Karl Popper on the empirical base of science

This part of my lecture might be described as an attack on empiricism, as formulated for example in the following classical statement of Hume’s: ‘If I ask you why you believe any particular matter of fact . . ., you must tell me some reason; and this reason will be some other fact, connected with it. But as you cannot proceed after this manner, in infinitum, you must at last terminate in some fact, which is present to your memory or senses; or must allow that your belief is entirely without foundation.’ [references omitted]

The problem of the validity of empiricism may be roughly put as follows; is observation the ultimate source of our knowledge of nature? And if not, what are the sources of our knowledge? . . .

The problem of the source of our knowledge has recently been restated as follows. If we make an assertion, we must justify it; but this means that we must be able to answer the following questions.

‘How do you know? What are the sources of your assertion?’ This, the empiricist holds, amounts in its turn to the question, ‘What observations (or memories of observations) underlie your assertion?’

I find this string of questions quite unsatisfactory.

First of all, most of our assertions are not based upon observations, but upon all kinds of other sources. ‘I read it in The Times‘ or perhaps ‘I read it in the Encyclopedia Britannica‘ is a more likely and a more definite answer to the question ‘How do you know?’ than ‘I have observed it’ or ‘I know it from an observation I made last year’.

‘But’, the empiricist will reply, ‘how do you think that The Times or the Encyclopedia Britannica got their information? Surely, if you only carry on your inquiry long enough, you will end up with reports of the observations of eyewitnesses (sometimes called "protocol sentences" or – by yourself – "basic statements"). Admittedly’, the empiricist will continue, ‘books are largely made from other books. Admittedly, a historian, for example, will work from documents. But ultimately, in the last analysis, these other books, or these documents, must have been based upon observations. Otherwise they would have to be described as poetry, or inventions, or lies, but not as testimony. It is in this sense that we empiricists assert that observation must be the ultimate source of our knowledge.’

Here we have the empiricist’s case, as it is still put by some of my positivists friends.

I shall try to show that this case is as little valid as Bacon’s; that the answer to the question of the sources of knowledge goes against the empiricist; and finally, that this whole question of ultimate sources – sources to which one may appeal, as one might to a higher court or a higher authority – must be rejected as based upon a mistake.

First I want to show that if you actually went on questioning The Times and its correspondents about the sources of their knowledge, you would in fact never arrive at all those observation by eyewitnesses in the existence of which the empiricists believes. You would find, rather, that with every single step you take, the need for further steps increases in snowball-like fashion.

Take as an example the sort of assertion for which reasonable people might simply accept as sufficient the answer ‘I read it in Time Times’; let us say that the assertion ‘The Prime Minister has decided to return to London several days ahead of schedule’. Now assume for a moment that somebody doubts this assertion, or feels the need to investigate its truth. What shall he do? If he has a friend in the Prime Minister’s office, the simple and most direct way would be to ring him up: and if this friend corroborates the message, then that is that.

In other words, the investigator will, if possible, try to check, or to examine, the asserted fact itself, rather than trace the source of the information. But according to the empiricist theory, the assertion ‘I have read it in The Times’ is merely a first step in a justification procedure consisting in tracing the ultimate source. What is the next step?

There are at least two next steps. One would be to reflect that ‘I have read it in The Times’ is also an assertion, and that we might ask ‘What is the source of your knowledge that you read it in The Times and not, say, in a paper looking very similar to The Times?’ The other is to ask The Times for the sources of its knowledge. The answer to the first question may be ‘But we have only The Times on order and we always get it in the morning’, which gives rise to a host of further questions about sources which we shall not pursue. The second question may elicit from the editor of The Times the answer: ‘We had a telephone call from the Prime Minister’s office.’ Now according to the empiricist procedure, we should at this stage ask next: ‘Who is the gentleman who received the telephone call?’ and then get his observation report; but we should also have to ask that gentleman: ‘What is the source of your knowledge that the voice you heard came from an official in the Prime Minister’s office?’, and so on.

There is a simple reason why this tedious sequence of questions never comes to a satisfactory conclusion. It is this. Every witness must always make ample use, in his report, of his knowledge of persons, places, things, linguistic usages, social conventions, and so on. He cannot rely merely upon his eyes or ears, especially if his report is to be of use in justifying any assertion worth justifying. But this fact must of course always raise new questions as to the sources of those elements of his knowledge which are not immediately observational.

This is why the programme of tracing back all knowledge to its ultimate source in observation is logically impossible to carry through: it leads to an infinite regress. (The doctrine that truth is manifest cuts off the regress. This is interesting because it may help to explain the attractiveness of that doctrine.)

I wish to mention, in parentheses, that this argument is closely related to another – that all observation involves interpretation in the light of our theoretical knowledge, or that pure observational knowledge, unadulterated by theory, would, if at all possible, be utterly barren and futile. [References omitted]

The most striking thing about the observationalist programme of asking for sources – apart from its tediousness – is its stark violation of common sense. For if we are doubtful about an assertion, then the normal procedure is to test it, rather than to ask for its sources; and if we find independent corroboration, then we shall often accept the assertion without bothering at all about sources

Of course there are cases in which the situation is different. Testing an historical assertion always means going back to sources; but not, as a rule, to the reports of eyewitnesses.

Clearly, no historian will accept the evidence of documents uncritically. There are problems of genuineness, there are problems of bias, and there are also such problems as the reconstruction of earlier sources. There are, of course, also problems such as: was the writer present when these events happened? But this is not one of the characteristic problems of the historian. He may worry about the reliability of a report, but he will rarely worry about whether or not the writer of a document was an eyewitness of the event in question, even assuming that this event was of the nature of an observable event. A letter saying ‘I changed my mind yesterday on this question’ may be most valuable historical evidence, even though changes of mind are unobservable (and even though we may conjecture, in view of other evidence, that the writer was lying).

As to eyewitnesses, they are important almost exclusively in a court of law where they can be cross-examined. As most lawyers know, eyewitnesses often err. This has been experimentally investigated, with the most striking results. Witnesses most anxious to describe an event as it happened are liable to make scores of mistakes, especially if some exciting things happen in a hurry; and if an event suggests some tempting interpretation, then this interpretation, more often than not, is allowed to distort what has actually been seen.

Hume’s view of historical knowledge was different: ‘… we believe’, he writes in the Treatise [references omitted], ‘that CAESAR was kill’d in the senate-house on the ides of March . . . because this fact is establish’d on the unanimous testimony of historians, who agree to assign this precise time and place to that event. Here are certain characters and letters present either to our memory or senses; which characters we likewise remember to have been us’d as the signs of certain ideas; and these ideas were either in the minds of such as were immediately present at that action, and receiv’d the ideas directly from its existence; or they were deriv’d from the testimony of others, and that again from another testimony . . . ’till we arrive at those who were eye-witnesses and spectators of the event.’ [references omitted]

It seems to me that this view must lead to the infinite regress described above. For the problem is, of course, whether ‘the unanimous testimony of historians’ is to be accepted, or whether it is, perhaps, to be rejected as the result of their reliance on a common yet spurious source. The appeal to ‘letters present either to our memory or senses’ cannot have any bearing on this or on any other relevant problem of historiography.

From "From Sources of Knowledge and Ignorance", Conjectures and Refutations, section xiii