Cornelius Castoriadis (1922-1997) was not only a powerful political thinker, economist, and psychoanalyst. He was also a militant activist, who in the late 1940s founded, along with Claude Lefort, “Socialisme ou Barbarie”, a French revolutionary group that had split from the Trotskyite Fourth International. Castoriadis and “Socialime ou Barbarie” were influential on the far left for two decades, although splits and disagreements among the group's members and followers characterized it.