Man is the Measure of All Things: Analysis of Socrates Versus Protagoras Argument

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Within Plato’s dialogue ‘Theaetetus’ (Burnyeat and Levett, 1990) Plato’s Socrates is concerned with epistemology - the theory of knowledge. In this essay, I will explain the arguments for the claim that knowledge is perception according to Protagoras, as well as Socrates’ refutations. Theatetus’ claim that knowledge is equivalent to perception implies for Plato’s Socrates that knowledge is simply relative to what one person perceives. Socrates quotes Protagoras’ book ‘Truth’, stating ‘Man is the measure of all things: of the things which are, that they are, and of the things which they are not, that they are not.’ (152a). Here, Socrates understands the Protagoras of Plato’s dialogue as claiming that there is no knowledge in itself, knowledge is simply what is perceived by one. Plato writes; ‘…As each thing appears to me, so it is for me, and as it appears to you, so it is for you…’ (152a). Things appear differently for different people, so whatever one individual perceives as true is true for them and so their knowledge is the truth. If one person perceives a room to be warm while another perceives the same room to be cold, both will be correct.

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There is no objective reality as we cannot understand another person’s experience and therefore cannot prove their claim wrong. Within the dialogue, Plato’s Socrates states; ‘…Nothing which you could rightly call anything or any kind of thing… Nothing ever is, but everything is coming to be.’ (152d). Plato’s Socrates gives several refutations for Protagoras’ argument. He first begins a weaker argument from authority. If knowledge is simply perception and man is the measure of all things then surely any animal capable of perception is equally wise. Here, Plato writes; ‘how could it ever be, my friend, that Protagoras was a wise man, so wise as to think himself fit to be a teacher of other men…’ (161d). The argument made here is that if Protagoras believes his epistemology to be true, then he surely cannot believe himself to be a teacher if all his students are assumed to be equally as knowledgeable as him. This brings authority into question. Nobody can ever be in error as what he or she perceives is truth for them and so the concept of expertise is abolished.

Humans no longer have authority over animals as both creatures are equally wise, although Socrates himself knows that this is not the case. His Socrates responds, in the voice of Protagoras; ‘…It would be a shocking thing if no man were wiser than a cow in a field.’ (162e). Within this argument, Plato’s Socrates is simply stating that accepting Protagoras’ philosophy as true means our notion of authority is false and we know our notion of authority is not false. Plato’s Socrates gives no justification for why our authority is not mistaken, he just simply states that this is the case. Without justification, this argument is weak and no more than an opinion. This argument also appears rather close-minded as by accepting what Plato has argued there is no consideration that our notion of authority could be incorrect, therefore adhering to our rigid beliefs about authority.

Plato also makes the assumption that humans and animals are capable of perception equally, whereas we do not know whether an animal is capable of having beliefs or even understanding the world with complexity. Due to this, the validity of Plato’s argument is reduced as it cannot be proved to be a fair comparison. Plato also seems to ridicule Protagorus, which does not provide a good counter to his argument. Within Philosophy and Phenomenological Research (Kattsoff, 1953), it is written; ‘The proper refutation of the Sophists cannot be ridicule or ad hominem arguments, but can only take the form of a new philosophical anthropology which successfully indicates the inadequacies of a sense-bound theory of man and his knowledge’.

Although this argument is weak, Plato does create a solid foundation for the issues regarding relativism. If Protagoras himself believes that what is true for one is not always true for another then relativism itself may be true for him but not to others. This lack of objectivity shows inconsistencies within his own epistemology. Similarly, if Protagorean relativism is accepted then we must also accept the assumption put forward that humans and animals are equally as wise, regardless of whether this is true or not. If Protagoras is no wiser than a pig or a cow, what reason do we have to believe his theory of knowledge? Also, despite the insufficient justification regarding authority, the argument holds strong deductive inference and so Plato’s argument is structurally sound from premise to conclusion.

Plato’s Socrates challenges his own argument by considering how Protagoras himself would respond. He concludes that by accepting Protagorean relativism we do not have to accept that all creatures are equally wise or that everyone is of equal authority. Plato writes; ‘…The man I call wise is the man who can change the appearances…’ (166d). Here it is argued that expertise is not based on having greater knowledge than another individual but being able to change and better how a situation appears to others. Using this way of thinking we can conclude that we can only understand better or worse beliefs, not true or false beliefs. Plato’s Socrates provides a stronger argument to the issue of authority. He argues that we can distinguish between true or false beliefs when concerning the future and making predictions. If a prediction is made it either will happen or will not happen, thus there is a true or false belief being predicted. This argument not only justifies the weaker argument that our notion of authority if not mistaken, but also refutes Protagoras’ claims.

Authority is given because one has greater knowledge than another, demonstrated by their future predictions, so true judgements can be made. Therefore, knowledge cannot be perception and Protagoras cannot be correct in his epistemology. This argument is much stronger in comparison due to the evidence Plato’s Socrates provides to back up his claims. A deductive inference is used to guarantee the conclusion that knowledge is not perception, as each premise is justified. Unlike the previous argument, the premise that true and false beliefs can be distinguished when regarding future predictions is sufficient justification that knowledge is not perception, as this is not based on opinion. The argument relies on objectivity, as the future is something that either will or will not happen. This raises the validity of his argument as it is held on a factual basis, unlike the previous argument which begs the question, due to it’s subjective premise.

However, Plato does understand that Protagorean relativism can be solved if we take a stance of Heraclitean flux, despite not being directly related. Due to this, Plato’s Socrates also offers a refutation to Heraclitus’ flux theory. This is the argument that everything is always in a sate of flux and change, so nothing can ever hold identity or being. Plato writes; ‘…No one of them is anything in itself; all things… are coming to be through association with one another, as the result of motion.’ (157a). According to this doctrine, two opposite beliefs do not contradict each other as neither of them has any true properties or identity. This philosophy support the theory of Protagorean relativism as if nothing has any identity then what one individual perceives as true must be true for them, as nothing holds any properties that would prove otherwise.

Therefore, man is the measure of all things as his entire reality is based on what he believes to be true. Plato also provides another argument to relativism. Plato argues that many believe in false beliefs but if all beliefs are true then their beliefs are false, while if not all beliefs are true then there are false beliefs (169d-170c). This conflicts with Protagorean relativism because either way false beliefs exist. His argument is valid through deductive inference and he also uses logic to come to his conclusion, objectively proving how the framework of Protagoras’ thinking collapses when issues such as this are considered. Plato’s Socrates also discusses the issue of relativism itself. Plato writes; ‘…Supposing he himself did not believe that man is the measure, any more than the majority of people, then this Truth he wrote is true for no one?’ (171a).

Here, Plato points out the fundamental problem that relativism faces - if man is the measure of all things, yet some do not believe this to be true, then man is not the measure of all things for them. Anyone who disagrees with Protagoras and believes him to be wrong must be right in Protagoras’ own view, as that belief is true for them. Therefore, Protagorean relativism must be deemed as incorrect by everyone, including Protagoras himself. Again, Plato uses logic and rationality to come to this conclusion. When considering the implications of Protagoras’ doctrine it becomes clear to Plato how inconsistent relativism truly is, as by putting his own truth into practice it becomes clear how broken it really is. As a result of this, Plato is able to create a sound and coherent argument against Protagoras. Plato’s Socrates, however, finds this theory to be insufficient. He argues that if we accept Heraclitus’ doctrine then we can never attribute meaningful qualities to anything, nor could there be knowledge of anything.

Although our beliefs may be true, they become meaningless. Also, if everything is in a constant state of change then our beliefs must also be subject to change, as if we believe an object to be red then we must also accept that, due to the state of flux the object is in, it is changing to become not-red, therefore our belief cannot be true. Plato writes; ‘…If all things are in motion, every answer, on whatever subject, is equally correct, both ‘it is thus’ and ‘it is not thus’.’ (183a). Due to this, both Protagorean relativism and Heraclitean flux cannot be correct as both rely on the theories that man is the measure of all things and all is flux. If we accept Heraclitus’ philosophy then neither of these can be true as nothing is anything - man cannot be the measure of all things as man cannot be anything due to his lack of identity or properties. However, it may still be argued that despite Plato’s refutations of Protagoras’ argument, he may have simply misunderstood what he was trying to construe. Although this is subjective in itself, it has been argued (Epps, 1964); ‘…Was he not simply stating the inescapably inherent subjectivity of all human knowledge and thinking?’. While this argument cannot be validated, it is still a possibility that Protagoras’ claim was simply misinterpreted. Here, Plato demonstrates a valid refutation for both theories.

The argument holds deductive inference as the premises, such as that nothing can be anything and therefore all is not flux, guarantee the conclusion that Heraclitean flux cannot be true and therefore neither can Protagorean relativism. Within Philosophy (Burrell, 1932), it is stated; ‘How can you prove an opinion to be false if falsehood is impossible, and impossible for the very simple reason that every opinion of every single person is true?’. Again, the argument is objective as it involves no subjective opinion as Plato uses logic to come to the conclusion of his argument. This is shown by the use of Heraclitus’ own language - all is flux yet nothing holds a single property. The two beliefs contradict each other and therefore cannot exist at the same time. Again, the use of logic makes the argument more valid. This strong refutation of Heraclitean flux therefore invalidates the argument for Protagorean relativism. Within his dialogue, Plato puts forward multiple different arguments against Protagoras. Although all of them are not equally strong as others, Plato acknowledges where his arguments are weak and counters them, such as his argument to authority and challenging it as Protagoras himself. By using logic and objectivity to structure his assertions, Plato is able to provide sound refutations to both Protagoras and Heraclitus, recognising the relationship between the two. Therefore, Plato provides several adequate arguments against Protagoras’ epistemology.


  1. Plato (1990). The Theaetetus of Plato. Translated by Levett, M. Revised by Burnyeat, M. Indianapolis: Hackett Publishing Company.
  2. Kattsoff, L. (1953). Man is the Measure of all Things. Philosophy and Phenomenological Research. Rhode Island: International Phenomenological Society, 13(4), pp. 454. Burrell, P. (1932).
  3. Man the Measure of All Things: Socrates versus Protagoras (I). Philosophy. Cambridge: Cambridge University Press, 7(25), pp. 34.
  4. Epps, P. (1964). Protagoras' Famous Statement. The Classical Journal. Minnesota: The Classical Association of the Middle West and South, Inc., 59(5), pp. 225

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