In spite of contemporary theoretical disagreement on the nature of language, there is a widespread informal agreement about what linguistic facts are. This article argues that a functional approach to language can provide the foundation for an explicit account of what the informal consensus implies. The account bridges the ‘internalist’ and the ‘externalist’ views of language by understanding mental constructs such as those involved in human languages as aspects of a dynamic social equilibrium. As in evolutionary biology, processes of selection can work to stabilize certain features while others vary; and this process (in which ‘function’ plays a key role) is a necessary supplement to a purely mentalist account. This position is seen as a development of the position of Searle on social facts and Keller on language change.