 Lewis’s (1975) take on human languages results from his attempt to solve an apparent paradox. On the one hand, languages are functions from sentences to meanings, and a theory of meaning for any language will describe such functions. On the other hand, a natural language is a social practice followed by a given population. It seems that languages (simpliciter) and natural languages belong to deeply different categories, raising the puzzle of how the two might be related. One difference is that there are indefinitely many languages, but the great majority are not even potentially natural languages. They could not be learned, acquired, or deployed by humans because they have an infinite lexicon, for example, or infinitely many rules of composition, or allow for an unlimited embedding of subordinate sentences. Another difference is that languages are abstract objects while a natural language appears to be a concrete thing that, by definition, is learned, acquired, and developed by members of a specific human population.