  But while traces are vital to the statement of many syntactic rules  and regularities, they are ‘empty categories’ —  they are not audible in the sentence as spoken. (See Chomsky 1981  and Lasnik and Uriagereka 1986 for more on traces and other empty  categories.) Traces (and other similarly abstract properties of  languages) thus raise a question for the theory of language  acquisition. For if, as Chomsky maintains, mastery of language  involves knowledge of rules stated in terms of sentences' syntactic  properties, and if those properties are not so to speak  ‘present’ in the data, but are rather highly abstract and ‘unobservable,’ then it becomes hard to see how children  could possibly acquire knowledge of the rules concerning them. As a  consequence, children's feat in learning a language appears  miraculous: how could a child learn the myriad rules governing  linguistic expression given only her exposure to the sentences spoken around   her?[4] 