Modern approaches began to develop in the 18th century when the classical discipline of rhetoric was gradually removed. During the 19th century linguistics came to be regarded as belonging to either psychology or biology, and such views remain the foundation of today's mainstream Anglo-American linguistics. They were however contested in the early 20th century by Ferdinand de Saussure who established linguistics as an autonomous discipline within social sciences. Following Saussure's concept, general linguistics consists of the study of language as a semiotic system which includes the subfields of phonology, morphology, syntax and semantics. The linguist's approach to these can be synchronic or diachronic.