Language is the human capacity for acquiring and using complex systems of communication, and a language is any example of such a system of complex communication. The scientific study of language is called linguistics. It is impossible to know precisely how many languages there are in the world, and the number depends on a partly arbitrary distinction between languages and dialects. However, estimates vary between around 6,000 and 7,000 languages in number. 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. 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.