Sartre's notion of 'Radical Freedom' said that everyone always has a choice, and every act is a free act. When people say they have 'no choice' but to do something, they are lying to themselves. He would point out situations such as a group of hikers who encounter a boulder blocking their path, leaving them 'no choice' but to turn back. Sartre claimed this was wrong, as they also could fling themselves off the mountain, killing themselves.
Sartre spent time in a Nazi war camp in World War II, and said that he was never more free than during that time, and it influenced many of his ideas on freedom in his post-war writings.
Jean-Paul Sartre and Albert Camus had a very public falling out and subsequent feud, largely due to Sartre's continued support of the Soviets and Communism after the discovery of the slave camps and other atrocities committed by Stalin. Sartre and Camus's feud essentially came from Camus rejecting Sartre's politics after the news came out of the massive Soviet slave camps. Sartre decided to continue his support of the Soviet Union against the capitalist West, and Camus thought this was an awful thing to do (most of us today would probably side with Camus, considering we have an even more full knowledge of what went on in the USSR).
He should have stuck with boinking Simone de Beauvoir.
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