Perhaps his (Chomsky's) most influential and time-tested contribution to the field is the claim that modeling knowledge of language using a formal grammar http://en.wikipedia.org/wiki/Formal_grammar accounts for the "productivity" or "creativity" of language. In other words, a formal grammar of a language can explain the ability of a hearer-speaker to produce and interpret an infinite number of utterances, including novel ones, with a limited set of grammatical rules and a finite set of terms. He has always acknowledged his debt to Pāṇini http://en.wikipedia.org/wiki/P%C4%81%E1%B9%87ini for his modern notion of an explicit generative grammar, although it is also related to rationalist http://en.wikipedia.org/wiki/Rationalist ideas of a priori http://en.wikipedia.org/wiki/A_priori_and_a_posteriori knowledge.
http://en.wikipedia.org/wiki/Chomsky http://en.wikipedia.org/wiki/Chomsky
