When we study human language, we are approaching what some might call the 'human essence,' the distinctive qualities of mind that are, so far as we know, unique to man and that are inseparable from any critical phase of human existence, personal or social. Hence the fascination of this study, and, no less, its frustration. The frustration arises from the coming to grips with the core problem of human language, which 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, that bear no simply physical resemblance and are in no simple way analogous to the expressions that constitute one's linguistic experience; and one is able ... to produce such expressions on an appropriate occasion, despite their novelty.... The normal use of language is, in this sense, a creative activity. This creative aspect of normal language use is one fundamental factor that distinguishes human language from any known system of animal communication.