All Thinkers

Karl Popper

Karl Popper was an Austrian-British philosopher of science and politics. He was born in 1902 in Vienna into a well-off, secular Jewish family that had converted to Lutheran Christianity. His parents loved books, and Popper grew up reading widely. As a young man he tried out many of the big intellectual movements of his time. He attended Marxist meetings, studied Freudian psychology, and worked briefly with the Adlerian school of psychology. He was struck by something that bothered him for the rest of his life. The followers of these movements seemed able to explain everything. Whatever happened in the world, Marxism, Freudianism, and Adlerian theory could fit it into their system. Popper began to wonder whether this was a strength or a weakness. He trained as a teacher and earned a doctorate in philosophy from the University of Vienna in 1928. In 1934 he published his first major book, Logic of Scientific Discovery, which made his name in philosophy of science. As Hitler rose to power, Popper, with his Jewish background, knew he had to leave Europe. In 1937 he took a teaching post in New Zealand. There, during the Second World War, he wrote The Open Society and Its Enemies, his great defence of liberal democracy. In 1946 he moved to the London School of Economics, where he stayed for the rest of his career. He was knighted in 1965. He wrote on philosophy of science, politics, the mind, and many other subjects. He died in 1994 in London at the age of 92.

Origin
Austria / United Kingdom
Lifespan
1902-1994
Era
20th century
Subjects
Philosophy Of Science Political Philosophy Epistemology 20th Century Philosophy Liberalism
Skills
Scientific Thinking Critical Thinking Ethical Thinking Research Skills Problem Solving
Why They Matter

Popper matters for three reasons. First, he changed how people think about science. Most earlier thinkers had said science works by gathering many observations and generalising from them. Popper said this was wrong. No matter how many white swans you see, you can never prove that all swans are white. But a single black swan disproves the rule. Real science, Popper argued, advances by trying to disprove its own theories, not to confirm them. A theory is scientific only if it can, in principle, be shown to be wrong. This idea, called falsifiability, is now central to how scientists and educators talk about scientific method.

Second, he wrote one of the most important defences of liberal democracy in the twentieth century. The Open Society and Its Enemies, written during the Second World War, attacks Plato, Hegel, and Marx as enemies of the open society. Popper argued that we cannot know in advance how to build a perfect society. We can only test reforms, learn from mistakes, and improve step by step. Democracy is valuable because it lets us correct errors without violence.

Third, he linked these two projects. Both science and democracy work by being open to criticism. Closed systems that cannot be questioned, whether scientific theories or political ideologies, are dangerous. Open systems that test their ideas and accept being wrong are the foundations of progress.

Key Ideas
1
How Do You Know If Something Is Science?
2
All Swans Are White
3
The Open Society
Key Quotations
"A theory which is not refutable by any conceivable event is non-scientific. Irrefutability is not a virtue of a theory (as people often think) but a vice."
— Conjectures and Refutations, 1963
This is one of Popper's clearest statements of his core idea. We tend to praise a theory for fitting all the facts. Popper said this was a warning sign, not a strength. A theory that fits everything tells us nothing, because it forbids nothing. A theory worth having makes risky predictions that could fail. If the predictions never fail no matter what happens, the theory is not really being tested. It is just being protected. For students, this is a powerful idea to apply to many claims they meet, in science, politics, and everyday life. When someone offers a theory that 'explains everything', the question to ask is: what would prove this theory wrong? If the answer is 'nothing could', be careful.
"We must plan for freedom, and not only for security, if for no other reason than only freedom can make security secure."
— The Open Society and Its Enemies, 1945
This is one of Popper's central political claims. Many governments offer security at the cost of freedom: obey us, and we will keep you safe. Popper thought this was a bad trade. Without freedom, security itself becomes fragile, because there is no way to correct mistakes by the people in charge. Free societies make mistakes too, but they can fix them. Closed societies cannot, and so eventually their security falls apart, often violently. The argument is still relevant today, when many regimes offer order in exchange for silence. For students, the line is a useful test. When leaders ask us to give up freedom for safety, what is the long-term price? Popper thought it was usually higher than it looked.
Using This Thinker in the Classroom
Scientific Thinking When teaching students how to tell science from pseudoscience
How to introduce
Use Popper's basic test: a theory is scientific if it could in principle be shown to be wrong. Compare a horoscope, which can fit any life events, with a weather forecast, which makes specific predictions that can fail. Discuss with students which is more scientific and why. Ask them to think of claims they meet on social media or in advertising. Could those claims be tested? What evidence would refute them? Popper's idea gives students a practical tool for evaluating the many confident claims they encounter. The test is not whether a claim sounds smart. The test is whether anything could prove it wrong.
Critical Thinking When introducing students to the limits of generalisation
How to introduce
Use the black swan story. For centuries, Europeans only saw white swans and concluded that all swans are white. Then black swans were found in Australia. Discuss with students: how do we know when our experience is enough to support a general claim? It usually isn't. 'I have never seen X' does not prove X does not exist. Popper's lesson: we should hold our generalisations loosely and stay alert for counter-examples. This applies to stereotypes about people, beliefs about how the world works, and our own assumptions about what is possible. Black swans are everywhere, once we look.
Ethical Thinking When discussing what makes societies free
How to introduce
Introduce Popper's distinction between open and closed societies. An open society lets people criticise the government, change laws, and try new ideas. A closed society fixes things in place, often around one ideology or leader. Discuss with students: what features of an open society do they value? What would they miss in a closed one? The point is not to celebrate any particular country as perfect. It is to think about which features of political life make peaceful change possible. Open societies are not perfect. They are correctable. Popper thought that was their main strength.
Further Reading

For a first introduction, Popper's own Conjectures and Refutations (1963) is more readable than his earlier technical works. Bryan Magee's Popper (Fontana Modern Masters, 1973) is a short, clear overview by a thinker who knew him personally. The BBC In Our Time podcast episodes on Popper and on falsifiability are good free starting points. The Stanford Encyclopedia of Philosophy entry on Popper is solid and free.

Key Ideas
1
Conjectures and Refutations
2
The Paradox of Tolerance
3
Piecemeal Engineering
Key Quotations
"Science must begin with myths, and with the criticism of myths."
— Conjectures and Refutations, 1963
Popper had a generous view of how science begins. He did not think science started with pure observation. He thought it started with bold guesses, even mythological stories about how the world worked. The first Greek thinkers told stories about water, fire, atoms, or invisible forces. Most of these stories were wrong. But they were the seeds of later theory. What turned myth into science was the willingness to criticise the stories, test them, and replace them. The mythologising impulse is not a problem. The refusal to question the myths is. For students, this idea is liberating. Bold imagination is part of science, not opposed to it. Wild guesses are useful, as long as we are willing to give them up when they fail. The danger is not having too many ideas. It is being too attached to the ones we have.
"True ignorance is not the absence of knowledge, but the refusal to acquire it."
— Widely attributed to Popper, with similar formulations across his works
This line has been quoted in many forms across Popper's work. The basic idea is consistent. There is nothing wrong with not knowing something. Everyone is ignorant of most things. What is shameful is refusing to learn when learning is available. People who cannot be told they are wrong are the dangerous ones, because they cut themselves off from improvement. The line connects directly to Popper's view of science. We progress by being willing to discover that we were wrong. People who refuse this, in science or in life, condemn themselves to repeating their errors. For students, this is a useful frame for their own learning. Mistakes and ignorance are normal. The willingness to face them and learn is what separates growth from stagnation.
Using This Thinker in the Classroom
Problem Solving When teaching students how progress actually happens
How to introduce
Walk students through Popper's pattern: problem, guess, test, refutation, new problem. Real progress in science, engineering, and life rarely comes from steady accumulation of certainties. It comes from bold guesses tested ruthlessly. Most guesses fail. The few that survive harsh testing become our current best ideas. Discuss with students how this applies to their own learning. When they study or solve problems, the willingness to try things and be wrong is more useful than waiting for certainty before starting. Popper's pattern is a model for productive failure. The aim is not to avoid mistakes. It is to make better mistakes, faster.
Critical Thinking When discussing utopian thinking
How to introduce
Introduce Popper's distinction between utopian and piecemeal engineering. Utopian engineering tries to redesign society from scratch according to a master plan. Piecemeal engineering tries one reform at a time and learns from results. Discuss with students: which approach is more likely to succeed? Why? What are the costs of each? Popper thought big utopian plans almost always fail because we do not know enough, and the cost of forcing them on people is very high. Smaller, testable reforms are humbler but more reliable. This is a useful frame for thinking about politics, social policy, and even personal goals. Big dreams are not bad, but they often need to be broken into testable steps.
Further Reading

For deeper reading, The Open Society and Its Enemies (Routledge editions) and The Logic of Scientific Discovery (Routledge) are essential. Anthony O'Hear's Karl Popper (Routledge, 1980) is a useful guide. The debates with Kuhn are gathered in Lakatos and Musgrave's Criticism and the Growth of Knowledge (1970), one of the great collections in philosophy of science. Popper's Unended Quest (1976), his intellectual autobiography, is a good way into his life and thought.

Key Ideas
1
The Limits of Falsificationism
2
The Attack on Plato
3
Three Worlds
Key Quotations
"Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them."
— The Open Society and Its Enemies, Volume 1, 1945, footnote 4 to Chapter 7
This is the famous statement of the paradox of tolerance. It appears in a footnote, not the main text. Popper does not say all intolerant views must be silenced. He says a tolerant society has the right to defend itself when intolerant movements refuse argument and use violence or threats. The crucial part of the passage is that he prefers to keep argument open as long as possible. Suppression is a last resort. The paradox is now invoked widely in debates about free speech, hate speech laws, and extremism. It is also misused. Some people quote it to justify silencing any view they dislike, which goes far beyond what Popper said. For advanced students, the line is a useful and controversial prompt. Where exactly is the line between tolerating wrong ideas and tolerating dangerous movements? Popper started a debate that is still live.
"Our knowledge can only be finite, while our ignorance must necessarily be infinite."
— Conjectures and Refutations, 1963
This is one of Popper's most beautiful statements about the human condition. Knowledge is finite. Ignorance is infinite. No matter how much we learn, the universe of what we do not know is much larger than what we do know. This is not a counsel of despair. For Popper, it was a reason for humility and also for hope. Each step of new knowledge opens up new questions we did not know existed. Science is not a march toward a final complete truth. It is an endless conversation between curiosity and reality. Closed systems that claim total knowledge are not just wrong; they misunderstand what knowledge is. For advanced students, the line is a useful corrective to confidence in any system, including modern science itself. The honest scientific attitude is to know that we still know very little, and to keep going anyway.
Using This Thinker in the Classroom
Research Skills When teaching students how scientific knowledge actually grows
How to introduce
Compare Popper's view of science with that of Thomas Kuhn. Popper said science advances by trying to refute its theories. Kuhn said real scientists usually defend their theories until a crisis builds, then everything changes at once in a paradigm shift. Discuss with students which view fits the history of science better. Most modern philosophers of science take elements from both. Real science has Popperian moments of bold conjecture and refutation, and Kuhnian periods of paradigm-defending normal work. Knowing the debate gives students a more realistic picture of what scientific progress looks like, and helps them understand why scientific consensus changes the way it does.
Ethical Thinking When discussing the paradox of tolerance
How to introduce
Read with students Popper's footnote on the paradox of tolerance. Discuss the careful position he actually took: tolerant societies can defend themselves against violent intolerance, but argument should be the first response, and suppression is a last resort. Then discuss how the line is sometimes used today. Some quote Popper to justify silencing any view they find objectionable, which goes far beyond his actual claim. Ask students: where is the line between protecting open argument and shutting down legitimate disagreement? The question is genuinely hard. Popper opened the conversation. He did not finish it. Working through it carefully is good practice for serious ethical thinking.
Common Misconceptions
Common misconception

Popper proved that science works by falsification.

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What to teach instead

He did not prove it. He proposed it as the best account of scientific method. The proposal has been deeply influential but also seriously challenged. Thomas Kuhn argued that real scientists rarely abandon a theory after a single failed test. Imre Lakatos showed that successful research programmes routinely defend their core theories from quick refutation. Paul Feyerabend went further, arguing that no fixed method describes how science actually works. Most contemporary philosophers of science accept some Popperian intuitions while rejecting strict falsificationism. Popper started one of the great twentieth-century debates about science, and his ideas remain influential, but the picture is more complicated than the simple slogan 'science works by falsification' suggests.

Common misconception

The paradox of tolerance means we should ban any view we find intolerant.

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What to teach instead

Popper said something more careful. He thought tolerant societies should keep argument open as long as possible. Most intolerant views can and should be answered in debate, not silenced. Suppression is a last resort, justified only when intolerant movements refuse argument and use threats or violence to silence others. The paradox is sometimes quoted today to justify shutting down views people simply dislike, which goes far beyond Popper's position. Reading the original footnote is important. Popper was a defender of free debate. He believed open argument almost always served tolerance better than censorship, and that protecting argument was itself a kind of tolerance.

Common misconception

Popper's attack on Plato is the standard view of Plato.

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What to teach instead

It is not. Popper read Plato as a proto-totalitarian theorist of the closed society. Most Plato scholars disagree, sometimes strongly. They argue that Popper ignored Plato's irony, his evolving thought across his career, and the difference between his characters and his own voice. The Republic is not a simple political manifesto. It is a complex philosophical dialogue with many layers. Popper's reading was bold and influential, but it is one reading among many, and it has been heavily criticised. Students who only know Plato through Popper get a distorted picture. Reading both is useful, but the Plato attack should not be taken as the final word on a thinker who has been read in many other ways for over two thousand years.

Common misconception

Popper was anti-science or skeptical of scientific knowledge.

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What to teach instead

He was not. Popper was a deeply pro-science thinker who admired Einstein and the scientific method enormously. His point was not that scientific knowledge is unreliable, but that it is provisional. Theories are our best current guesses, always open to revision when better evidence comes. This is not skepticism of science. It is a clearer description of what scientific knowledge actually is. Some readers, mistakenly, take 'theories can never be proven' as a license to dismiss scientific findings. Popper would have rejected this. Provisional knowledge based on rigorous testing is still our best knowledge. The difference between 'we have not finally proven this' and 'we have no good reason to believe it' is crucial. Popper insisted on the first, never the second.

Intellectual Connections
In Dialogue With
Thomas Kuhn
Kuhn's The Structure of Scientific Revolutions (1962) gave a different picture of how science works. While Popper said scientists test and try to refute their theories, Kuhn said real scientists usually defend their theories within a 'paradigm' until a crisis forces a revolutionary shift. The Popper-Kuhn debate became one of the great conversations in twentieth-century philosophy of science. Most contemporary philosophers draw on both. Reading them together gives students the central debate about how scientific progress actually works.
Complements
Hannah Arendt
Arendt's The Origins of Totalitarianism (1951) and Popper's The Open Society and Its Enemies (1945) are the two great twentieth-century critiques of totalitarianism by central European Jewish exiles. Both saw Nazism and Stalinism as new political phenomena requiring new tools to understand. Both feared closed political systems that could not be questioned. They differed in style and method, but their concerns overlapped. Reading them together gives students a rich double picture of mid-twentieth-century political thought in defence of freedom.
In Dialogue With
Karl Marx
Popper attacked Marx in The Open Society and Its Enemies as one of the great architects of closed-society thinking. He saw Marxism as offering total, unfalsifiable explanations and a master plan that justified violence. Marxists have pushed back hard, arguing that Popper read Marx unfairly and ignored the open, scientific elements of Marxist analysis. The Popper-Marx confrontation is one of the major intellectual battles of the twentieth century. Reading them together helps students see both why Marxist thought has been so influential and why critics like Popper found it dangerous.
Develops
John Locke
Locke had argued in the seventeenth century that government depended on the consent of the governed and could be peacefully changed. Popper extended this tradition into the twentieth century, with a sharper account of why open societies matter and why their enemies threaten them. Both thinkers were defenders of liberal constitutional government. Popper's idea that we should evaluate political systems by whether they can be peacefully reformed, not by their utopian ideals, builds directly on Locke's tradition. Reading them together shows the long line of liberal political thought that runs from the seventeenth to the twentieth century.
In Dialogue With
Sigmund Freud
Popper used Freudian psychoanalysis as one of his main examples of pseudoscience. He argued that Freud's theories explained everything and could be refuted by nothing. Defenders of Freud have pushed back, arguing that Popper's test of falsifiability is too strict and that psychoanalysis offers genuine if different kinds of insight. The Popper-Freud encounter is a useful test case for thinking about what counts as science, what counts as useful knowledge, and where the lines between them lie. Reading them together helps students think critically about both the strengths and the limits of Popper's falsificationism.
Anticipates
Esther Duflo
Duflo, the Nobel-winning economist of poverty, works in a way Popper would have admired. She and her collaborators run randomised controlled trials of small policy interventions to see what actually reduces poverty. This is piecemeal engineering in action: trying specific reforms, testing them carefully, learning from failures, scaling what works. The approach is humbler than grand development theory and more effective. Reading Popper alongside Duflo shows how his philosophical framework continues to shape practical policy work today, especially among economists and policymakers committed to evidence-based reform.
Further Reading

For research-level engagement, Malachi Hacohen's Karl Popper: The Formative Years 1902-1945 (2000) is the standard intellectual biography. The Cambridge Companion to Popper, edited by Jeremy Shearmur and Geoffrey Stokes (2016), gathers leading scholars. The journal Philosophy of Science regularly publishes work in the Popperian tradition and its critics. For Popper's politics, Jeremy Shearmur's The Political Thought of Karl Popper (1996) is essential. The Karl Popper Charitable Trust maintains his archive and supports ongoing scholarship.