 This is the context in which introducing talk of I-languages can be useful, according to Chomsky (1986: 22–3; 2000: 4–5). Language acquisition can be thought of as a development through states of the brain from S0 through intermediate states into a relatively stable mature state, SM. S0 is the initial state common to all humans, idealizing away from individual linguistic impairments and the like. Subsequent states arise through exposure to a particular linguistic environment. Nothing said so far requires that these states be thought of as representational states we could call “knowing a language”. But if we describe being-in-state SL as knowing a language L, knowledge that a linguist might in principle make explicit in a theory of L, language acquisition can be described—usefully—as a matter of children evolving through various stages of knowledge en route to acquiring adult competence. This description is useful because the empiricist/nativist debate can now be couched as a debate over what linguistic information must already be known by someone in S0 if information supplied by the linguistic environment is to culminate in knowledge of the mature language M. Empiricists claim that nothing much is needed, that S0 is a “blank slate” to be filled in using environmental data. Nativists claim that plenty of information must already be provided, in the form of innate knowledge of a language dubbed Universal Grammar (UG) by Chomsky. We each come predisposed to acquire only certain languages, the humanly possible ones that can grow out of UG.