54. Protagoras – The First Sophist and Philosophical Revolutionary

The philosophy of the most well-known Sophist, Protagoras, included a phrase 'man is the measure.'

“Man is the measure of all things.”

This famous dictum is familiar to most of us, yet I imagine that most people have no idea who uttered those words. You can probably guess by the title of this post that it was none other than the philosopher Protagoras. But what did he mean by it and why are there different interpretations of such a simple phrase?

In post 53 I discussed the Sophist movement, from the negative perception that they were charlatans to the positive perspective that they were teachers of rhetoric and other subjects who provided a well-needed service in ancient Athens.

In this post I will discuss a third element of the Sophists and that is that they were philosophers in their own right. As such, I will focus on their actual philosophies; I use the plural because Sophism was not a homogenized movement. There was no single school or organization of Sophism that propagated specific doctrines like what eventually would eventually emerge with Plato’s Academy or Aristotle’s Lyceum.1 Even though the Sophists shared no common beliefs, there were certain ideas and themes that were common to their teachings.

The Life of Protagoras

The best place to start would be in discussing the life of Protagoras, not just because he was the most well-known Sophist, but also because he was the founder of the movement as well. We know the names of only 30 Sophists but there were certainly many more, considering the vast educational, political, and legal needs of major cities such as Athens and other Greek city-states for which the Sophists were the sole source of such services.2

One thing that is certain about Protagoras is that like many of the ancient philosophers, we know very little about his life. He was born in Abdera, Thrace, c. 490 B.C. He eventually made his way to Athens where he spent most of this life and career. He greatly influenced moral and political thought in Athens and became an advisor to Pericles, general and statesman, as well as other wealthy influential Athenians.3 He even drafted a constitution at the behest of Pericles for the neophyte Greek colony of Thurii in 444. He was said to have been a pupil of the Atomist Democritus.

Protagoras apparently was the first philosopher to earn an income as a paid teacher of rhetoric. This seems to be cast by many as a negative aspect of the Sophists. I view this more as a positive since I see more of an entrepreneurial spirit than an opportunistic one. An entrepreneur sees an unmet need that others do not see and then seeks to fill it. There were obvious abuses, but this shouldn’t taint the good that the Sophists like Protagoras accomplished.

The Tragic End of Protagoras

Unfortunately for Protagoras, the arc of his life ended on a tragic note.

Protagoras’ ideas could be categorized into three categories: the study of the correct usage of words, epistemology, and agnosticism.4 As the saying goes, “Two out of three ain’t bad.” Well, it was bad for Protagoras for his agnosticism did not play too well for the Athenian audience.

According to Diogenes Laertius, the 4th century A.D. Greek biographer, Protagoras was indicted in Athens for blasphemy because of his book entitled Concerning the Gods.5 Unlike Xenophanes before him, he did not malign the gods but simply professed his ignorance on the matter. The following is just the introduction to his book:

“Concerning the gods, I am not able to know to a certainty whether they exist or whether they do not. For there are many things that prevent one from knowing, especially the obscurity of the subject and the shortness of the life of man.”6

As a result of just this introduction, he was indicted for blasphemy and his books were burned in the public square. Anyone who possessed any of his writings was summoned to bring them forth and have them burned also. Before he was able to be brought to trial, he escaped and eventually drowned while crossing to Sicily c. 420. This was the first recorded occurrence of cancel culture.

The Philosophy of the Presocratics

In post 53, I discussed the main differences between the Presocratic philosophers and the Sophists who came after them. Please see that article if you would like a little more background information for what I will expound upon here.

Prior to the Sophists, the Presocratic philosophers were chiefly interested in Object, trying to determine the ultimate principle or arché of the universe.7 They posited some interesting theories but failed at achieving their aim. As a result of these deficiencies, a certain skepticism or mistrust developed in relation to cosmological theories.

Because of this, if real progress were to be made in philosophy, there would have to be a shift from Object to Subject as a point of focus. Plato’s advances in reconciling unity and change in the universe were only made possible by the shift in emphasis from Object to Subject, and this shift first appeared among the Sophists. These thinkers started to realize that the greatest mystery lay not out there in the cosmos somewhere, but in man himself. The Greek tragedian Sophocles wrote around this time:

“Miracles in the world are many, but there is no greater miracle than man.”8

– Sophocles, Antigone, 332ff

So, the Sophists were not just pragmatists who met educational needs in Athens, but were philosophers in their own right who filled in the lacuna left to them by the Presocratics. As much as Aristotle may have maligned the Sophists, he really has them to thank for providing the shift in thinking that made Plato’s philosophy possible. The Sophists provided the needed bridge between the Presocratic philosophers and the Golden Age of Greece.

As mentioned earlier, rather than searching for some overarching principle of the universe, the Sophists were empiricists. They accumulated vast amounts of knowledge based on their empirical observations; they were the first encyclopedists or polymaths.9 From there, they would try to formulate general principles, taking more of an inductive rather than deductive approach. Their conclusions were partly theoretical and partly practical.

In summary, the Presocratics were more speculative. They were interested in finding objective truth, whereas the Sophists were more practical, which led them to naturally become teachers rather than theoreticians or philosophers in the traditional sense. They weren’t as interested in finding out the truth as they were in teaching and applying it. Their sense of “virtue,” then, was more of a political, practical kind than an ideal sort. It was one of intellect and ability – and one that Socrates would become famous for questioning.

Protagoras the Pioneer

Diogenes tells us that Protagoras had an impressive list of firsts. He was the first person to posit the idea that there are two sides to every argument, exactly opposite to one another.10 He was also the first person who demanded a fee for his services, and a handsome fee at that. He charged 100 minae, which is roughly equivalent to $35,000 today, the modest cost of a year’s tuition for many colleges.11 The next time that a college professor critiques the Sophists for charging exorbitant fees, he or she should think twice!

According to Diogenes:

“He was the first person to give a precise definition of the parts of time; and who explained the value of opportunity, and who instituted contests of argument, and who armed disputants with the weapon of sophism.”12

He was also the first person to divide discourse into four parts – entreaty, interrogation, answer, and injunction – and he was the first person to have regular discussions on specific topics, the equivalent of the modern-day seminar.

Diogenes also says something curious about Protagoras. He attributes the invention of the “Socratic” dialogue to Protagoras, claiming that he developed these from the reasonings of Socrates’ pupil Antisthenes.

The Correct Use of Words

As stated above, Protagoras made contributions to philosophy in three areas. I already mentioned his agnosticism; now I want to discuss what is called “orthoepeia,” or the correct usage of words. In Plato’s dialogue Phaedrus, Socrates references a work of Protagoras to this effect entitled Orthoepeia (The Correctness of Language). Unfortunately, this work has been lost.13

In Plato’s dialogue Protagoras, Protagoras interprets a poem of Simonides and pays particular attention to the literal meaning of words and the writer’s intent.14 Later sources claimed that Protagoras was the first to discuss grammar in the modern perspective of syntax. In short, Protagoras was a wordsmith. He loved words!

This type of word analysis would become particularly useful in the interpretation of court documents such as wills and deeds. Because of this, it can be said that Protagoras’ work in the correct and precise use of language contributed to the development of jurisprudence. Along these lines, Plutarch gives an account that vividly illustrates this point.

Death by Javelin

In Plutarch’s vignette, Protagoras is a guest in the house of the great statesman Pericles. As people do when they congregate, they were discussing current events, and in this case it was a recent tragedy that occurred during an athletic event.15 Apparently, an athlete had thrown a javelin and unfortunately killed someone. Plutarch’s interesting account is as follows:

“For instance, a certain athlete had hit Epitomes the Pharsalian with a javelin, accidentally, and killed him, and Pericles, Xanthippus said, squandered an entire day discussing with Protagoras whether it was the javelin, or rather the one who hurled it, or the judges of the contests, that ‘in the strictest sense’ ought to be held responsible for the disaster.”

– Plutarch’s Lives, Pericles, 36.3

They spent the entire day discussing who was culpable for this untimely death. One fact was clear: There was an accidental death on a track field. What wasn’t so clear was who was culpable morally or legally, if anyone.16 Was it the umpire, the javelin thrower, or the javelin itself? It is remarkable that Protagoras’ discussion of legal culpability sounds more like a modern than an ancient topic of debate. This led me to once again seek the perspective of my anonymous attorney friend, who made his debut in post 53:

“One can readily see in Plutarch’s vignette some of the same fundamental questions as are raised and hopefully solved for by modern western jurisprudence. The fields of tort law and criminal law, in particular, concern themselves with questions of cause and effect, material harm, and legal/moral culpability for same. The western legal tradition, including as it is embodied in final expressions of laws passed by legislatures or statutory laws, and those derived from the rulings of judges or common law, addresses the significance of human agency as a proximate cause of good or “evil” in the world. Whether in any circumstance, the human agent acted freely and knowingly, the degree to which their action caused a material result, and whether that result was desirable or undesirable for the particular individuals directly affected, as well as the society at large, lie at the heart of tort law and criminal law jurisprudence. 

“Among other things, we see these principles at work in familiar legal concepts, such as ‘duty, breach, causation, and damage,’ proximate cause, materiality, negligence, criminal intent, duress, mitigating circumstances, affirmative defenses, cruel and unusual punishment, and deterrence. All of these being fundamental principles at work in determining questions like, “What really caused Epitomes to die?” “Who, if anyone, should account for that death?” and “What should that accounting be?” At the heart of all of these questions lies the innate human desire to realize justice, order, and security (properly tempered by mercy) in the society at large, in our personal relations with others, and hopefully, within ourselves.”

Man is the Measure of All Things

By far, the above metaphysical assertion is Protagoras’ signature saying, similar to Heraclitus’ assertion that “No one steps into the same river twice.”

The actual quote as found in Plato’s Theaetetus is as follows:

“[T]he measure of all things, of the existence of the things that are and the non-existence of the things that are not.”

– from Plato’s dialogue Theaetetus, 152a

Because of this phrase, some have christened him the “father of relativism.” But what exactly did he mean by this? The exact meaning to even ancient readers was not clear and thus was a subject of much debate. Modern thinkers oftentimes add to the confusion. Because if its ambiguity, Plato devotes a long discussion to it in his Theaetetus. This is important because it gives us a good idea of how the Greeks at the time understood it. To help us understand this phrase better, we should briefly explore this entertaining and at times humorous dialogue.

Plato’s Theaetetus

Theaetetus was a Platonic dialogue from which we contemplate the philosophy of Protagoras via his 'man is the measure' phrase.

Of all of Plato’s dialogues, Theaetetus is his greatest work on epistemology, which makes it the most appropriate place to talk about Protagoras’ “man is the measure” assertion. Some would argue that this is Plato’s greatest dialogue period.17 It was written in about 396 B.C. and is classified as one of his middle to later dialogues.

Plato wrote Theaetetus in his familiar type of dialogue structure. Socrates is the main interlocutor since he is the one asking the questions. The two people in the hot seat are a brilliant young mathematician named Theaetetus and his older, less brilliant tutor, Theodorus.18

Like many of Plato’s other dialogues, it is an aporetic, a dialogue that ends in an impasse.19 Plato structures the dialogue around the discussion of three types of knowledge – all of which are rejected – so by the end of the dialogue, we are certain of what knowledge is not. As such, Socrates brings a dramatic end to this dialogue:

“If after this you ever undertake to conceive other thoughts, Theaetetus, and do conceive, you will be pregnant with better thoughts than these by reason of the present search, and if you remain barren, you will be less harsh and gentler to your associates, for you will have the wisdom not to think you know that which you do not know. So much and no more my art can accomplish; nor do I know aught of the things that are known by others, the great and wonderful men who are today and have been in the past. This art, however, both my mother and I received from God

Theaetetus 201b-c

In true Socratic fashion, Socrates encourages Theaetetus to be content with not knowing, for wisdom is defined in part as knowing what we do not know. This is also the path of humility, for those who are aware of their ignorance “will be less harsh and gentler” to their associates. Socrates’ philosophical path led to humility and not the hubris that we so often associate with philosophers. And this shouldn’t be surprising because he attributes his wisdom to God.20 This is definitely Socrates and not just Plato speaking through Socrates. Plato, as a student of Socrates, knew his teacher well.

Knowledge as Perception

The three types of knowledge posited (and refuted) in Theaetetus are as follows:

  1. Knowledge is perception.
  2. Knowledge is true judgement.
  3. Knowledge is true judgement with an account.21

Let’s deal with the knowledge as perception discussion as found in Theaetetus:

Socrates: “And so, Theaetetus, begin again and try to tell us what knowledge is. And never say that you are unable to do so; for if God wills it and gives you courage, you will be able.”

Theaetetus: “I think, then, that he who knows anything perceives that which he knows, and, as it appears at present, knowledge is nothing else than perception.”

Socrates: “Good! Excellent, my boy! That is the way one ought to speak out. But come now, let us examine your utterance together, and see whether it is a real offspring or a mere wind-egg. Perception, you say, is knowledge?”

Theaetetus: “Yes!”

– Plato, Theaetetus, 151a-e

The table is set now for discussing knowledge as only perception. From this point Socrates immediately pivots to discuss Protagoras’ “man is the measure” statement:

Socrates: “And, indeed, if I may venture to say so, it is not a bad description of knowledge that you have given, but one which Protagoras also used to give. Only, he has said the same thing in a different way. For he says somewhere that man is ‘the measure of all things, of the existence of the things that are and the non-existence of the things that are not.’ You have read that, I suppose?”

Theaeteus: “Yes, I have read it often.”

Socrates: “Well, is not this about what he means, that individual things are for me such as they appear to me, and for you in turn such as they appear to you—you and I being ‘man’?”

Theaetetus: “Yes, that is what he says.”

– Plato, Theaetetus, 151e-152b

For those old enough to remember the old Perry Mason television show, it always appeared that Perry’s client was guilty until he started interrogating the key witness, asking him or her questions in the Socratic fashion. He then proceeded to dismantle the witness piece by piece through his interrogation. You get the sense that Socrates is about to dismantle Theaetetus’ argument:

Socrates: “It is likely that a wise man [Protagoras] is not talking nonsense; so let us follow after him. Is it not true that sometimes, when the same wind blows, one of us feels cold, and the other does not? or one feels slightly and the other exceedingly cold? . . . Then in that case, shall we say that the wind is in itself cold or not cold or shall we accept Protagoras’ saying that it is cold for him who feels cold and not for him who does not?”

– Plato, Theaetetus, 152b

A question often asked about Protagoras’ “man is the measure” statement is whether he is talking about man in the singular as an individual or in the plural as mankind. Our most important source for this statement is Plato, but there are also two other sources: Aristotle and Sextus Empericus.22 It is interesting that all three understand “man” to be “each individual.”

Theaetetus’ argument is that two people who are together in a certain location may perceive the temperature differently. In other words, one might be hot and the other cold. This is universal and leads to thermostat battles in almost every household. According to Pythagoras, then each person is the “measure” of the perception of temperature in his environment.

Socrates moves in for the kill. He apparently has many objections to this view:

Socrates: “Perception, then, is always of that which exists and, since it is knowledge, cannot be false.”

– Plato, Theaetetus, 152c

This is where the arguments start to fall apart. If perception is knowledge of what exists, then it cannot be false. But if that is the case, then how does one explain contradictory perceptions? Now Socrates starts to undermine this philosophy of Protagoras:

Socrates: “By the Graces! I wonder if Protagoras, who was a very wise man, did not utter this dark saying to the common herd like ourselves, and tell the truth in secret to his pupils.”

– Plato, Theaetetus, 152c

The humor of his sarcasm is very effective here. Now he moves in with the coup de grâce:

Socrates: “I will tell you and it is not a bad description, either, that nothing is one and invariable, and you could not rightly ascribe any quality whatsoever to anything, but if you call it large it will also appear to be small, and light if you call it heavy, and everything else in the same way, since nothing whatever is one, either a particular thing or of a particular quality; but it is out of movement and motion and mixture with one another that all those things become which we wrongly say ‘are’—wrongly, because nothing ever is, but is always becoming. And on this subject all the philosophers, except Parmenides, may be marshalled in one line—Protagoras and Heraclitus and Empedocles…”

– Plato, Theaetetus, 152d-e

According to Socrates, if Protagoras’ doctrine was true, then oneness could not exist. On this point, he surprisingly calls on Parmenides as his expert witness. Plato had a lot of respect for Parmenides, even devoting an entire dialogue to him. But this is even more remarkable considering, as I discussed in my Parmenides series, that Parmenides’ doctrine of Oneness denies reality.

By lumping Protagoras in with Heraclitus and Empedocles, he seems to be equating Protagoras’ “man is the measure” doctrine with Universal Flux and Eternal Recurrence, respectively. Therefore, if all is change and relative, then “nothing is one and invariable, and you could not rightly ascribe any quality whatsoever to anything.” In other words, the universe would be absurd.

Closing Arguments

We must remember that Plato’s concern, in Socratic fashion, was to discover the truth by showing the limits of the various philosophies that were contending for the truth, and this extended to the thoughts introduced by the Sophists.23 In this way, Plato seems to oversimplify Protagoras’ “man is the measure” statement, only limiting to sense perceptions, whereas Aristotle and Sextus Empericus broaden the interpretation to include opinions and moral judgments. Even Plato, in another section of Theaetetus, does the same:

Socrates: “Well then, Protagoras, what shall we do about the [‘man is the measure’] doctrine? Shall we say that the opinions which men have are always true, or sometimes true and sometimes false? For the result of either statement is that their opinions are not always true, but may be either true or false. Just think, Theodorus, would any follower of Protagoras, or you yourself, care to contend that no person thinks that another is ignorant and has false opinions? . . . Do not myriads of men on each occasion oppose their opinions to yours, believing that your judgement and belief are false?”

– Plato, Theaetetus, 170c-d

So, why is Plato doing this? He seems to contradict himself in interpreting Protagoras. We have to remember that Plato likes to test his readers. In the above case, he simply set up a strawman argument to see if we would catch the fallacy of the argument. The argument is fallacious because it contains the false premise that Protagoras’ “measure” was limited to sense perceptions and not “opinions, beliefs, and judgments.” Part of Plato’s purpose for his dialogues was not simply to convey truth but to train our reasoning skills so that we could pursue the truth. Consider his dialogues a philosophical boot camp, if you will.

As you can see, we are leaving the world of the Sophists as philosophers and entering the world of Plato. This is why I introduced the Platonic dialogues in this post and the previous post – to whet your appetite as we move toward Plato.


What about Protagoras’ “man is the measure” statement? By formulating that statement, Protagoras is leading us into the world of the theory of knowledge or epistemology. How do we know what we know? And this is a big can of worms indeed.

This is why in the next post I will continue this discussion of the “man is the measure” statement and use that as a doorway to discuss epistemology. This is very relevant for us since a great deal of the problems that afflict us in the 21st century go back to the major philosophical sea change ushered in by Descartes. By shifting the philosophical emphasis from that of metaphysics – of which epistemology is just a tool – to epistemology itself, he led the modern age into an extreme skepticism from which we have yet to recover. Instead of analyzing the house under construction, we analyze the tool box.

The Philosophy of Protagoras

Finally, if Protagoras’ simple statement of “man is the measure” can plunge us into the depths of philosophy, he couldn’t have been that much of a charlatan, but rather a philosopher in his own right.

I close with the following poem of Diogenes eulogizing Protagoras:

I hear accounts of you Protagoras, That, traveling far from Athens, on the road, You, an old man, and quite infirm, did die. For Cecrops’ city drove you forth to exile; But you, though ‘scraping dread Minerva’s might, Could not escape the outspread arms of Pluto.”24

Any comments? Please leave them below and don’t forget to subscribe. Thank you!

Deo Gratias

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From Amazon: “The Theaetetus is a seminal text in the philosophy of knowledge, and is acknowledged as one of Plato’s finest works. Cast as a conversation between Socrates and a clever but modest student, Theaetetus, it explores one of the key issues in philosophy: what is knowledge? Though no definite answer is reached, the discussion is penetrating and wide-ranging, covering the claims of perception to be knowledge, the theory that all is in motion, and the perennially tempting idea that knowledge and truth are relative to different individuals or states. The inquirers go on to explore the connection
between knowledge and true judgement, and the famous threefold definition of knowledge as justified true belief. Packed with subtle arguments, the dialogue is also a work of literary genius, with an unforgettable portrait of Socrates as a midwife of wisdom.”

Footnotes and Endnotes:

  1. Taylor, C.C.W. and Mi-Kyoung Lee, “The Sophists”The Stanford Encyclopedia of Philosophy (Fall 2020 Edition), Edward N. Zalta (ed.)
  2. Kerferd, George Briscoe. “Sophist”Encyclopedia Britannica, 6 May. 2020
  3. Poster, Carol, “Protagoras”, The Internet Encyclopedia of Philosophy
  4. Ibid.
  5. Copleston S.J., Frederick, A History of Philosophy, Book One, “Volume 1 Greece and Rome”, p.87, An Image Book, Doubleday, 1985
  6. Laertius, Diogenes, Lives of Eminent Philosophers, Translated by C.D. Yonge, p. 334, Digireads.com Publishing, 2020
  7. Copleston S.J., Frederick, A History of Philosophy, p. 81
  8. Copleston S.J., Frederick, A History of Philosophy, p. 82
  9. Ibid.
  10. Laertius, Diogenes, Lives of Eminent Philosophers, Translated by C.D. Yonge, p. 334
  11. Vickers, Michael. “Golden Greece: Relative Values, Minae, and Temple Inventories.” American Journal of Archaeology, vol. 94, no. 4, Archaeological Institute of America, 1990, pp. 613–25; Of course this is a very rough estimate for it is very difficult to compare monetary values between two very different societies separated by more two millennia.
  12. Laertius, Diogenes, Lives of Eminent Philosophers, Translated by C.D. Yonge, p. 334
  13. Bonazzi, Mauro, “Protagoras”, The Stanford Encyclopedia of Philosophy (Fall 2020 Edition), Edward N. Zalta (ed.), cf. Phaedrus 267c
  14. Poster, Carol, “Protagoras”, The Internet Encyclopedia of Philosophy and for a fascinating discussion of this topic see “Chapter 2. Simonides’ Ode to Scopas in the Protagoras“, The Center for Hellenistic Studies, November 2, 2020
  15. Stone, Ferdinand Fairfax. “A Problem for Pericles.”
  16.  California Law Review, vol. 59, no. 3, California Law Review, Inc., 1971, pp. 769–83
  17. Chappell, Sophie-Grace, “Plato on Knowledge in the TheaetetusThe Stanford Encyclopedia of Philosophy (Spring 2021 Edition), Edward N. Zalta (ed.)
  18. Chappell, Sophie-Grace, “Plato on Knowledge in the TheaetetusThe Stanford Encyclopedia of Philosophy (Spring 2021 Edition), Edward N. Zalta (ed.)
  19. Ibid.
  20. It is interesting that Socrates often refers to “God” in the singular and not to the “gods” in the plural. In the polytheistic age in which he lived, I believe that this is because he was aware of the one true God. For further discussion on this, please see my earlier posts on Socrates.
  21. Chappell, Sophie-Grace, “Plato on Knowledge in the Theaetetus
  22. Bonazzi, Mauro, “Protagoras”, The Stanford Encyclopedia of Philosophy
  23. Ibid.
  24. Laertius, Diogenes, Lives of Eminent Philosophers


Copleston S.J., Frederick, A History of Philosophy, Book One, “Volume 1 Greece and Rome”, An Image Book, Doubleday, 1985

The First Philosophers, A new translation by Robin Waterfield, Oxford University Press, 2000, reissued 2009

The Greek Sophists, by John Dillon (Translator, Introduction), Tania Gergel (Translator, Introduction), Penguin Classics; 1st edition, October 28, 2003

Laertius, Diogenes, Lives of Eminent Philosophers, Translated by C.D. Yonge, Digireads.com Publishing, 2020

Moser, Paul K., The Oxford Handbook of Epistemology, Oxford University Press, October 27, 2005

Moss, Jessica, Plato’s Epistemology: Being and Seeming, Oxford University Press, March 11, 2021

Plato, Complete Works, edited by  John M. Cooper and D.S. Hutchinson, ‎ Hackett Publishing Co., 1848 pages, May 1, 1997

Plato, Theaetetus, Oxford World Classics, 1st Edition, Translated by John McDowell and Lesley Brown, April 1, 2014

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