Sidney Hook was an American philosopher of pragmatism known for his contributions to the philosophy of history, the philosophy of education, political theory, and ethics. After embracing communism in his youth, Hook was later known for his criticisms of totalitarianism, both fascism and Marxism–Leninism. A social democrat, Hook sometimes cooperated with conservatives, particularly in opposing Marxism–Leninism. After World War II, he argued that members of such groups as the Communist Party USA and Leninists like democratic centralists could ethically be barred from holding the offices of public trust because they called for the violent overthrow of democratic governments.