Karl Popper

Sir Karl Popper CH FRS FBA (28 July 1902 – 17 September 1994) was an Austrian and British philosopher and a professor at the London School of Economics.
He is considered one of the most influential philosophers of science of the 20th century. He also wrote on social and political philosophy, especially the evils of totalitarian ideas and politics. Popper is known for the idea of empirical falsification.
Life.
Karl Popper was born in Vienna (then in Austria-Hungary) in 1902 to middle-class parents of Jewish origins, both of whom had converted to Christianity. Popper received a Lutheran upbringing and was educated at the University of Vienna. His father had 12,000–14,000 volumes in his personal library. 
In 1919, he became attracted to Marxism. He joined the Association of Socialist School Students and also became a member of the Social Democratic Party of Austria, which was at that time a party that fully adopted the Marxist ideology. He soon stopped believing in Marxism, and was a supporter of social liberalism for the rest of his life.
Falsification.
Popper, brought up in Vienna, was well aware of the Vienna Circle. That school of logical positivism, led by Moritz Schlick, defined knowledge (especially scientific knowledge) as being propositions which could be verified. Popper thought this was quite wrong. In his opinion, science grew indirectly, by wrong ideas being "falsified". This he worked up in great detail in a series of books, of which "The logic of scientific discovery" is the most famous. All philosophy of science since then has had to deal with this issue, the "criterion". By 'criterion' is meant: what is it that makes a theory truly scientific, as opposed to simply common sense or opinion?
The open society.
Popper's work on political philosophy is also of great importance. Marx claimed knowledge of an historical process, where societies evolved from one state to another, until they reached a final state. This type of thought is known as 'historicism'. Popper argued that the growth of human knowledge partly causes the evolution of human history. Since "no society can predict its own future states of knowledge", it follows that no science can predict human history.
Popper's great works in defence of the liberal society were "The open society and its enemies" and "The poverty of historicism". His allies in this struggle were Friedrich Hayek, Ludwig von Mises and Milton Friedman.
The paradox of tolerance.
Although Popper was an advocate of toleration, he thought intolerance should not be tolerated. If tolerance allowed intolerance to succeed completely, tolerance itself would be threatened. In "The open society and its enemies: The spell of Plato", he argued that:
The Three worlds doctrine.
The Three Worlds Doctrine was invented by Popper and is used in philosophy to understand the condition in which we find ourselves as human beings.
It assumes the existence of three worlds. They are
This way of looking at reality is used by many philosophers to explain the relationship between the physical and spiritual worlds.
It was first described by the Austrian-British philosopher Karl Popper in a lecture given in August 1967.