In the years following the appearance of Noam Chomsky's book, "Syntactic Structures," in 1957, transformational grammarians modified and improved his initial model of language. The notion of a deep structure of meaning underlying a sentence's surface structure was revised to embody elements representing negation, command, and interrogation, and to reveal subordination and coordination. "Meaning-changing" transformations became unnecessary. The complicated subcategorization system was replaced by a non-hierarchical model which represented the characteristic features of individual words in a part of the deep structure called the lexicon. Although a 1965 book by Chomsky, "Aspects of the Theory of Syntax," incorporated these modifications, linguists were still not satisfied with the adequacy of deep structure in representing some areas of meaning. They sought a deeper, more abstract level of structure "where the various semantic notions would be separated out" and where a concern with sentences would give way to a concern with propostions. Nevertheless, the essential human mystery of language remains. (LK)