Plato against the Sophists: Is Man the Measure of All Things?

Why was Plato so opposed to Sophistic thought? In this article we explore his interaction with the Sophists, and especially with Protagoras.

Feb 16, 2023By Luke Dunne, BA Philosophy & Theology

plato against the sophists


Who were the Sophists? Who was Protagoras? What did he mean by “Man is the measure of all things”? This article goes over two separate criticisms – that Protagoras’ argument is self-defeating, and that accepting it leads to solipsism. In explaining these criticisms, some reasonable counter arguments are considered, many of which hinge on the consistency of empiricist philosophers devaluing the importance of truth and knowledge in favor of a more practical approach.


This article also represents an introduction to certain general aspects of Sophistic thought. We will consider criticisms of Protagorean philosophy found in Plato, and several further criticisms which are either implicit in Plato’s work or developed separately.


What Is a Sophist?

democritus protagoras
‘Democritus and Protagoras’ by Salvator Rosa, 1663, via the Hermitage Museum.


Protagoras was a professional sophist, and so his work – as for most sophists – was oriented towards the main goals of his profession, which were largely political in nature. Sophists taught certain technical skills – such as rhetoric, a practice with many and elaborate conventions in Ancient Greece – and to offer more general advice to those with political ambitions.


It is worth observing that ‘political ambition’ was probably more ubiquitous in many Greek cities than it is today, and a large proportion of the well-born men who were full citizens, and therefore functionally or explicitly legal persons, engaged themselves in the public lives of their cities.

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The proliferation, however unevenly, of democracy in the Greek world was doubtless an important catalyst for this widespread political engagement. Protagoras lived throughout most of the 5th century BC, and by the 4th century more than half of roughly a thousand Greek city states was a democracy, or had been in the past.


Protagoras’ Life

ribera protagoras painting
‘Protagoras’ by Jusepe de Ribera, 1637, from Wikimedia Commons.


Protagoras was born in Abdera in Thrace, in the far north-east of Greece, on the coast of the Aegean Sea. Some Greeks testify that he came from a relatively poor family, whereas another (Philostratus, in his Lives of the Sophist) tells us that Protagoras’ family housed Xerxes, King of Kings of the Persian Empire.


Philostratus also reports that Protagoras was thus accorded a rare honor; to be educated by the Persian magi, or wise men.


Many of the testimonies of Protagoras’ life seem to be pejorative, yet accidentally give the impression of a rather well-traveled, romantic figure. Whatever the truth of Philostratus’ story – which was likely told so as to explain some of Protagoras’ views which were antithetical to the conventions of Greek thought – he appears to have had some substantial contact with Middle-Eastern thought.


Protagoras’ Political Experience

plato leonidas drosis marble statue
A modern day statue of Plato by Leonidas Drosis, via Wikimedia Commons.


Protagoras traveled extensively around Greece, and also appears to have met and gained the trust of Pericles, one of Athens’ most famous generals and leaders, who is said to have appointed him to draft the constitution of Thurii. Thurii was a new Athenian colony in southern Italy, in what would eventually be known as Magna Graecia, or ‘Greater Greece’, to the Romans.


Few philosophers can claim to have had as explicit and extensive connection with the public life of their state, and that of many foreign ones, than Protagoras. His first hand experience of politics and the diversity of human social organization would be considered remarkable today; in the Greek world, it appears to be so extraordinary as to leave him something of an outcast among his contemporaries.


Itinerance and Inconsistency

athenian acropolis photograph
A photograph of the Athenian acropolis taken in 2006, via Wikimedia Commons.


A criticism levied by Socrates in the Timaeus is that the very itinerance of the sophists is what makes them ill equipped to understand the needs of any one city. Socrates argues as follows:


“I’ve always thought that sophists as a class are very well versed in making long speeches and doing many other fine things. But because they wander from one city to the next and never settle down in homes of their own, I’m afraid their representations of those philosopher-statesmen would simply miss their mark.”


This feeds into a broader kind of criticism of the sophist, one which the Eleatic Stranger makes (and Plato appears to encourage the reader to accept as valid): that because the sophist sells his expertise in the construction of arguments, he is compelled to accept whichever position those paying him prefer.


On Being an Argumentative Gun for Hire

allegory rethoric hyre
‘Allegory of Rhetoric’ by Laurent de La Hyre, 1650, via Encyclopedia Britannica.


This seems a rather thin argument: is it really true that practicing constructing arguments for a position you happen not to believe in makes you a less consistent philosopher?


There are arguments one can make against ‘charity’ in philosophy (that is, the value of giving one’s opponents the benefit of the doubt when characterizing their arguments). Nonetheless, few people are willing to entirely discount that part of great philosophy is an ability to offer a substantial account of one’s opponents arguments, in part to understand what motivates them in the first place.


Presumably, part of learning that skill involves learning how to think through arguments for a point of view which you yourself do not actually hold. Additionally, there are arguments about arguments themselves which sophists like Protagoras make which are not at all affected by the particular arguments they might be called to make by whoever happens to hire them.


Man as the Measure of All Things 

tischbein nine muses painting
‘The Nine Muses – Polyhymnia (Rhetoric)’ by Johann Heinrich Tischbein, 1781, from Wikimedia Commons.

Protagoras offers one of the most famous philosophical arguments, and he does so in a single sentence which also constitutes one of only a few genuine fragments of his work which has passed down to us directly. It runs as follows:


“Of all things the measure is man: of those that are, that they are; and of those that are not, that they are not.”


There is every reason to think that this doctrine is intentionally ambiguous, but we can use its interpretation in another dialogue of Plato’s, namely the Theaetetus, to infer how some of Protagoras’ contemporaries may have understood it. Plato paraphrases Protagoras thus:


“… just as each thing appears to me, so too it is for me, and just as it appears to you, so too again for you”.


Because this point in the Theaetetus is committed to investigating a separate thesis, that of whether knowledge is perception, we can understand the interpretation of Protagoras here as strictly empiricist. Both Aristotle and Sextus Empiricus acknowledge a broader interpretation of this quote, one which appears to relativise all opinions and judgments along with sensation.


plato cunego etching
An etching of Plato by Cunego, 1783, via the Wellcome Collection.


Socrates tries to explain Protagoras’ thesis using the analogy of weather: if the wind appears warm to me, it is warm, and if it appears cold to another person, then it is cold.


‘Man is the measure of all things’ means that, in any given case, knowledge is determined by our own sensations. Does Protagoras use the word ‘man’ to offer an abstraction; that is, attempting to offer an account of knowledge acquisition in general? Or, is he rather making the opposite kind of movement, away from abstraction and towards a historically constituted subject?


Jaap Mansfeld argues that, although experience is at the core of Protagoras’ argument, it is not a claim about knowledge as such, but a claim about the validity of any particular individual’s experience, and thus takes against all abstraction about knowledge, truth and reality as such. This is a subtle difference, and Socrates’ misunderstanding haunted other empiricist philosophers for the next 2,500 years.


This kind of relativising argument faces two further kinds of objection. Firstly, that it is self-defeating. The basic thought behind this is simple, and has been levied as more or less every attempt at relativization or the postulation of the subject in philosophical history. It says simply, ‘if what is true is only true for me, and I judge that Protagoras’ thesis is false for me, then Protagoras’ thesis is false’.


Sophists and Solipsism

plato herm marble sophists
A marble bust of Plato from the Altes museum in Berlin, 1st Century AD, via Wikimedia Commons.


The retort available to Protagoras is a fairly simple one, and runs something along these lines: ‘if what is false is false for you only, then your holding my thesis to be false has no bearing on whether I or anyone else should follow you in doing so’.


In other words, Protagoras can consistently defend himself by arguing against any overall or total conception of truth, and in favor of exclusively truth relative to historically constituted subjects.


The charge which then threatens is that of solipsism, which is really a claim about the possibility of productive discourse and mutually acceptable conclusions. If truth is relative to particular individuals, how can Protagoras avoid the kind of intellectual loneliness while positing the impossibility of a general criterion of truth?


Arguably, the correct empiricist approach to all of this (both the initial objection that Protagoras’ thought is self-defeating, and this new objection about solipsism) is to deny not only the existence of absolute or total conceptions of truth, but the importance of truth as a concept in any guise. Human beings are capable of collaborating and find mutual understanding without sharing a criterion of truth as such; that is the essence of practical philosophy, philosophy orientated towards actions.


We can also test for ourselves whether we feel ourselves to be wholly separated from those with whom we have not agreed on a standard of truth and objectivity beforehand, or whether the assumption of such a thing is really necessary to avoid becoming solipsistic.

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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.