And it is a fact that my social theory (which favours gradual and piecemeal reform, reform controlled by a critical comparison between expected and achieved results) contrasts with my theory of method, which happens to be a theory of scientific and intellectual revolutions. 
Category: The Myth of the Framework
Herodotus seems to have been one of those somewhat rare people whose minds are broadened by travel. At first he was no doubt shocked by the many strange customs and institutions which he encountered in the Middle East. But he learned to respect them, and to look on some of them critically, and to regard others as the results of historical accidents: he learned to be tolerant, and he even acquired the ability to see the customs and institutions of his own country through the eyes of his barbarian hosts.
This is a healthy attitude. But it may lead to relativism, that is, to the view that there is no absolute or objective truth, but rather one truth for the Greeks, another for the Egyptians, still another for the Syrians, and so on. 
[T]heories are steps in our search for truth – or to be both more explicit and more modest, in our search for better and better solutions of deeper and deeper problems (where ‘better and better’ means, as we shall see, ‘nearer and nearer to the truth’). [154-5]
Some laws and customs can be very cruel, while others provide for mutual help and the relief of suffering. Some countries and their laws respect freedom while others do less, or not at all. These differences are most important, and they must not be dismissed or shrugged off by a cultural relativism, or by the claim that different laws and customs are due to different standards, or different ways of thinking, or different conceptual frameworks, and that they are therefore incommensurable or incomparable. On the contrary, we should try to understand and to compare. We should try to find out who has the better institutions. And we should try to learn from them. 
It is most important to see that a critical discussion always deals with more than one theory at a time. For in trying to assess the merits or demerits even of one theory, it always must try to judge whether the theory in question is an advance: whether it explains things that we have been unable to explain so far – that is to say, with the help of older theories. 
I shall now very briefly criticize Bacon’s anti-theoretical dogma and his view of science […].
1. The idea that we can purge our minds of prejudices at will and so get rid of all preconceived ideas or theories, prior to, and preparatory to, scientific discovery, is naïve and mistaken. It is mainly through scientific discovery that we learn that certain of our ideas such as those of the flat earth or the moving sun are prejudices. We discover the fact that one of the beliefs we held was a prejudice only after the advance of science has led us to discard it. For there is no criterion by which we could recognize prejudices in anticipation of this advance.
2. The rule ‘Purge yourself of prejudice!’ can therefore have only the dangerous result that, after having made an attempt or two, you may think that you have succeeded – with the result, of course, that you will stick more tenaciously to your prejudices and dogmas, especially to those of which you are unconscious.
3. Moreover, Bacon’s rule was ‘purge your mind of all theories!’ But a mind so purged would not only be a pure mind: it would be an empty mind.
4. We always operate with theories, even though more often than not we are unaware of them. The importance of this fact should never be played down. Rather, We should try, in each case, to formulate explicitly the theories we hold. For this makes it possible to look out for alternative theories, and to discriminate critically between one theory and another.
5. There is no such thing as a ‘pure’ observation, that is to say, an observation without a theoretical component. All observation – and especially all experimental observation – is an interpretation of facts in the light of some theory or other. 
Bacon’s observationism and his hostility to all forms of theoretical thought were revolutionary, and were felt to be so. They became the battle cry of the new secularized religion of science, and its most cherished dogma. This dogma had an almost unbelievable influence upon both the practice and the theory of science, and this influence is still strong in our own day. 
This criterion of demarcation between empirical and non-empirical theories I have also called the criterion of falsifiability or the criterion of refutability. It does not imply that irrefutable theories are false. Nor does it imply that they are meaningless. But it does imply that, as long as we cannot describe what a possible refutation of a certain theory would be like, that theory may be regarded as lying outside the field of empirical science.
The criterion of refutability or falsifiability may also be called the criterion of testability. For testing a theory, like testing a piece of machinery, means trying to fault it. Thus a theory that we know in advance cannot possibly be faulted or refuted is not testable. 
Science begins with observation, says Bacon, and this saying is an integral part of the Baconian religion. It is still widely accepted, and still repeated ad nauseam in the introductions to even some of the best textbooks in the field of the physical and biological sciences.
I propose to replace this Baconian formula by another one.
Science, we may tentatively say, begins with theories, with prejudices, superstitions, and myths. Or rather, it begins when a myth is challenged and breaks down – that is, when some of our expectations are disappointed. But this means that science begins with problems, practical problems or theoretical problems.