Universal Grammar (UG) is a theory in linguistics, usually credited to Noam Chomsky, proposing that the ability to learn grammar is hard-wired into the brain. It is sometimes known as 'mental grammar', and as opposed to other 'grammars', e.g. prescriptive, descriptive and pedagogical. The theory suggests that linguistic ability manifests itself without being taught (see the poverty of the stimulus argument), and that there are properties that all natural human languages share. It is a matter of observation and experimentation to determine precisely what abilities are innate and what properties are shared by all languages.