In the theory of transformational grammar, sentences have two ways they can be represented: deep and surface structure. Deep structure refers to the underlying meaning of a sentence as it is represented and comprehended in the brain. It serves as a counterpoint to surface structure, which is the actual written or spoken form of the sentence. This concept was created by Noam Chomsky in his 1957 book, Syntactic Structures, which formulated the theory of transformational grammar. According to this theory, humans use transformations, a type of cognitive process, to map structural relationships between sentence referents understood in the linguistic regions of the brain and the actual content of a sentence that is seen or heard.
The concept of deep structure contends that information related to each component of a sentence, such as its subjects and predicates, is codified into abstract pieces inside of the brain. Sentences that are different in terms of their surface structure, such as "The boy kicked the ball," and "The ball was kicked by the boy," can have the same deep structure. The reason for this is because the component pieces for each sentence are related in the same way in the brain, so that humans can understand the sentences as semantically equivalent, even though they are syntactically different. With ambiguous sentences, such as "I have seen driving man," with only one surface structure, multiple structural interpretations can be created by rearranging the component pieces, such as "I have seen a man driving," or "I have seen a man who normally drives."
Deep structure, as described by Chomsky, was subject to certain rules that are innate in the human brain. These include transformational rules for deriving the meaning of the surface structure of a sentence, such as adding an implied object to a sentence: the command "Just drive!" becomes an instruction for "Drive the car," through the rule of addition, for example. Through other transformations, the deep structure of a thought is converted into grammatically correct sentences that can be understood by the listener or reader. These rules, as well as the ability to maintain abstract ideas in the brain, are innate, according to the theory, so people do not have to be taught to encode language in terms of deep structure; it is a process that occurs automatically. Although the concept of structures remains important in linguistics, most linguists no longer believe that deep structure is the only way that humans derive meaning from language.