Sir Karl Popper (1902-1994) was an Austrian-born philosopher, later British citizen. His work was wide-ranging but he is particularly known for his contributions in the philosophy of science and political philosophy.
Philosophy of Science
In science Popper proposed a falsifiability criterion for the demarcation of science from pseudoscience as a result of his analysis of universal laws. Universal laws, which science hopes to describe, have a logical structure which precludes certain existential statements: universal laws are a negation of an existential proposition. That is to say that if an assertion is made that "all x's are y" (a universal statement) this the same assertion as "no x that is not y exists".
Popper's example was that of the colour of swans. The universal statement "all swans are white" implies the negative existential statement "no swans of a colour other than white exist". To verify the negative existential we would need to trawl through the universe to ensure that no non-white swan existed anywhere in time or space. Conversely if we saw just one black swan we could confirm the existential statement "black swans exist" and refute the universal "all swans are white".
Popper was uncompromising in his rejection of any attempt to take confirming instances as increasing the legitimacy, likelihood, reasonableness or probability of the universal statement. No matter how many white swans you see you are no more able to assume that all swans are white than if you had seen just the one white swan. The ease with which “confirming” instances can be found led Popper to the conclusion that an attempted verification of a universal law is worse than useless: to be science universal laws must be capable of being put to the test, to be falsifiable.
A refugee from Austria after the rise of Nazism, Popper dedicated the war years to the study of the roots of totalitarian thought. The “Open Society and its Enemies” analysed the contribution, as he saw it, of Plato, Hegel and Marx to an anti-rational tribalistic “closed society”. The closed society was, for Popper, marked by a lack of criticism of established ideas (established societal norms being treated not as societal norms but as “taboo” or “the natural order”) together with a collectivism that subjected the individual to the group goal.
Popper also saw “historicist” views and underpinning much of the thought behind the “closed” society. “Historicism”, dealt with in “The Poverty of Historicism” and criticised throughout the “Open Society”, both sees historical laws of development and bases much of its morality on these supposed laws.