[I]rrationalists are dangerously mistaken when they suggest that there is any knowledge, of whatever kind, or source, or origin, which is above or exempt from rational criticism. 
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. 
But in a static society that beginning of infinity never happens. Despite the fact that I have assumed nothing other than that people try to improve their lives, and that they cannot transmit their ideas perfectly, and that information subject to variation and selection evolves, I have entirely failed to imagine a static society in this story.
For a society to be static, something else must be happening as well. One thing my story did not take into account is that static societies have customs and laws – taboos – that prevent their memes from changing. They enforce the enactment of the existing memes, forbid the enactment of variants, and suppress criticism of the status quo. However, that alone could not suppress change. First, no enactment of a meme is completely identical to that of the previous generation. It is infeasible to specify every aspect of acceptable behaviour with perfect precision. Second, it is impossible to tell in advance which small deviations from traditional behaviour would initiate further changes. Third, once a variant idea has begun to spread to even one more person – which means that people are preferring it – preventing it from being transmitted further is extremely difficult. Therefore no society could remain static solely by suppressing new ideas once they have been created.
That is why the enforcement of the status quo is only ever a secondary method of preventing change – a mopping-up operation. The primary method is always – and can only be – to disable the source of new ideas, namely human creativity. So static societies always have traditions of bringing up children in ways that disable their creativity and critical faculties. That ensures that most of the new ideas that would have been capable of changing the society are never thought of in the first place.
How is this done? The details are variable and not relevant here, but the sort of thing that happens is that people growing up in such a society acquire a set of values for judging themselves and everyone else which amounts to ridding themselves of distinctive attributes and seeking only conformity with the society’s constitutive memes. They not only enact those memes: they see themselves as existing only in order to enact them. So, not only do such societies enforce qualities such as obedience, piety and devotion to duty, their members’ sense of their own selves is invested in the same standards. People know no others. So they feel pride and shame, and form all their aspirations and opinions, by the criterion of how thoroughly they subordinate themselves to the society’s memes. [381−2]
A static society involves – in a sense consists of – a relentless struggle to prevent knowledge from growing. 
In his epistemological sensibility, Popper follows in the footsteps of the philosophers of the 18th-century Enlightenment. Their common starting point may be summarised in the following principle: The price of acquiring any knowledge at all is that it will be somehow distorted by the conditions of its acquisition; hence, criticism is the only universally reliable method. 
I admit that the natural sciences are in danger of stifling mental growth, instead of furthering it, if they are taught as technologies (the same is probably true of painting and of poetry); and that they should be treated (like painting and poetry) as human achievements, as great adventures of the human mind, as chapters in the history of human ideas, of the making of myths (as I have explained elsewhere), and of their criticism. 
Above all, our theory explains why simplicity is so highly desirable. To understand this there is no need for us to assume a ‘principle of economy of thought’ or anything of the kind. Simple statements, if knowledge is our object, are to be prized more highly than less simple ones because they tell us more; because their empirical content is greater; and because they are better testable. 
The a posteriori evaluation of a theory depends entirely upon the way it has stood up to severe and ingenious tests. But severe tests, in their turn, presuppose a high degree of a priori testability or content. Thus the a posteriori evaluation of a theory depends laregely upon its a priori value: theories which are a priori uninteresting—of little content—need not be tested because their low degree of testability excludes a priori the possibility that they may be subjected to really significant and interesting tests.
On the other hand, highly testable theories are interesting and important even if they fail to pass their test; we can learn immensely from their failure. Their failure may be fruitful, for it may actually suggest how to construct a better theory. [143-4]
We start, I say, with a problem – a difficulty. It is perhaps a practical problem, or a theoretical problem. Whatever it may be, when we first encounter the problem we cannot, obviously, know much about it. At best, we have only a vague idea what our problem really consists of. How, then, can we produce an adequate solution? Obviously, we cannot. We must first get better acquainted with the problem. But how?
My answer is very simple: by producing a very inadequate solution, and by criticizing this inadequate solution. Only in this way can we come to understand the problem. For to understand a problem means to understand why it is not easily soluble – why the more obvious solutions do not work. We must therefore produce these obvious solutions and try to find out why they will not do. In this way, we become acquainted with the problem. And in this way we may proceed from bad solutions to slightly better ones – provided always that we have the ability to guess again. [97-8]
The tension between our knowledge and our ignorance is decisive for the growth of knowledge. It inspires the advance of knowledge, and it determines its ever-moving frontiers.
The word ‘problem’ is only another name for this tension or rather, a name denoting various concrete instances of it.
As I suggested above, a problem arises, grows, and becomes significant through our failures to solve it. Or to put it another way, the only way of getting to know a problem is to learn from our mistakes.
This applies to pre-scientific knowledge and to scientific knowledge.
My view of the method of science is, very simply, that it systematizes the pre-scientific method of learning from our mistakes. It does so by the device called critical discussion.
My whole view of scientific method may be summed up by saying that it consists of these three steps:
1. We stumble over some problem.
2. We try to solve it, for example by proposing some theory.
3. We learn from our mistakes, especially from those brought home to us by the critical discussion of our tentative solutions – a discussion which tends to lead to new problems.
Or in three words: problems – theories – criticism.
I believe that in these three words the whole procedure of rational science may be summed up. [100-1]
It is the fashion of the present time to disparage negative logic—that which points out weaknesses in theory or errors in practice, without establishing positive truths. Such negative criticism would indeed be poor enough as an ultimate result; but as a means to attaining any positive knowledge or conviction worthy the name, it cannot be valued too highly; and until people are again systematically trained to it, there will be few great thinkers, and a low general average of intellect, in any but the mathematical and physical departments of speculation. On any other subject no one’s opinions deserve the name of knowledge, except so far as he has either had forced upon him by others, or gone through of himself, the same mental process which would have been required of him in carrying on an active controversy with opponents. That, therefore, which when absent, it is so indispensable, but so difficult, to create, how worse than absurd is it to forego, when spontaneously offering itself! If there are any persons who contest a received opinion, or who will do so if law or opinion will let them, let us thank them for it, open our minds to listen to them, and rejoice that there is some one to do for us what we otherwise ought, if we have any regard for either the certainty or the vitality of our convictions, to do with much greater labour for ourselves. [ch. II, 57-8]