  Part of this sharpening is the result of Chomsky’s important methodological distinction between competence and performance. Chomsky argued that a scientific approach to language needed to focus on the specific mental representations that underlie linguistic behavior (‘linguistic competence’), and not on the behavior itself (‘linguistic performance’). Linguistic performance, he argued, is scientifically intractable, because it is the result of too many idiosyncratic interacting factors. We would do better to take on the much more circumscribed question: what is the system of rules (the grammar) that generates all the allowable sentences? It soon became clear that even if we set aside the performance systems involved in real linguistic behavior, the rules of the grammar were themselves very complicated, often unintuitive, and abstract, in that they involved categories and constructs that were at a significant remove from the data. The idea that children could simply ‘pick up’ these rules by attending to what is associated with what in their language environment was just not plausible (but we will see in section 3 that this claim continues to be challenged). Yet every normal child does in fact learn a language, and so does somehow master these rules. So either the general learning system that the child wields is somehow more powerful than the Associationist-Empiricist had assumed, or the Nativist is right and there is some innate language-specific information that ‘greases the wheels’ of language acquisition. To resist the Nativist conclusion, the Empiricist has to return to the drawing board to develop a more powerful general learning theory. Chomsky developed the Nativist position and termed the innate information ‘universal grammar’ or ‘linguistic theory’. This is the essence of Chomsky’s famous Poverty of the Stimulus argument, which in an important way provided a measure of the challenge that Empiricism faces. The Empiricist-Nativist debate was no longer ‘you-say-experience-I-say-innate’ affair; it looked to many to be a matter of ‘put up or shut up’, and the burden was on the Empiricist to do the putting up.