For a rationalist cognitive scientist, a grammar is a finite mental object, systematically assigning abstract structures to all the well-formed expressions of a language — that is, to each member of a set that, for natural languages (such as Chinese or Italian), is infinite and discrete. Infinite, because every speaker of a language can produce and understand an unlimited number of new grammatical sentences. Discrete, because continuous modification of a sentence to change it into another is impossible. No sentence could be halfway between “It's a good car, but they don't sell it” and “It's a good car, but they don't tell it.”
A grammar capable of generating complex structures for all well-formed sentences of a natural language must have recursive rules, because phrasal constituents can contain other phrasal constituents of the same or higher kinds (“The young doctor's three beautiful sisters” is a noun phrase containing another noun phrase; “The spy who came in from the cold” is a noun phrase containing a sentence). Moreover, structural rules of sentence formation can be applied recursively to embed relative clauses embedding other relative clauses, without limit (as in “This is the cat that killed the rat that ate the malt that lay in the house that Jack built”). Because such grammars are finite, whereas the languages they generate are infinite and contingently shaped by use, it is advantageous, and methodologically cogent, to consider the concept of grammar as primary, and that of language as derived.
