Jean-Paul Charles Aymard Sartre (21 June 1905 – 15 April 1980) was a French existentialist philosopher, playwright, novelist, screenwriter, political activist, biographer, and literary critic. He was one of the leading figures in 20th century French philosophy, particularly Marxism, and was one of the key figures in literary and philosophical existentialism. His work continues to influence fields such as Marxist philosophy, sociology, critical theory and literary studies. Sartre was also noted for his long non-monogamous relationship with the feminist author and social theorist Simone de Beauvoir. He was awarded the 1964 Nobel Prize in Literature but refused it.
The main idea of Jean-Paul Sartre is that we are, as humans, "condemned to be free." This theory relies upon his belief that there is no creator, and is formed using the example of the paper knife. Sartre says that if one considered a paper knife, one would assume that the creator would have had a plan for it: an essence. Sartre said that human beings have no essence before their existence because there is no Creator. Thus: "existence precedes essence". This forms the basis for his assertion that since one cannot explain their own actions and behaviour by referencing any specific human nature, they are necessarily fully responsible for those actions. "We are left alone, without excuse".
Source: Wikipedia, "Jean-Paul Sartre", available under the CC-BY-SA License.