Cognitive grammar is a cognitive approach to language developed since 1976 by Ronald Langacker. Langacker develops the central ideas of cognitive grammar in his seminal, two-volume Foundations of cognitive grammar, which became a major departure point for the emerging field of Cognitive Linguistics. Cognitive grammar treats human languages as consisting solely of semantic units, phonological units, and symbolic units (conventional pairings of phonological and semantic units). Like construction grammar (developed by Langacker's student Adele Goldberg), and unlike many mainstream linguistic theories, cognitive grammar extends the notion of symbolic units to the grammar of languages. Langacker further assumes that linguistic structures are motivated by general cognitive processes. In formulating his theory, he makes extensive use of principles of gestalt psychology and draws analogies between linguistic structure and aspects of visual perception.