In 1957, Noam Chomsky published Syntactic Structures, in which he developed the idea that each sentence in a language has two levels of representation — a deep structure and a surface structure.  The deep structure represented the core semantic relations of a sentence, and was mapped on to the surface structure (which followed the phonological form of the sentence very closely) via transformations. Chomsky believed that there would be considerable similarities between languages' deep structures, and that these structures would reveal properties, common to all languages, which were concealed by their surface structures. However, this was perhaps not the central motivation for introducing deep structure. Transformations had been proposed prior to the development of deep structure as a means of increasing the mathematical and descriptive power of context-free grammars. Similarly, deep structure was devised largely for technical reasons relating to early semantic theory. Chomsky emphasizes the importance of modern formal mathematical devices in the development of grammatical theory:
Development of basic concepts
Though transformations continue to be important in Chomsky's current theories, he has now abandoned the original notion of Deep Structure and Surface Structure. Initially, two additional levels of representation were introduced (LF — Logical Form, and PF — Phonetic Form), and then in the 1990s Chomsky sketched out a new program of research known as Minimalism, in which Deep Structure and Surface Structure no longer featured and PF and LF remained as the only levels of representation.
To complicate the understanding of the development of Noam Chomsky's theories, the precise meanings of Deep Structure and Surface Structure have changed over time — by the 1970s, the two were normally referred to simply as D-Structure and S-Structure by Chomskyan linguists. In particular, the idea that the meaning of a sentence was determined by its Deep Structure (taken to its logical conclusions by the generative semanticists during the same period) was dropped for good by Chomskyan linguists when LF took over this role (previously, Chomsky and Ray Jackendoff had begun to argue that meaning was determined by both Deep and Surface Structure).
Innate linguistic knowledge
Terms such as "transformation" can give the impression that theories of transformational generative grammar are intended as a model for the processes through which the human mind constructs and understands sentences. Chomsky is clear that this is not in fact the case: a generative grammar models only the knowledge that underlies the human ability to speak and understand. One of the most important of Chomsky's ideas is that most of this knowledge is innate, with the result that a baby can have a large body of prior knowledge about the structure of language in general, and need only actually learn the idiosyncratic features of the language(s) it is exposed to. Chomsky was not the first person to suggest that all languages had certain fundamental things in common (he quotes philosophers writing several centuries ago who had the same basic idea), but he helped to make the innateness theory respectable after a period dominated by more behaviorist attitudes towards language. Perhaps more significantly, he made concrete and technically sophisticated proposals about the structure of language, and made important proposals regarding how the success of grammatical theories should be evaluated.
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