To: Simon A.
From: Geoffrey Klempner
Subject: Xenophanes' reflections on the limits of human knowledge
Date: 21 February 2002 13:29

Dear Simon,

Thank you for your e-mail of 12 February with your second essay for the Ancient Philosophy program, in response to the question, 'What is it to "know" that something is the case? Can the truth of a scientific theory ever be known? Illustrate your answer by reference to Xenophanes' reflections on the limits of human knowledge.'

The life of Pyrrho of Elis (365-270 BC) overlaps with that of Aristotle (384-322 BC), so Pyrrho came long after the Presocratic philosophers. However, it is a good idea to contrast, as you have done, his ideas with those of the Presocratic philosopher Xenophanes, and to speculate on how Pyrrho and Xenophanes would debate the question of the limits of human knowledge.

First, however, we need a working definition of knowledge. This is the first thing that the question asks for. If we don't have an idea of what would *count* as knowing something, we are not going to get very far in investigating the question of whether we can know the truth of a scientific theory, or the more general question of the limits of human knowledge, or indeed whether knowledge is attainable at all.

Knowledge is different from a belief which happens, by accident, to have hit on the truth. For example, suppose that the universe really is made of water. Then Thales' belief was true. But Thales' didn't *know* that the universe was made of water. He just guessed and got lucky.

Similarly, to take a case from everyday life, someone asks me the way to the market and I confidently give directions, not wishing to seem ignorant. In fact, my directions were correct, but I didn't know they were correct. I guessed, and my guess just happened be right.

You've obviously taken the point about the link between Xenophanes' argument for the existence of one god, and his argument for scepticism. How does this affect the question whether, or how we can know the truth of a scientific theory? You say, 'Thus he came out against over-zealous thinkers who took their beliefs for the truth and urged them to moderate their assertions and subject them to rigorous and objective testing as to whether they resemble the truth.'

There are two ideas here:

1. Testing must be rigorous and 'objective' rather than subjective, in the sense of producing results that different investigators are able reliably to agree on. Clearly, the Presocratic physicists had serious shortcomings in this area.

2. The result of this process is still not truth, but only something that 'resembles' the truth. What does that mean? A scientific theory that has withstood rigorous testing may not the final truth, but we can still *count* on it as a reliable guide to action. We can reasonably expect similar results in the future to the results we have obtained many times in the past.

Is this Xenophanes? We know that putting ideas 1. and 2. into practice has very real practical benefits. However, Xenophanes did not have an example of 'good' science to point to. So his message is predominantly negative, while avoiding the extreme scepticism of Pyrrho. So although one cannot read 1. and 2. into what Xenophanes says, it is reasonable to suppose that he would have agreed to these two principles, had he known what we know.

The contrast with Pyrrho is very striking. I am not sure about the argument you attribute to Pyrrho. The argument from illusion was stated by Descartes in the First Meditation in the seventeenth century. Pyrrho's main argument, as I understand it, is simply that equally persuasive reasons can be given for or against any given belief.

Whereas Xenophanes' moderate scepticism can be seen as a spur to scientific inquiry, the scepticism advanced by Pyrrho seems self-defeating. And indeed we are told that he and his followers made the attempt to live life according to the principles of scepticism, avoiding beliefs of any kind, and 'living by nature'.

You ask, 'How could Pyrrho be sceptical about others and be sure about his theory of scepticism?' I used to think that all Pyrrho could meet this criticism by arguing, 'Either we know one thing - the truth of scepticism - or we know nothing. Either way, there is no other knowledge.' However, it seems to me now that this is wrong, and that the objection is in fact a serious criticism of his position. It is not sufficient in order to answer your question, for Pyrrho to acknowledge that he can be wrong about his sceptical theory, as you suggest.

This is the reason. If Pyrrho is wrong in his extreme scepticism, then we have nothing to worry about. If he is right, then we do have something to worry about. In that case, what we are presented with is not the Pyrrhonian argument for scepticism as such but rather a pair of alternatives, whose *probability* we are free to judge:

Alternative A. Pyrrho's argument is valid. Therefore we know nothing, and science is a waste of effort.

Alternative B. Pyrrho's argument is invalid. Therefore we should continue pursuing science.

Having accepted that he might be wrong, Pyrrho cannot plead that there is, say, a fifty per cent chance that he is right. We can confidently say, 'Science seems to work. That's good enough for us,' and assign a 99.9 per cent probability to Alternative B!

All the best,

Geoffrey