Universal grammar (UG), in modern linguistics, is the theory of the innate biological component of the language faculty, usually credited to Noam Chomsky. The basic postulate of UG is that there are innate constraints on what the grammar of a possible human language could be. When linguistic stimuli are received in the course of language acquisition, children then adopt specific syntactic rules that conform to UG. The advocates of this theory emphasize and partially rely on the poverty of the stimulus (POS) argument and the existence of some universal properties of natural human languages. However, the latter has not been firmly established, as some linguists have argued languages are so diverse that such universality is rare. It is a matter of empirical investigation to determine precisely what properties are universal and what linguistic capacities are innate. Argument The theory of universal grammar proposes that if human beings are brought up under normal conditions (not those of extreme sensory deprivation), then they will always develop language with certain properties (e.g., distinguishing nouns from verbs, or distinguishing function words from content words). The theory proposes that there is an innate, biologically determined language faculty that knows these rules, making it possible for children to learn to speak. This faculty does not know the vocabulary of any particular language (so words and their meanings must be learned), and there remain several parameters which can vary freely among languages (such as whether adjectives come before or after nouns) which must also be learned. Evidence in favor of this idea can be found in studies like Valian (1986), which show that children of surprisingly young ages understand syntactic categories and their distribution before this knowledge shows up in production. As Chomsky puts it, "Evidently, development of language in the individual must involve three factors: genetic endowment, which sets limits on the attainable languages, thereby making language acquisition possible; external data, converted to the experience that selects one or another language within a narrow range; [and] principles not specific to the Faculty of Language." Occasionally, aspects of universal grammar seem describable in terms of general details regarding cognition. For example, if a predisposition to categorize events and objects as different classes of things is part of human cognition, and directly results in nouns and verbs showing up in all languages, it could be assumed that rather than this aspect of universal grammar being specific to language,