Linguistics is the scientific study of language.There are three aspects to this study: language form, language meaning, and language in context. The earliest activities in the description of language have been attributed to Pāṇini (fl. 4th century BCE), with his analysis of Sanskrit in Ashtadhyayi. In linguistics, human language is a system of sounds, symbols, and meaning. Phonetics is the study of acoustic, visual, and articulatory properties in the production and perception of speech and non-speech sounds. The study of language meaning, on the other hand, deals with how languages encode relations between entities, properties, and other aspects of the world to convey, process, and assign meaning, as well as to manage and resolve ambiguity. While the study of semantics typically concerns itself with truth conditions, pragmatics deals with how context influences meanings. Grammar is a system of rules which govern the language of a particular speech community. It encompasses of sound, meaning, and gestures, and includes phonology (how sounds and gestures function together), morphology (the formation and composition of words), and syntax (the formation and composition of phrases and sentences from words). In the early 20th century Ferdinand de Saussure distinguished between the notions of langue and parole in his formulation of structural linguistics. According to him, parole is the specific utterance of speech, whereas langue refers to an abstract phenomenon that theoretically defines the principles and system of rules that govern a language. This distinction resembles the one made by Noam Chomsky between competence and performance, where competence is individual’s ideal knowledge of a language, while performance is the specific way in which it is used. In classical Indian philosophy of language, the Sanskrit philosophers like Patanjali and Katyayana had distinguished between sphota (light) and dhvani (sound). In the late 20th century, French philosopher Jacques Derrida distinguished between the notions of speech and writing. The formal study of language has also led to the growth of fields like psycholinguistics, which explores the representation and function of language in the mind; neurolinguistics, which studies language processing in the brain; and language acquisition, which investigates how children and adults acquire a particular language. Linguistics also includes the study of other aspects like the influence of social, cultural, historical and political factors on language. The study of such cultural discourses and dialects is the domain of sociolinguistics, which looks at the relation between linguistic variation and social structures, as well as that of discourse analysis, which examines the structure of texts and conversations. Research on language through historical and evolutionary linguistics focuses on how languages change, and on the origin and growth of languages, particularly over an extended period of time. Corpus linguistics takes naturally occurring texts or films (in signed languages) as its primary object of analysis, and studies the variation of grammatical and other features based on such corpora. Stylistics involves the study of patterns of style: within written, signed, or spoken discourse. Language documentation combines anthropological inquiry with linguistic inquiry to describe languages and their grammars. Lexicography covers the study and construction of dictionaries. Computational linguistics applies computer technology to address questions in theoretical linguistics, as well as to create applications for use in parsing, data retrieval, machine translation, and other areas. People can apply actual knowledge of a language in translation and interpreting, as well as in language education – the teaching of a second or foreign language. Policy makers work with governments to implement new plans in education and teaching which are based on linguistic research. Areas of study related to linguistics include semiotics (the study of signs and symbols both within language and without), literary criticism, translation, and speech-language pathology.