  At the same time as the behaviorist program in psychology was waning  under pressure from Chomsky and others, linguists were abandoning  what is known as ‘American Structuralism’ in the theory  of syntax. Like the behaviorists, the structuralists (e.g., Harris,  1951) refused to postulate irreducibly theoretical entities; they  insisted that syntactic categories (such as ‘noun phrase’ (‘NP’) or ‘verb phrase’ (‘VP’),  etc.) be reducible to properties of actual utterances (collected in  ‘corpora’ — lists of things people have said). In  his landmark book, Syntactic Structures (1957), however,  Chomsky argued that because corpora can contain only finitely many  sentences, no attempt at reduction can succeed. Linguists need  theoretical constructs that capture regularities going beyond the set of actual utterances, and that allow them to predict the properties  of novel utterances. But if the category NP, for instance, is to  include noun phrases that haven't been uttered yet, the meaning of  noun phrase can't be exhausted by what's in the corpus: the  structuralists' positivistic strictures on theoretical kinds are  misguided.