 Lewis, we saw, thinks a language is an arbitrary pairing of sentences and meanings. This arbitrariness is overcome through the co-ordination embodied in a conventional practice, an E-language in Chomsky’s terminology. Chomsky does not deny the existence of some linguistic arbitrariness, manifest in linguistic diversity but he holds that this diversity is limited by human psychology (1995a: 15f). On a “learning” model of language development, which Chomsky rejects, infants are little scientists observing and inferring the linguistic conventions prevailing among adult users (e.g., to be truthful and trusting in some L). But these learners could not possibly rule out all but one of the languages that are logically compatible with the data. Even if the child was exceptionally smart, the evidence available in the typical linguistic environment is insufficiently rich—there is a “poverty of stimulus”. If the information in a child’s environment is too poor to account for developments in her competence, anterior knowledge must be providing her with mental stirrups (see Laurence and Margolis 2001 and the entry on  innateness and language).