"All organisms, from the simplest to the most complex, learn by trial and error. But there is a profound difference in the way this method is applied. The difference between the amoeba and Einstein is that, although both make use of the method of trial and error elimination, the amoeba dislikes erring while Einstein is intrigued by it. The amoeba, when it makes a mistake, dies or suffers; it has no interest in its errors except to avoid them. Einstein, on the other hand, is fascinated by his mistakes: he consciously searches for them, because he knows that we learn from our errors. This conscious critical attitude, this readiness to learn from mistakes, is what distinguishes human knowledge from that of other animals, and it is what makes science possible." —Karl Popper, Objective Knowledge: An Evolutionary Approach (1972), Chapter 7, p. 247

"No rational argument will have a rational effect on a man who does not want to adopt a rational attitude." — Karl Popper, The Open Society and Its Enemies (1945)

"The growth of knowledge depends entirely upon disagreement." –– Karl Popper.

“True ignorance is not the absence of knowledge, but the refusal to acquire it.” –– Karl Popper.

"Knowledge never begins from nothing, but always from some background knowledge—knowledge which at the moment is taken for granted—together with some difficulties, some problems. These problems arise from the clash between expectations (which are part of the background knowledge) and new experiences, or from internal contradictions within the background knowledge itself." — Karl Popper, Objective Knowledge: An Evolutionary Approach (1972), Chapter 3, p. 121.

"We must become the makers of our fate, not its prophets." –– Karl Popper

"The greatest advances in human thought have often come from those who dared to challenge the established powers, not from those who conformed to them." —Karl Popper, The Open Society and Its Enemies (1945)

"We must replace the idea of destiny with the idea of responsibility; it is not fate that shapes our future, but our own rational efforts." —Karl Popper, The Open Society and Its Enemies (1945)

"Critical reason is the only alternative to violence so far discovered. But it is an alternative which requires courage, patience, and a willingness to face unpleasant truths. The task is difficult, and the fight is not always successful; but it is not hopeless." — Karl Popper, The Open Society and Its Enemies, 1945.

“All life is problem solving.” — Karl Popper.

"All observations are theory-impregnated. There is no pure, disinterested, theory-free observation.” — Karl Popper.

"There are no ultimate sources of knowledge." — Karl Popper.

"Every intellectual has a very special responsibility. He has the privilege and the opportunity of studying. In return, he owes it to his fellow men (or ‘to society’) to represent the results of his study as simply, clearly and modestly as he can. The worst thing that intellectuals can do—the cardinal sin—is to try to set themselves up as great prophets vis-à-vis their fellow men and to impress them with puzzling philosophies. Anyone who cannot speak simply and clearly should say nothing and continue to work till he can do so." — Karl Popper, The Open Society and Its Enemies, Vol. 2 (1945), p. 298

"Optimism is a duty. The future is open. It is not predetermined. No one can predict it, except by chance. We all contribute to determining it by what we do. We are all equally responsible for its success." — Karl Popper.

"Our knowledge grows through trial and error-elimination." –– Karl Popper.

"The role of the educator is to encourage critical thinking and to help the student to develop the ability to think for himself." — Karl Popper, Unended Quest: An Intellectual Autobiography (1976)

"All laws and theories are conjectures, or tentative hypotheses." — Karl Popper.

“Error correction is the most important method in technology and learning in general. In biological evolution, it appears to be the only means of progress.” — Karl Popper, All Life Is Problem Solving, p. 100

"I disbelieve in specialization and in experts. By paying too much respect to the specialist, we are destroying the commonwealth of learning, the rationalist tradition, and science itself." — Karl Popper, Realism and the Aim of Science (1996), p. 8

"The quest for certainty is an intellectual vice; it leads to dogmatism and the suppression of criticism, which are the enemies of progress and freedom." —Karl Popper, Objective Knowledge (1972)

"The spirit of science is criticism." –– Karl Popper.

"All laws, all theories, remain essentially tentative, or conjectural, or hypothetical, even when we feel unable to doubt them any longer.” — Karl R. Popper, Conjectures and Refutations, Ch. 1, p. 51 (1962)

"Ignorance is not a simple lack of knowledge but an active aversion to facing the possibility of being wrong, which blocks the growth of understanding." — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge, 1963, p. 25

"There are all kinds of sources of our knowledge; but none has authority." — Karl Popper.

"Freedom is not a supplier of goods, nor a guarantor of happiness. Those who expect such things from freedom misunderstand its nature. We do not choose political freedom because it promises us this or that. We choose it because it makes possible the only dignified form of human coexistence, the only form in which we can be fully responsible for ourselves. Without freedom, we cannot speak of morality, for morality presupposes the possibility of choice; and without freedom, we cannot speak of responsibility, for responsibility presupposes the possibility of acting otherwise. This is why an open society, a society that values freedom, is the only society in which human beings can live as moral agents." —Karl Popper, The Open Society and Its Enemies, Volume 1: The Spell of Plato (1966 ed.), Chapter 12, p. 200

"Our knowledge can only be finite, while our ignorance must necessarily be infinite. We do not aim at establishing truth, in the sense of certainty, but at finding better theories by eliminating the false ones. Our theories are like nets: we try to make the mesh ever finer, to catch more of reality, though we can never grasp it all." — Karl Popper, Objective Knowledge: An Evolutionary Approach, 1972, p. 29 (Oxford University Press edition)

"The discovery that one is in error is one of the best things that can happen to a man, provided he acknowledges it and tries to correct it; for it is only through the recognition of our mistakes that we can grow in knowledge and improve our actions." —Karl Popper, The Open Universe (1982).

"I may be wrong and you may be right, and by an effort, we may get nearer to the truth." — Karl Popper.

"It is not his possession of knowledge, of irrefutable truth, that makes the man of science, but his persistent and recklessly critical quest for truth." — Karl R. Popper, The Logic of Scientific Discovery.

"All things living are in search of a better world." –– Karl Popper.

"knowledge is an adventure of ideas" –– Karl Popper.

“Thus I am turning the tables on those who think that observation must precede expectations and problems; and I even assert that for logical reasons, observation cannot be prior to all problems, although obviously it will often be prior to some problems—for example to those problems which arise from an observation that disappoints some expectation or refutes some theory. The fact that observation cannot precede all problems may be illustrated by a simple experiment which I wish to carry out, by your leave, with yourselves as experimental subjects. My experiment consists of asking you to observe, here and now. I hope you are all co-operating, and observing! However, I fear that at least some of you, instead of observing, will feel a strong urge to ask: ‘WHAT do you want me to observe?’            If this is your response, then my experiment was successful. For what I am trying to illustrate is that, in order to observe, we must have in mind a definite question which we might be able to decide by observation. Darwin knew this when he wrote: ‘How odd it is that anyone should not see that all observation must be for or against some view . . .’” –– Karl Popper, Objective Knowledge (1979): An evolutionary approach, Revised edition, Chapter 7: Evolution and the Tree of Knowledge, p. 259

"Freedom is no mere ideology but a way of life which makes life better and more worth living." –– Karl Popper.

“True education does not consist in being taught facts, but in being taught to ask questions—to doubt, to criticize, and to seek answers through rational discussion.” — Karl Popper, The Myth of the Framework: In Defence of Science and Rationality, 1994, p. 43

“Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.” — Karl Popper, Objective Knowledge.

"Science is the search for truth and its aim is the approximation to the truth." –– Karl Popper.

"The fact that we are fallible, and that we may err even when we are most sincere and most careful, is, I think, one of the most important lessons we can learn from the history of science." — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge, 1963, p. 247 (Routledge edition)

"Never can an authority admit that the intellectually courageous, i.e. those who dare to defy his authority, may be the most valuable type." –– Karl Popper, The Open Society and Its Enemies, Volume One: The Spell of Plato, page 113.

"This is the deepest reason for the fallibility of man. Human mistakes are made because one can sometimes give good reasons for a thing to be true, and bad reasons for a thing to be true, but never conclusive reasons for a thing to be true. One cannot generally distinguish between truth and falsehood by a criterion. This does not mean that we shouldn’t try to find truth, and it does not mean that we shouldn’t get nearer and nearer to the truth. It doesn’t even mean that we cannot know that we have got nearer to the truth. We can know we have got nearer to the truth, but it means that we can never know that we have reached the truth. Truth is something we haven’t got in our pocket." — Karl R. Popper

“The method of science is the method of bold conjectures and ingenious and severe attempts to refute them.” — Karl Popper, Objective Knowledge: An Evolutionary Approach (1972)

"The further we progress in knowledge, the more clearly we can discern the vastness of our ignorance." –– Karl Popper, The Myth of the Framework (1994), Ch. 2, p. 35.

Whenever we propose a solution to a problem, we ought to try as hard as we can to overthrow our solution, rather than defend it. — Karl Popper.

"To err is human. All human knowledge is fallible and therefore uncertain." — Karl Popper.

"The bucket theory of the mind assumes that our knowledge comes from the accumulation of observations, as if the mind were a bucket into which impressions are poured. But this view is mistaken. All observation is preceded by a conjecture, a hypothesis, or an expectation. We are born with innate expectations—not innate ideas in the old sense, but dispositions which determine how we shall react to stimuli. These expectations are activated and modified by experience, particularly when we actively explore the world. This is the searchlight theory of knowledge: our mind is not a passive receptacle but an active searchlight, projecting expectations and hypotheses onto the world, and learning from the feedback it receives. This active process is the basis of all learning and discovery, from the simplest forms of animal behavior to the most advanced scientific theories." —Karl Popper, Objective Knowledge: An Evolutionary Approach (1972), Appendix, p. 344

"It should be our pride to teach ourselves as well as we can to speak as clearly and unpretentiously as possible, and to avoid like the plague the suggestion that we are in the possession of knowledge which is too deep to be clearly and simply expressed." —Karl Popper, The Myth of the Framework: In Defence of Science and Rationality (1994), Chapter 1, p. 33

"Philosophers are as free as others to use any method in searching for truth. There is no method peculiar to philosophy." — Karl Popper, The Logic of Scientific Discovery, 1959.

"We are not students of some subject matter but students of problems. And problems may cut right across the borders of any subject matter or discipline." — Karl Popper, Unended Quest (1976)

“The more we learn about the world, and the deeper our learning, the more conscious, specific, and articulate will be our knowledge of what we do not know.” –– Karl Popper, Conjectures and Refutations, Chapter 1, p. 28 (Routledge, 5th edition, 1989).

"There is no such thing as a pure observation, an observation which is not theory-impregnated." — Karl Popper, Objective Knowledge: An Evolutionary Approach (1972).

"The history of science, like the history of all human ideas, is a history of irresponsible dreams, of obstinacy, and of error. But science is one of the very few human activities—perhaps the only one—in which errors are systematically criticized and fairly often, in time, corrected." — Karl Popper, Conjectures and Refutations, p. 215 (1963).

"The conflict between rationalism and irrationalism has become the most important intellectual, and perhaps even moral, issue of our time." — Karl Popper, The Open Society and Its Enemies, Volume 2, Chapter 24, p. 545

"It is our duty to help those who need help; but it cannot be our duty to make others happy, since this does not depend on us, and since it would only too often mean intruding on the privacy of those towards whom we have such amiable intentions." —Karl Popper, The Open Society and Its Enemies, Vol. 1, p. 235 (1945)

"All knowledge is theory–impregnated, including our observations." –– Karl Popper

“Bold ideas, unjustified anticipations, and speculative thought, are our only means for interpreting nature.” — Karl Popper, The Logic of Scientific Discovery, Ch. 10, p. 280 (1959)

"For I believe that there is only one excuse for a lecture: to challenge. It is the only way in which speech can be better than print." — Karl Popper, Unended Quest (1992), p. 142

"We can now see that the growth of knowledge depends entirely upon disagreement. For science grows through a process of mutual criticism, and thus through the elimination of error. It is through disagreement—through the clash of opinions and the critical discussion of alternatives—that we approach the truth. This is why I have always emphasized the importance of bold conjectures and their critical testing: without disagreement there can be no criticism, and without criticism there can be no progress. The idea that we can ever reach agreement on a framework of assumptions within which all rational discussion must proceed is, I believe, an illusion—an illusion which has been fostered by the long tradition of philosophical thought that I have criticized." —Karl Popper, The Myth of the Framework: In Defence of Science and Rationality (1994), Chapter 1, p. 34

"The future is not a repetition of the past. We live in a world of emerging novelty. Each moment brings the unexpected, the unpredicted." — Karl Popper, The Open Universe (1982), p. 162

"Science must begin with myths, and with the criticism of myths; neither with the collection of observations, nor with the invention of experiments, but with the critical discussion of myths, and of magical techniques and practices." — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge (1963), p. 50

“We must not think that the war of ideas is a war that can be avoided; it is forced upon us by the dogmatist who cannot tolerate criticism. The war of ideas is a war worth fighting; it is the only war that can secure peace. For peace is not the absence of conflict, but the presence of rational discussion.” — Karl Popper, Conjectures and Refutations (1963), p. 25.

"Science must begin with myths, and with the criticism of myths." — Karl R. Popper, Conjectures and Refutations.

"We are not passive receptors of sense data, but active organisms. Because we react to our environment not always merely instinctively, but sometimes consciously and freely. Because we can invent myths, stories, theories; because we have a thirst for explanation, an insatiable curiosity, a wish to know. Because we not only invent stories and theories, but try them out and see whether they work and how they work. Because by a great effort, by trying hard and making many mistakes, we may sometimes, if we are lucky, succeed in hitting upon a story, an explanation, which 'saves the phenomena'; perhaps by making up a myth about 'invisibles', such as atoms or gravitational forces, which explain the visible. Because knowledge is an adventure of ideas." — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge (1963), p. 95.

"I maintain that he who teaches that not reason but love should rule opens the way for those who rule by hate." –– Karl Popper, The Open Society and Its Enemies, Vol. 2, page 223.

"Science must begin with myths, and with the criticism of myths; neither with the collection of observations, nor with the invention of experiments, but with the critical discussion of myths, and of magical techniques and practices." — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge (1963), p. 50.

“Induction, i.e. inference based on many observations, is a myth. It is neither a psychological fact, nor a fact of ordinary life, nor one of scientific procedure.” — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge, 1962.

"The surest path to intellectual perdition: the abandonment of real problems for the sake of verbal problems." — Karl R. Popper, Unended Quest: An Intellectual Autobiography, Chapter 7: "A Long Digression Concerning Essentialism," p. 16 (Routledge Classics, 2002)

"The best thing that can happen to a scientific theory is that it is refuted, because this is how we learn and advance our knowledge." —Karl Popper, Conjectures and Refutations (1963)

"Once we abandon the idol of certainty, we also destroy one of the chief defences of obscurantism which bars the progress of science. For this worship of certainty hampers both the boldness of our conjectures and the rigour and honesty of our tests." — Karl Popper, The Logic of Scientific Discovery, 1959, p. 281 (Hutchinson edition)

“The more we learn about the world, and the deeper our knowledge penetrates, the more we become aware of the vastness of our ignorance.” — Karl R. Popper, Conjectures and Refutations.

"It is wrong to think that belief in freedom always leads to victory; we must always be prepared for it to lead to defeat. If we choose freedom, then we must be prepared to perish along with it." — Karl Popper, The Open Society and Its Enemies, Vol. 2 (1945), Ch. 12, p. 225

"There is no such thing as the scientific method, in the sense of a set of rules that, if followed, will guarantee success in science. Science proceeds by trial and error, by conjectures and refutations, and this is all we can say about its method." — Karl Popper, Conjectures and Refutations, 1963, p. 51 (Routledge edition)

"Every solution of a problem raises new unsolved problems; the more so the deeper the original problem and the bolder its solution. The more we learn about the world, and the deeper our learning, the more conscious, specific, and articulate will be our knowledge of what we do not know—our knowledge of our ignorance. For this, indeed, is the main source of our ignorance: the fact that our knowledge can be only finite, while our ignorance must necessarily be infinite." — Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge (1963), p. 28.

“By criticizing our theories we can let our theories die in our stead.” — Karl Popper

"It is complete nihilism to propose laying down arms in a world where atom bombs are around. It is very simple: there is no way of achieving peace other than with weapons." — Karl Popper, The Lesson of This Century (1997), p. 22

"The more we try to return to the heroic age of tribalism, the more surely do we arrive at the Inquisition, at the Secret Police, and at a romanticized gangsterism." — Karl Popper, The Open Society and Its Enemies, Vol. 1.

"We do not know: we can only guess. And our guesses are guided by the unscientific, the metaphysical faith in laws, in regularities which we can uncover—discover. In our infinite ignorance we are also infinitely fallible. Our knowledge is finite, but our ignorance is infinite." ––Karl Popper, "Conjectures and Refutations": The Growth of Scientific Knowledge, Chapter 1, p. 28 (1963)

"Science is one of the very few human activities — perhaps the only one — in which errors are systematically criticized and fairly often, in time, corrected." — Karl R. Popper, Conjectures and Refutations.