<P> Chomsky argued that the human brain contains a limited set of constraints for organizing language . This implies in turn that all languages have a common structural basis: the set of rules known as "universal grammar". </P> <P> Speakers proficient in a language know which expressions are acceptable in their language and which are unacceptable . The key puzzle is how speakers come to know these restrictions of their language, since expressions that violate those restrictions are not present in the input, indicated as such . Chomsky argued that this poverty of stimulus means that Skinner's behaviourist perspective cannot explain language acquisition . The absence of negative evidence--evidence that an expression is part of a class of ungrammatical sentences in a given language--is the core of his argument . For example, in English, an interrogative pronoun like what cannot be related to a predicate within a relative clause: </P> <Dl> <Dd> * "What did John meet a man who sold?" </Dd> </Dl> <Dd> * "What did John meet a man who sold?" </Dd>

Universal grammar proposes a base number of rules govern all human languages