Louis Pierre Althusser was one of the most influential Marxist philosophers of the 20th century. He advanced claims about Marxist philosophy during the 1960s that were widely discussed. However, interest in his perspective declined in the 1970s due to reversals in his theoretical positions and the fortunes of Marxism. Nonetheless, the theory of ideology that Althusser developed has been widely used in the social sciences and humanities and provided a foundation for post-Marxist philosophy.