Thomas asked:

Could you give me some pointers regarding the refutation of Protagorean relativism in Plato's
Theaetetus?

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The great Sophist, Protagoras, is remembered for being the thinker who claimed that 'Man is the
measure of all things.' Here is a more accurate rendition, courtesy of Jonathan Barnes The
Presocratic Philosophersp. 541:

Of all things a measure is man - of the things that are, that they are; of the things that are not,
that they are not.

In unit 14 of The First Philosophers I remark:

Any notion of a world beyond our ken whose nature we can only speculate about, or of a reality
behind appearances is demolished in one stroke. To accept Protagoras' claim is effectively to consign
all previous philosophical thought to the dustbin. It is what we human beings perceive with our
senses, what we are able to establish from our limited access to things - on the basis of where we
stand here and now - that 'measures' or determines how things are in reality.

It might occur to you that if Protagoras' view were accepted, one of the things that would have to be
consigned to the dustbin is Plato's Theory of Forms! Many, though, would today agree with
Protagoras' hostility to metaphysics. On my reading, Protagoras sounds like any good empiricist.

In his great dialogue Theaetetus,Plato suggests a more extreme reading. All human knowledge,
according to Protagoras, is made up of judgements of how things seemor appearto this or that
person. - What if that were true?

SOCRATES It sometimes happens, doesn't it, that when the same wind is blowing one of us feels
cold and the other not? Or that one feels slightly cold and the other very?...So it looks as though
things are, for each person, the way he perceives them.

THEAETETUS That seems plausible.
John McDowell tr. OUP 1973, 152 B.

Having gained this admission from the young Theaetetus, Socrates points out the patent absurdity of
this claim: if the truths in which human knowledge consists are ultimately based on each person's
subjective judgement of how things seem to them, the result is wholesale contradiction. - The wind is
freezing, cold and not cold all at the same time! Alluding to the Presocratic philosopher Heraclitus,
Socrates describes the result as a world in constant flux: 'nothing ever is, but things are always
coming to be' (152 E).

Later on in the dialogue, Plato suggests another, even more radical, way of explaining Protagoras. In
'measuring' things all we are doing is seeking out good experiences and escaping bad ones. 'In
education...a change must be effected from one of two conditions to the better one; but whereas a
doctor makes the change with drugs, a Sophist does it with things he says' (167 A). We shall naturally
prefer those Sophists whose influence leads us to have pleasurable experiences rather than those
whose influence leads us to have painful experiences. The most successful of these teachers - as
measured in the universal currency of pleasure and pain - will be the ones most deserving of fame
and its financial rewards.

This extreme form of pragmatism comes up against the classic response. Whether or not a given
belief leads to good consequences for the person holding that belief is a matter of objective fact, and
not merely question of whether the belief that that is so leads to good consequences. If you are ill,
you go to the doctor you believe is actually going to cure you, not the one who will only make you
thinkthat you have been 'cured'.

Finally, comes the knock-out punch:

SOCRATES Protagoras agrees that everyone has in his judgements the things which are. In doing
that, he's surely conceding that the opinion of those who make opposing judgements about his own
opinion - that is, their opinion that what he thinks is false - is true.
McDowell 171 A

In other words, by Protagoras' own doctrine, for the 'man' who thinks that 'Man is the measure of all
things' is false, it isfalse.

As for pointers, I think we should be sceptical about Plato's understandably hostile reading. Today,
one would describe Protagoras as holding an 'anti-realist' theory of truth. The anti-realist denies that
we have any notion of a 'truth' that transcends human powers of verification. As one who has
struggled with this question for a number of years, I can vouch that this is a pretty difficult theory to
refute.

Geoffrey Klempner