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Protagoras: Man is the measure of all things


To: Charles R.
From: Geoffrey Klempner
Subject: Protagoras: Man is the measure of all things
Date: 24th August 14:26

Dear Charles,

Thank you for your email of 16 August, with your fifth and final essay for the Ancient Philosophy program, in response to the question, 'What do you think Protagoras meant by his statement, 'Man is the measure'? In the light of your interpretation, how fair is the account that Plato gives of Protagoras' doctrine in the 'Theaetetus'?'

Well done for completing your program.

You've given a very thorough and exhaustive account of Plato's objections to the doctrine which he attributes to Protagoras, that 'knowledge is perception'. The big puzzle for me, however, is why you think that Plato's response is 'fair', given the way you have explained Protagoras' statement, 'Man is the measure of all things'.

You offer two alternatives, which we can call the philosophical version and the sophistical version.

According to the sophistical version, what Protagoras is saying is simply 'the only things that matter, that are of any importance, any relevance to day to day life, are those things we can perceive.' This might have been plausible in Protagoras' day, but in the present time would be considered atavistic, ignorant nonsense. Germs can't be seen, nor can high activity ultra-violet rays penetrating holes in the ozone layer. We live in a world full of unseen wonders and dangers (the microchip that powers this computer is one of the wonders).

According to the philosophical version, 'human experience attained through the activation of the human senses is a trustworthy and reliable basis for human knowledge.' All scientific knowledge would be classed under this heading, including knowledge of germs and electrons.

What I have just quoted from your essay is the classic statement of empiricism. There is no such thing as 'metaphysical' knowledge. All knowledge is physical knowledge, based on observation and experiment. Hume gives the classic statement, which has come to be known as 'Hume's Fork', in Section 12 Part III of his 'Essay Concerning Human Understanding':

'If we take in our hand any volume, of divinity or school metaphysics for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.'

But Plato doesn't give this interpretation a second thought. He takes Protagoras to be saying, much more simply and brutally, that knowledge just *is* perception, in the sense that whatever seems to be X, is X so far as the perceiver is concerned. There is no distinction between appearance and reality. Everything is just as it appears.

Criticizing this theory is like shooting fish in a barrel. It's so easy. Yet Plato isn't simply setting out to reduce Protagoras' ideas to absurdity. He wants us to consider that they are in some sense *true* of the phenomenal world, the world of 'sights and sounds'. This is the cosmos as Heraclitus described it, where nothing 'is' but rather is in a process of 'becoming'.

How can there be any stability? There cannot, if *all* that exists is the phenomenal world. In other words, what Plato is working towards is the idea that in order for there to be such a thing as knowledge we need the Forms, in addition to the phenomenal world of 'appearances'.

The doctrine he attributes to Heraclitus omits one very important feature of the theory: the Logos: in other words, he is as unfair to Heraclitus as he is to Protagoras. Plato's Forms take the role which Heraclitus gives to the Logos. The 'world of Heraclitean flux' is the world that would exist, per impossibile, if there were no Forms (or, as Heraclitus would say, if there were no Logos).

So where do we stand? Remember that this is a dialogue. Socrates is seeking assent from his interlocutor to a series of propositions, and drawing the conclusions from those statements. We need not draw the conclusion that the skewed interpretations of Protagoras and Heraclitus were intended to be the final word. It is sufficient that an audience would readily understand where Plato was going with his critique of 'knowledge is perception'.

You observe the irony in the fact that Protagoras, who gives every impression of taking a 'realist' view, as any good empiricist would, is led towards idealism. This is the classic predicament in modern philosophy -- witness Hume's 'Scepticism with Regard to the Senses' and Berkeley's 'Immaterialism'. I don't think Protagoras did see this far ahead: his empiricism wasn't sufficiently worked out. -- That is, assuming that he was stating the doctrine of empiricism. As I explain in unit 14, there are other readings, though none of which justify Plato's reading according to which knowledge simply 'is' perception.

All the best,