The core problem of human language ... I take to be this: having mastered a language, one is able to understand an indefinite number of expressions that are new to one's experience, they bear no simple physical resemblance and are in no simple way analogous to the expressions that constitute one's linguistic experience; and one is able, with greater or less facility, to produce such expressions on an appropriate occasion, despite their novelty, and independently of detecting stimulus configurations, and to be understood by others who share this mysterious ability.