Author

Karl Popper

24 key ideas across 3 books, each explained in our own words.

Conjectures and Refutations

IDEA 1
Knowledge grows by guess and test
Popper argues that knowledge grows through conjectures, bold attempts to explain the world, and refutations, attempts to prove them wrong. Learning from mistakes is the engine of progress. Criticism is welcome.
IDEA 2
Einstein took a real risk
Popper was impressed when Einstein's theory of relativity predicted that light would bend near the sun. The 1919 eclipse observations could have disproved the theory, which made it genuinely scientific.
IDEA 3
Some theories explain everything
Popper contrasts Einstein's theory with Freud's psychoanalysis, Adler's psychology and Marxism. These theories seemed to explain every possible case, which Popper saw as a weakness, not a strength. Nothing could prove them wrong.

IDEA 1
Science needs a dividing line
Popper asks how to tell science apart from non-science, a question he calls the problem of demarcation. He argues that the answer is not whether a theory can be proved, but whether it could in principle be shown false.
IDEA 2
Scientific theories must be falsifiable
Popper argues that a theory is scientific only if it makes predictions that could turn out wrong. A theory that fits every possible observation, he says, tells us nothing about the world.
IDEA 3
Induction cannot justify science
Popper agrees with Hume that no number of observations can prove a general law. Seeing many white swans does not prove all swans are white, but seeing one black swan can disprove it.

IDEA 1
Freedom depends on open societies
Popper defends the open society, one where people can criticise their leaders, change laws peacefully and make their own choices. He wrote the book during the Second World War as a defence of democracy against totalitarianism.
IDEA 2
Closed societies resist change
Popper contrasts open societies with closed societies, which rely on tradition, tribal loyalty and fixed rules. Closed societies discourage criticism and treat the group as more important than individuals. Change feels threatening there.
IDEA 3
History has no fixed destiny
Popper attacks historicism, the belief that history follows inevitable laws that can be predicted. He argues that such beliefs justify tyranny, because leaders claim to know where history must go.

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