  As we saw in §1.1, one of the conclusions Chomsky drew from his  (1959) critique of the Skinnerian program was that language cannot be learned by mere association of ideas (such as occurs in  conditioning). Since language mastery involves knowledge of grammar,  and since grammatical rules are defined over properties of utterances that are not accessible to experience, language learning must be more like theory-building in science. Children appear to be ‘little  linguists,’ making highly theoretical hypotheses about the  grammar of their language and testing them against the data provided  by what others say (and do):