One of the two guiding principles of cognitive approaches to grammar. The usage-based thesis holds that the mental grammar of the language user (his or her knowledge of language) is formed by the abstraction of symbolic units from situated instances of language use: an utterance. An important consequence of adopting the usage-based thesis is that there is no principled distinction between knowledge of language and use of language (competence and performance in generative grammar terms), since knowledge of language is knowledge of how language is used. The usage-based thesis is central not just to cognitive approaches to grammar but approaches to both language change and language acquisition which take a cognitive linguistic perspective, as developed, for instance, by Michael Tomasello and William Croft.