"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.'" —Karl Popper, Conjectures and Refutations.

"Utopian rationalism is a self-defeating rationalism. However benevolent its ends, it does not bring happiness, but only the familiar misery of being condemned to live under a tyrannical government." —Karl Popper, Conjectures and Refutations.

"There are no subject matters; no branches of learning—or, rather, of inquiry: there are only problems, and the urge to solve them." ––Karl Popper, Realism and the Aim of Science.

“Theories are nets cast to catch what we call ‘the world’: to rationalize, to explain, and to master it.” —Karl Popper, The Logic of Scientific Discovery.

"Science is a critical activity. We examine our hypotheses critically. We criticize them so that we can find errors, in the hope of eliminating the errors and thus getting closer to the truth." —Karl Popper, In Search of a Better World.

"To err is human means not only that we must constantly struggle against error, but also that, even when we have taken the greatest care, we cannot be completely certain that we have not made a mistake." —Karl Popper, In Search of a Better World.

"But have we not destroyed the environment with our natural science? No! We have made great mistakes – all living creatures make mistakes. It is indeed impossible to foresee all the unintended consequences of our actions. Here science is our greatest hope: its method is the correction of error." —Karl Popper, In Search of a Better World.

“New ideas should be regarded as precious and should be carefully nursed—especially if they seem to be a bit wild. I do not suggest that we should be eager to accept new ideas just for the sake of their newness. But we should be anxious not to suppress a new idea, even if it does not appear to us to be very good. There are many examples of neglected ideas, such as the idea of evolution before Darwin, or Mendel's theory. A great deal can be learned about obstacles to progress from the history of these neglected ideas.” —Karl Popper, The Myth of the Framework.

“Cultural relativism and the doctrine of the closed framework are serious obstacles to the readiness to learn from others. They are obstacles to the method of accepting some institutions, modifying others, and rejecting what is bad.” ––Karl Popper, The myth of the framework.

"I am a rationalist because I see in the attitude of reasonableness the only alternative to violence." —Karl Popper, Conjectures and Refutations.

"The growth of knowledge—or the learning process—is not a repetitive or a cumulative process but one of error-elimination. It is Darwinian selection, rather than Lamarckian instruction." ––Karl Popper, Objective Knowledge.

"What we attempt in science is to describe and (so far as possible) explain reality. We do so with the help of conjectural theories; that is, theories which we hope are true (or near the truth), but which we cannot establish as certain or even as probable (in the sense of the probability calculus), even though they are the best theories which we are able to produce and may therefore be called 'probable' as long as this term is kept free from any association with the calculus of probability." —Karl Popper, Objective Knowledge.

"The course of human history is strongly influenced by the growth of human knowledge." —Karl Popper, The Poverty of Historicism.

"Do not allow your dreams of a beautiful world to lure you away from the claims of men who suffer here and now. Our fellow men have a claim to our help; no generation must be sacrificed for the sake of future generations, for the sake of an ideal of happiness that may never be realized." —Karl Popper, Conjectures and Refutations.

"Criticism is, in a very important sense, the main motive force of any intellectual development. Without contradictions, without criticism, there would be no rational motive for changing our theories: there would be no intellectual progress." —Karl Popper, Conjectures and Refutations.

"In order to learn to avoid making mistakes we must learn from our mistakes. To cover up mistakes is, therefore, the greatest intellectual sin." —Karl Popper, In Search of a Better World.

"We cannot predict, by rational or scientific methods, the future growth of our scientific knowledge." —Karl Popper, The Poverty of Historicism.

"Western civilization is, in spite of all the faults that can quite justifiably be found with it, the most free, the most just, the most humanitarian and the best of all those we have ever known throughout the history of mankind. It is the best because it has the greatest capacity for self-criticism, and so, for improvement." —Karl Popper, In Search of a Better World.

"Rational belief, if there can be said to be such a thing, consists in giving preference to what is preferable in the light of critical arguments. So this again is not a question of belief, but a question of argument, and of the objective state of the critical debate." —Karl Popper, Unended Quest.

“For if we are uncritical we shall always find what we want: we shall look for, and find, confirmations, and we shall look away from, and not see, whatever might be dangerous to our pet theories. In this way it is only too easy to obtain what appears to be overwhelming evidence in favour of a theory which, if approached critically, would have been refuted.” —Karl Popper, The Poverty of Historicism.