Karl Popper
Philosopher of science: a theory is scientific only if it could be proven false; defender of the open society
3 videos analysed · 7 quotes checked word for word · updated 2026-10-22
Key facts
- Born in Vienna in 1902, he left for New Zealand in 1937 and settled in London in 1946. Wikipédia 2026 · Karl Popper
- Knighted by Queen Elizabeth II in 1965; he died in 1994. Wikipédia 2026 · Karl Popper
Positions
click a row to see the evidenceIn a 1969 interview he said: "I don't know whether God exists or not", and found agnosticism, admitting that we do not know and keep searching, "all right".
He rejects both classic answers: knowledge does not come from piling up observations (he accepts Hume's critique of induction), nor from pure reason. It comes from bold guesses tested against experience.
Fallibilism: all knowledge is provisional, made of conjectures that must be tested and can be replaced. He refuses, however, to call this scepticism: we can still get closer to the truth. In a 1988-89 television interview he repeats that science aims at the truth, but that, since we cannot trust ourselves to have found it, we hunt for our own errors; relativism is answered not by dogmatic certainty but by progress.
With the neuroscientist John Eccles (The Self and Its Brain, 1977) he defended an interactionist dualism: mental properties are distinct from physical ones and act on them. He was not a dualist of two substances, and he added a "World 3" of objective ideas.
An indeterminist: the future is open. He saw freedom "not just chance but, rather, the result of a subtle interplay" between randomness and selective control. Interviewed by Ernst Gombrich in 1988, he says there are natural laws "even though not really deterministic natural laws", and traces the idea of historical determinism back to natural determinism.
- He rejects determinism: there are natural laws, but not strictly deterministic ones. He sees historical determinism as an offshoot of natural determinism. Video 1988 · Karl Popper - Uncertain Truth -…
If the world is a clockwork, the whole future is already fixed.
He rejects determinism: there are natural laws, but not strictly deterministic ones, and historical determinism is an offshoot of natural determinism.
Criticizes : Social existence determines consciousness
His whole philosophy is "critical rationalism": every belief, his own included, must remain open to criticism. In an archive interview he calls "an attitude of conscious criticism" the decisive element of science.
He denies that history has a direction or laws: there is no historical necessity, and its course depends on human actions we cannot foresee. It is up to us to give it aims.
No sourced position yet on: Status of morality, Human nature.
On the big questions
Does God exist?Agnosticism
"I don't know whether God exists or not."
The question on the site: all the argumentsAre we free?Free will
Indeterminism: freedom as an interplay of chance and control.
- He rejects determinism: there are natural laws, but not strictly deterministic ones. He sees historical determinism as an offshoot of natural determinism. Video 1988 · Karl Popper - Uncertain Truth -…
If the world is a clockwork, the whole future is already fixed.
He rejects determinism: there are natural laws, but not strictly deterministic ones, and historical determinism is an offshoot of natural determinism.
Criticizes : Social existence determines consciousness
The question on the site: all the argumentsIs the mind the brain?The mind is not matter
Interactionism with Eccles: the mind acts on the brain.
The question on the site: all the argumentsCan science explain everything?True, but not everything
Science never proves; it tests, refutes and gets closer to the truth.
- No number of observations can prove a general law, but a single counter-example can refute it. So science cannot prove; it can only test. Article 2022 · Karl Popper
- Science does not start from piled-up observations but from bold guesses. Then we try to find the hypothesis's weaknesses, imagining circumstances in which it would not work. Video 1989 · Karl Popper on Falsification &…
- Science aims at the truth, but we cannot trust ourselves to have reached it. That is why we hunt for our own errors, with a conscious attitude of criticism. Video 1989 · Karl Popper on Falsification &…
- Against relativism, he does not claim to hold the truth in his pocket. He answers that we have made definite progress. Video 1988 · Karl Popper - Uncertain Truth -…
A failed prediction never refutes a single theory, only a bundle of theory and background assumptions.
He came to accept that theories predict only together with auxiliary assumptions, and that adjusting them is normal in science. What matters is whether a change is genuine or only ad hoc, made to escape refutation.
Scientists do not really try to refute their theories: science advances through paradigm shifts.
See 1 more objections
No method is better than another, and no one can claim to be closer to the truth.
He answers that he does not claim to hold the truth in his pocket, but that we have made definite progress.
Defends : A theory is scientific if it can be refutedCriticizes : Scientific revolutions: changing paradigm is not proving
The question on the site: all the argumentsDoes history have a direction?No direction
In 1988 he said that about the future "we know nothing but that it is open", not predetermined, and that it depends on our efforts.
- The future is open and not predetermined; it depends on our efforts whether it will be better or worse than now. We are the ones making it. Video 1988 · Karl Popper - Uncertain Truth -…
- There is no necessity of progress ("progressivism"): we can always regress, so we must work directly for a better world, not for "progress" as such. Video 1988 · Karl Popper - Uncertain Truth -…
- He rejects determinism, historical determinism being an offshoot of natural determinism; the course of history depends on the growth of knowledge, which cannot be predicted. Video 1988 · Karl Popper - Uncertain Truth -…2
Defends : The course of history cannot be predicted: the poverty of historicism
The question on the site: all the argumentsIn their words
we try really to find the weaknesses of the hypothesis
I don't know whether God exists or not.
Unlimited tolerance must lead to the disappearance of tolerance.
How the ideas evolved
Click a step for details1919Marx, Freud, Adler... and Einstein
As a young man in Vienna he notices that some theories seem confirmed by everything, while Einstein's makes risky predictions that could fail. This contrast becomes the seed of his criterion.
1934The Logic of Scientific Discovery
Logik der Forschung criticises inductivism and logical positivism and proposes falsifiability as the line between science and non-science.
1945The Open Society and Its Enemies
Written in exile in New Zealand, the book attacks the philosophical roots of totalitarianism and historicism, and defends a society of free individuals who respect each other's rights.
1946London School of Economics
He moves to the LSE in London, where he teaches logic and scientific method; Paul Feyerabend is among his students.
1977The Self and Its Brain
With John Eccles he defends an interactionist view of mind and brain, and a two-stage model of freedom.
1978Changing his mind on Darwin
He had called Darwinism "a metaphysical research program". He later writes: "I have changed my mind about the testability and logical status of the theory of natural selection."
1988Uncertain Truth on Channel 4
In a television series he is interviewed by Ernst Gombrich, John Eccles and Anthony Quinton. He rejects historical determinism and answers relativism with the idea of real, though never final, progress.
In their words« while I think there are natural laws even though not really deterministic natural laws »Interviewed by Gombrich, he rejects determinism, historical determinism being an offshoot of natural determinism. watch
« about the future we know nothing but that it is open that it is not predetermined and that it will depend on our efforts whether it will be better or worse than it is now »History has no predetermined course: the future is open and depends on our efforts. watch
« there is no such a thing as progressivism that is to say and necessity to progress »Against historicism: there is no necessity of progress; we can always regress, so we must work for a better world. watch
1989Criticism as the heart of science
In an interview he says that beyond trial and error, what is decisive in science is conscious criticism: we deliberately create the circumstances in which our hypothesis could fail.
In their words« conscious criticism an attitude of conscious criticism »In an interview he calls conscious criticism the decisive element of science, beyond trial and error. watch
« we try to find out its weaknesses we think about it and think perhaps under such and such circumstances our hypothesis will not work »He describes testing a hypothesis by deliberately creating the circumstances in which it could fail. watch
« we are aiming at the truth and that is our goal »Science aims at the truth, but since we cannot trust ourselves to have found it, we search for our own errors. watch
The arguments 7
The site's arguments they defend or criticize, and why. Click a title to see the full argument.
His own argument: the course of history depends on the growth of knowledge, which cannot be predicted, so history cannot be predicted either. In 1988 he adds that there is no necessity of progress: "we can always also regress".
His own criterion: a theory is scientific if possible observations could contradict it. In 1989: "we try to find out its weaknesses", imagining circumstances in which the hypothesis will not work, then testing them.
He accepts Hume's critique: experience can never verify a universal law. But he denies that science really uses induction, so the problem does not lead to scepticism.
Against the idea that science starts from piling up observations: it starts from conjectures.
Kuhn's Structure of Scientific Revolutions denied that science grows by testing and refuting; Popper's approach was overshadowed by it.
He rejects Marx's historical materialism and holds that there are no laws of history to predict the future.
The Open Society and Its Enemies attacks Plato's ideal city as a forerunner of closed, totalitarian societies.
Allies and opponents
David HumeInfluence
He takes over Hume's critique of induction, but draws an optimistic conclusion from it.
Albert EinsteinInfluence
Einstein's risky, refutable predictions were his model of a scientific theory.
Paul FeyerabendSparring partner
His former student at the LSE, who later rejected any fixed scientific method.
JJohn EcclesClose
The Nobel-winning neuroscientist with whom he wrote The Self and Its Brain.
Thomas KuhnDisagrees
Kuhn denied that science grows step by step through refutations.
Ludwig WittgensteinDisagrees
Popper saw him as a rival and his "nemesis", and attacked the focus of British philosophy on language.
Karl MarxDisagrees
He rejects historical materialism and any claim to predict history.
Georg Wilhelm Friedrich HegelDisagrees
Volume 2 of The Open Society presents Hegel's philosophy of history as a source of modern totalitarianism.
PlatoDisagrees
Target of The Open Society and Its Enemies.
Signature ideas
The asymmetry of proof
"An exception, far from 'proving' a rule, conclusively refutes it": a thousand white swans do not prove that all swans are white, one black swan proves they are not.
The paradox of tolerance
"Unlimited tolerance must lead to the disappearance of tolerance": a tolerant society may have to defend itself against intolerant movements.
The three worlds
World 1: physical things. World 2: our mental states. World 3: the objective contents of thought, such as theories and problems, which act back on the other two.
Closest and furthest
how it is computedAmong the philosophers
Among today's voices
Take the Compass: 5 minutes to find out which philosophers you are closest to.
All sources (9)
- Karl Popper Stanford Encyclopedia of Philosophy · 2022
- Karl Popper Wikipedia · 2026
- The Open Society and Its Enemies Wikipedia · 2026
- Karl Popper on Falsification & Science (1989) YouTube (republication) · 1989
- Karl Popper - Uncertain Truth - 1/6 (Channel 4, 1988) YouTube (republication) · 1988
- Karl Popper - Uncertain Truth - 2/6 (Channel 4, 1988) YouTube (republication) · 1988
- Karl Popper on Falsification & Science (1989) YouTube (republication) · 1989
- Karl Popper on Falsification & Science (1989) YouTube (republication) · 1989
- Karl Popper - Uncertain Truth - 2/6 (Channel 4, 1988) YouTube (republication) · 1988