The processing of utterances in human language (natural language as opposed to programming language) in order to extract meaning and respond appropriately. The main natural language studied has been written English, although there has also been work on other languages and on speech (see speech understanding). The processing requires both syntactic knowledge about the language concerned and semantic knowledge of the relationship between the utterance and what it means, usually in a knowledge base containing an internal representation of the world. Grammatical and semantic rules are used to analyze the utterance into logical formulae or semantic networks, where the meaning representation can be used by a reasoning system. See also discourse understanding, natural-language processing.