Diderot and d’Alembert’s Encyclopédie thus defined the philosophe as one who ‘trampling on prejudice, tradition, universal consent, authority, in a word, all that enslaves most minds, dares to think for himself’. 
Yet the traditional question of the authoritative sources of knowledge is repeated even today — and very often by positivists, and by other philosophers who believe themselves to be in revolt against authority.
The proper answer to my question ‘How can we hope to detect and eliminate error?’ is, I believe, ‘By criticizing the theories or guesses of others and — if we can train ourselves to do so — by criticizing our own theories or guesses.’ (The latter point is highly desirable, but not indispensable; for if we fail to criticize our own theories, there may be others to do it for us.) This answer sums up a position which I propose to call ‘critical rationalism’. It is a view, an attitude, and a tradition, which we owe to the Greeks. It is very different from the ‘rationalism’ or ‘intellectualism’ of Descartes and his school, and very different even from the epistemology of Kant. Yet in the field of ethics, of moral knowledge, it was approached by Kant with his principle of autonomy. This principle expresses his realization that we must not accept the command of an authority, however exalted, as the basis of ethics. For whenever we are faced with a command by an authority, it is for us to judge, critically, whether it is moral or immoral to obey. The authority may have power to enforce its commands, and we may be powerless to resist. But if we have the physical power of choice, then the ultimate responsibility remains with us. It is our own critical decision whether to obey a command; whether to submit to an authority.
Kant boldly carried this idea into the field of religion: ‘…in whatever way’, he writes, ‘the Deity should be made known to you, and even … if He should reveal Himself to you: it is you … who must judge whether you are permitted to believe in Him, and to worship Him.’
In view of this bold statement, it seems strange that Kant did not adopt the same attitude — that of critical examination, of the critical search for error — in the field of science. I feel certain that it was only his acceptance of the authority of Newton’s cosmology — a result of its almost unbelievable success in passing the most severe tests — which prevented Kant from doing so. If this interpretation of Kant is correct, then the critical rationalism (and also the critical empiricism) which I advocate merely puts the finishing touch to Kant’s own critical philosophy. And this was made possible by Einstein, who taught us that Newton’s theory may well be mistaken in spite of its overwhelming success.
So my answer to the questions ‘How do you know? What is the source or the basis of your assertion? What observations have led you to it?’ would be: ‘I do not know: my assertion was merely a guess. Never mind the source, or the sources, from which it may spring — there are many possible sources, and I may not be aware of half of them; and origins or pedigrees have in any case little bearing upon truth. But if you are interested in the problem which I tried to solve by my tentative assertion, you may help me by criticizing it as severely as you can; and if you can design some experimental test which you think might refute my assertion, I shall gladly, and to the best of my powers, help you to refute it.’ [34-5]
Thus Kant’s reply to Hume came near to being right; for the distinction between an a priori valid expectation and one which is both genetically and logically prior to observation, but not a priori valid, is really somewhat subtle. But Kant proved too much. In trying to show how knowledge is possible, he proposed a theory which had the unavoidable consequence that our quest for knowledge must necessarily succeed, which is clearly mistaken. When Kant said, ‘Our intellect does not draw its laws from nature but imposes its laws upon nature’, he was right. But in thinking that these laws are necessarily true, or that we necessarily succeed in imposing them upon nature, he was wrong. Nature very often resists quite successfully, forcing us to discard our laws as refuted; but if we live we may try again. 
In Wirklichkeit aber nimmt mit sehr wenigen Ausnahmen jemand, der in eine Partei eintritt, gehorsam jene Geisteshaltung an, die er später folgendermaßen ausdrücken wird: „Als Monarchist, als Sozialist meine ich, dass …“ Das ist so bequem! Denn es heißt, nicht zu denken. Es gibt nichts Bequemeres als nicht zu denken.
Das dritte Merkmal der Parteien – nämlich dass sie Maschinen zur Fabrikation kollektiver Leidenschaft sind – ist so augenscheinlich, dass es keiner Erläuterung bedarf. Die kollektive Leidenschaft ist die einzige Energie, die den Parteien für die Propaganda nach außen hin und für den Druck, den sie auf die Seele jedes Mitglieds ausüben, zur Verfügung steht.
Man gibt zu, dass der Parteiengeist blind macht, dass er taub macht für die Gerechtigkeit und dass er selbst rechtschaffene Leute zum grausamsten Wüten gegen Unschuldige hinreißt. Man gibt es zu, doch man denkt nicht daran, die Organismen abzuschaffen, die einen solchen Geist fabrizieren. [28-9]