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Thinkers

Karl Popper

Analytic · Europe · 1902 to 1994

Austrian-born thinker who held that science advances by testing bold guesses against evidence. He defended open societies.

What it is

Karl Popper (1902 to 1994) was an Austrian and British philosopher of science, born in Vienna. He is best known for the idea that a theory counts as scientific only if some possible observation could show it to be false, and for his defence of open, liberal society against totalitarian thinking.

He left Austria for New Zealand in 1937 and taught there through the Second World War. After the war he moved to England, taught at the London School of Economics, and was knighted in 1965.

The main ideas

  • Falsifiability. A scientific theory takes a risk. It says things that observation could prove wrong. A claim that no possible observation could ever contradict is not science, however well it fits what we see. He called this the line between science and non-science.
  • Why psychoanalysis and Marxism failed the test. He argued that these theories, as they were then held, could explain any outcome and so could never be caught out. He contrasted them with Einstein's theory, which made risky predictions.
  • Induction. He accepted the old criticism by Hume that repeated observation cannot prove a general law, and held that science does not actually work that way.
  • Critical rationalism. His name for his philosophy, which turns away from the idea that science begins with plain observation and grows by induction.
  • Against historicism. History does not unfold according to a knowable law towards a fixed goal, so claims to know where it is heading give no ground for sweeping plans.
  • The open society. In The Open Society and Its Enemies (1945) he defended liberal democracy and criticised totalitarian thinking, Marxism among it.
  • Three worlds. He distinguished the physical world, the world of mental states, and the world of the products of the mind, such as theories, each with its own standing.

What it looks like in practice

Picture someone who hears a claim that a certain diet works for everyone, and that when it fails, the person must not have followed it properly. Popper's test would ask what result, if it came, the claimer would accept as showing the diet does not work. If the answer is none, the claim has stopped being testable. This is an illustration of the idea, not a saying of Popper.

Books and moments

  • The Logic of Scientific Discovery is his book setting out falsifiability. It appeared in English in 1959.
  • The Open Society and Its Enemies (1945) is his defence of liberal society and his criticism of totalitarianism.
  • The move to New Zealand (1937) took him out of Austria for the war years.

Questions and criticism

  • Do scientists really work this way? Thomas Kuhn argued that real scientists do not follow a falsificationist method, and many came to see that account as the better one.
  • Is observation neutral? What one observes is shaped by the theory one holds, which makes a clean test of a theory hard.
  • Is falsifiability too strict a boundary? Some theories are hard to test in isolation yet seem to belong in science, so it is still discussed whether the line is drawn in the right place.

Close neighbours

He wrote against the logical positivists of the Vienna Circle, who wanted a different test for what is meaningful. Bertrand Russell admired his work. The page on Russell sets out a thinker who admired him.

Written by epchē in our own words, and checked against independent sources on 9 October 2026. The sources are listed below.

Who they connect to

Influenced

  • Abdolkarim Soroush · He took up Popper's view that all knowledge is open to correction, and applied it to religious thought.
  • Imre Lakatos · He began from Popper's view that science grows by criticism, and gave it a subtler form.
  • Paul Feyerabend · He began as a follower of Popper and later turned against his rules for science.

Influenced by

  • David Hume · Popper began from Hume's problem of induction and gave a new answer.
  • Immanuel Kant · Popper described himself as a Kantian who replaced certainty with criticism.

Reacted against

  • Auguste Comte · Popper criticised Comte's belief that history follows laws that can be foretold. (scholars disagree about this link)
  • G. W. F. Hegel · Popper attacked Hegel as an apologist for the state and a source of historicism.
  • Karl Marx · Popper argued that Marxism is unfalsifiable and that historical prophecy is unsound.
  • Ludwig Wittgenstein · Popper denied that philosophy is only the clearing up of linguistic confusions.
  • Plato · The Open Society and Its Enemies criticises Plato as a forerunner of closed rule.

Reacted against by

  • Paul Feyerabend · He argued that Popper's rules for testing theories would have stopped many great advances.
  • Thomas Kuhn · He argued that scientists rarely try to disprove their framework, as Popper said they should, until it is in crisis.

Runs parallel to

Other Analytic thinkers

Sources we used

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