Chomsky is credited with the creation of the theory of generative grammar, considered to be one of the most significant contributions to the field of linguistics made in the 20th century. He also helped spark the cognitive revolution in psychology through his review of B. F. Skinner’s Verbal Behavior, in which he challenged the behaviorist approach to the study of behavior and language dominant in the 1950s. His naturalistic approach to the study of language has affected the philosophy of language and mind. He is also credited with the establishment of the Chomsky hierarchy, a classification of formal languages in terms of their generative power. Beginning with his critique of the Vietnam War in the 1960s, Chomsky has become more widely known for his media criticism and political activism, and for his criticism of the foreign policy of the United States and other governments.