(vii) Reflexivity

　 Human languages can be used to describe themselves. The language used to talk about language is called metalanguage. When linguists write grammars or lexicographers compile dictionaries they must use metalanguage. When we teach languages as a subject we also have to use it. No evidence exists that suggests that any other species write grammars or compile dictionaries or teach the communication system to outsiders.

　 Linguists have observed other characteristics, but the ones discussed above are more striking, particularly the first two. These are universal features possessed by all human languages. Although some animal communication systems possess, to a very limited degree, one or another of these features except creativity and duality, none is found to have all the features. On this basis linguists tend to conclude that human languages are qualitatively different from animal communication systems. Therefore, the foregoing definition of language seems to be valid.