Language is the human ability to acquire and use complex systems of communication, and a language is any specific example of such a system. The scientific study of language is called linguistics. Questions concerning the philosophy of language, such as whether words can represent experience, has been debated since Gorgias and Plato in Ancient Greece. Thinkers such as Rousseau have argued that language originated from emotions while others like Kant have held that it originated from rational and logical thought. 20th-century philosophers such as Wittgenstein argued that philosophy is really the study of language. Major figures in linguistics include Ferdinand de Saussure and Noam Chomsky. Estimates of the number of languages in the world vary between 5,000 and 7,000. However, any precise estimate depends on a partly arbitrary distinction between languages and dialects. Natural languages are spoken or signed, but any language can be encoded into secondary media using auditory, visual, or tactile stimuli – for example, in graphic writing, braille, or whistling. This is because human language is modality-independent. Depending on philosophical perspectives regarding the definition of language and meaning, when used as a general concept, “language” may refer to the cognitive ability to learn and use systems of complex communication, or to describe the set of rules that makes up these systems, or the set of utterances that can be produced from those rules. All languages rely on the process of semiosis to relate signs to particular meanings. Oral and sign languages contain a phonological system that governs how symbols are used to form sequences known as words or morphemes, and a syntactic system that governs how words and morphemes are combined to form phrases and utterances. Human language has the properties of productivity, recursivity, and displacement, and relies entirely on social convention and learning. Its complex structure affords a much wider range of expressions than any known system of animal communication. Language is thought to have originated when early hominins started gradually changing their primate communication systems, acquiring the ability to form a theory of other minds and a shared intentionality. This development is sometimes thought to have coincided with an increase in brain volume, and many linguists see the structures of language as having evolved to serve specific communicative and social functions. Language is processed in many different locations in the human brain, but especially in Broca’s and Wernicke’s areas. Humans acquire language through social interaction in early childhood, and children generally speak fluently when they are approximately three years old. The use of language is deeply entrenched in human culture. Therefore, in addition to its strictly communicative uses, language also has many social and cultural uses, such as signifying group identity, social stratification, as well as social grooming and entertainment. Languages evolve and diversify over time, and the history of their evolution can be reconstructed by comparing modern languages to determine which traits their ancestral languages must have had in order for the later developmental stages to occur. A group of languages that descend from a common ancestor is known as a language family. The Indo-European family is the most widely spoken and includes English, Spanish, Portuguese, Russian, and Hindi; the Sino-Tibetan family, which includes Mandarin Chinese, Cantonese, and many others; the Afro-Asiatic family, which includes Arabic, Amharic, Somali, and Hebrew; the Bantu languages, which include Swahili, Zulu, Shona, and hundreds of other languages spoken throughout Africa; and the Malayo-Polynesian languages, which include Indonesian, Malay, Tagalog, Malagasy, and hundreds of other languages spoken throughout the Pacific. Academic consensus holds that between 50% and 90% of languages spoken at the beginning of the twenty-first century will probably have become extinct by the year 2100.