this theory, implicitly or otherwise.


Socrates certainly sees the appeal of this theory, and quotes several supporting examples – the body, for instance, tends to decay when it is left in a state of permanent rest, whereas exercise (that is, movement) makes it both strong and durable.


A Theory of Becoming 

protagoras ribera painting
Protagoras, Jusepe de Ribera, 1637, via Wikimedia Commons


In light of this, Socrates restates Theaetetus’ theory in light of this relation to the theory of becoming:


“According to this theory, black or white or any other color will turn out to have come into being through the impact of the eye upon the appropriate motion; and what we naturally call a particular color is neither that which impinges nor that which is impinged upon, but something which has come into being between the two, and which is private to the individual percipient.”


It is here that Socrates finds his line of attack on this way of thinking. He asks Theaetetus whether, “it possible…for any thing to become bigger or more in number in any other way than by being increased?”


It is important to catch the purpose of his question. The theory that knowledge is perception is parasitic on the theory that claims that everything which we call large will reveal itself to be small, and nothing is just any one kind of thing. What Socrates is getting at is that there are contexts in which questions of size and number are not wholly relativized – that is, once we make the initial relativization and cease to talk about number in itself, but rather increase, then we are compelled to accept certain constants.



democritus protagoras painting
Democrito e Protagora, Salvator Rosa, 1663-1664, via Wikimedia Commons


Things might be large relative to one thing and small relative to another: things cannot be said to be big or small in every context. However, things do not get bigger by themselves without there being some increase: this is true in every conceivable context.


Indeed, conceivability (or something quite like it) is the test which Socrates has for the strength of his counter-example, claiming that were Theaetetus to claim that it is possible for things to become bigger in number by some way other than being increased, then, “the tongue will be safe from refutation but the mind will not”.


Socrates then goes on to make a related methodological claim. Namely, that the point of our undertaking such an investigation is simply to look at our thoughts themselves in relation to themselves, and see what they are—whether, in our opinion, they agree with one another or are entirely at variance.


Indeed, Socrates goes on to observe that whenever we say that we perceive something, we are literally perceiving some thing: “it is impossible to become percipient, yet percipient of nothing.” It is an implication, therefore, that the terms ‘being’ and ‘becoming’ must always be applied to something.


Common Sense 

socrates louvre statue
Socrates, Lysippos, 1st Century AD, via Wikimedia Commons


This strategy of refutation is closely related to another strategy, which we can generalize as a refutation on the grounds of common sense, or at least on the grounds of what we ubiquitously hold to be true.


It is in this vein that Socrates makes the following attack on Protagoras’ thesis that ‘man is the measure of all things’. He notes first that humans are not the only creatures capable of perception; pigs, baboons and other even more ‘out-of-the-way’ animals are also capable of this. So why not, as Socrates points out, say that a pig or a baboon is rather the measure of all things?


As Theaetetus agrees, it is extremely hard to accept this thesis, although Socrates then – semi-seriously – advocates that initial ‘plausibility’ need not be the mark of truth. It is followed by another, more rigorous objection; that we take one who knows something to know it even when he is not perceiving it. To put it another way – perception and memory are not one and the same, and yet it seems bizarre to say that one knows something only as they directly perceive it, and not once they have learned and remembered it.



school of athens raffael
Raphael, The School of Athens, 1511, via Musei Vaticani.


Indeed, it would seem to be an implication of the theory that “knowledge is perception” that once one has shut their eyes, they cease to know what they did know just a moment ago, when their eyes were open. It is this point which concludes Socrates’ analysis of Theaetetus’ definition of knowledge as perception:


“we showed that a man who has seen something, and then shuts his eyes, remembers but does not see it; and that showed that he does not know the thing at the very time that he remembers it. We said that this was impossible.”


It is worth saying by way of conclusion that there are various ways of amending Theaetetus’ definition so as to save it from this objection, and yet preserve that which makes it appealing. Here is one, which I derive from David Hume. We should not claim that there is a qualitative distinction between perception and memory, because perceptions impress upon us a copy, which though weaker or less distinct, is the imprint of the original perception. In a certain sense, memory becomes a feature of perception, a version of it, weaker still, but also more fluid.

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By Luke DunneBA Philosophy & TheologyLuke is a graduate of the University of Oxford's departments of Philosophy and Theology, his main interests include the history of philosophy, the metaphysics of mind, and social theory.