Chomsky created the theory of generative grammar. This is one of the most important contributions to the field of linguistics made in the 20th century. He also helped start the cognitive revolution in psychology through his review of B. F. Skinner's "Verbal Behavior". He challenged the behaviorist way of looking at behavior and language. This was the main approach used in the 1950s. His natural approach to the study of language also changed the philosophy of language and mind. He also invented the Chomsky hierarchy, a way of looking at formal languages in terms of their power to explain language.