Linguists differentiate between descriptive and explanatory theories of language. Descriptive theories describe the structures of a language, avoiding comparisons to other languages. Explanatory theories aim to explain the phenomena and internal structures of a language as well as how a language evolved. While the distinction is not widely recognized, generative grammar aims to be an explanatory theory. Chomsky also views that a single theory can simultaneously serve as both a descriptive and explanatory theory by explaining innate linguistic knowledge.