We should be thankful for every serious attempt to criticize our positions. Praise is of no intellectual value. Victories in debates are of no intellectual value. Yet if we can succeed even in getting a little clearer about our problems, we should be very happy with that intellectual achievement. 
Rationality, for Popper, is to be identified with openness to criticism; and each individual is to be valued a s a source of possible criticism. Objectivity, rather than being regarded as the attribute of the particular, wise individual is regarded as a social product – a product of critical discussion. 
Poppers Botschaft ist klar. „Wir können nicht wissen“, sagt er, „wir können nur mutmaßen.“ Da keine wissenschaftliche Theorie endgültig beweibar ist, kommt es darauf an, immer erneut und mit ganzer Kraft zu prüfen, ob akzeptierte Theorien falsch sind, irrig oder widerlegt. Um dies zu tun, müssen wir die Bedingungen rationaler, kritischer Auseinandersetzung aufrechterhalten, unter denen es möglich bleibt, verschiedener Auffassung zu sein. Was für unser Wissen gilt, gilt auch für unser Verhalten und unsere Politik. Da niemand alle Antworten kennt, müssen wir vor allem sicherstellen, daß es möglich bleibt, unterschiedliche Antworten zu geben. 
The phenomenon of human knowledge is no doubt the greatest miracle in our universe. It constitutes a problem that will not soon be solved, and I am far from thinking that the preset volume makes even a small contribution to its solution. But I hope that I have helped to restart a discussion which for three centuries has been bogged down in preliminaries.
Since Descartes, Hobbes, Locke, and their school, which includes not only David Hume but also Thomas Reid, the theory of human knowledge has been largely subjectivist: knowledge has been regarded as a specially secure kind of human belief, and scientific knowledge as a specially secure kind of human knowledge.
The essays in this book break with a tradition that can be traced back to Aristotle—the tradition of this commonsense theory of knowledge. I am a great admirer of common sense which, I assert, is essentially selfcritical. But while I am prepared to uphold to the last the essential truth of commonsense realism, I regard the commonsense theory of knowledge as a subjectivist blunder. This blunder has dominated Western philosophy. I have made an attempt to eradicate it, and to replace it by an objective theory of essentially conjectural knowledge. This may be a bold claim but I do not apologize for it. [Preface]
No doubt the idea which inspires the inductive style—the idea of adhering strictly to the observed facts and of excluding bias and prejudice—is laudable. And no doubt those trained to write in this way are unaware that this laudable and apparently safe idea is itself the mistaken result of a prejudice—worse still, of a philosophical prejudice—and of a mistaken theory of objectivity. (Objectivity is not the result of disinterested and unprejudiced observation. Objectivity, and also unbiased observation, are the result of criticism, including the criticism of observational reports. For we cannot avoid or suppress our theories, or prevent them from influencing our observations; yet we can try to recognize them as hypotheses and to formulate them explicitly, so that they may be criticized.)