The terms were first introduced by the linguist Kenneth Pike, who argued that the tools linguists have developed for describing linguistic behaviors could be adapted to the description of any human social behavior. The challenge facing linguists is to describe in a way that will make that language intelligible (in other words, so that someone can learn that language and communicate in it effectively), while also allowing systematic comparisons with other languages. In order to describe a language in a way that facilitates comparisons, linguists have generated a set of signs that represent a total repertoire of sounds used in human languages, through which they can represent any language phonetically. When a phonetic description of a language is combined with a phonemic description, however, linguists learn that in any language there are differences in sounds that make no difference in local meaning; for example, some english speakers pronounce t-a-l-k as "tawk," others as "tahk." In this case, native speakers may be aware of the difference, which for them does not change the meaning of the word but which does manifest a difference in accent, which may reveal information about the speaker's geographic, class, or ethnic origins. In some cases (for example, the difference between the aspirated p in pat, and the unaspirated p in tap) native speakers may not even recognize the difference in sound. Conversely, differences that are unimportant in one language may be crucially important in another language.