The theory of language structures first proposed in Chomsky’s Syntactic Structures (1957). Just as physics studies the forms of physically possible processes, so linguistics should study the form of possible human languages. This would define the limits of language by delimiting the kinds of processes that can occur in language from those that cannot. The result would be a universal grammar, from which individual languages would derive as, in effect, different ways of doing the same thing. Variations such as vocabulary and principles governing word order would be revealed as different applications of the same underlying rules. Tacit knowledge of this universal grammar is pre-programmed, an innate biological endowment of normal human infants. The argument with which Chomsky supported the claim for such an endowment is known as the argument from the ‘poverty of stimulus’: it is argued that language-learning proceeds so fast in response to such a relatively slender body of ‘data’ that the infant must be credited with an innate propensity to follow the grammar of everybody else. The extent to which this argument treats the infant as a theorist or ‘little linguist’ has been much debated (see also language of thought hypothesis). In general, formal approaches to language in philosophiy have been more interested in the possibility of unravelling concealed semantic structure, rather than in the more purely grammatical problems of linguistics.