Knowledge of Language
The Competence-Performance Distinction
In his Aspects of the Theory of Syntax (1965) Noam Chomsky proposed to seperate two aspects of language: competence and performance. The former refers to the knowledge an idealized speech community has about a language, i.e. the system of rules and principles that underlies the use of language; the latter, on the other hand, is the use of that knowledge of language. Chomsky held that linguistic theory describes competence:
There are a number of things that are noteworthy: First, linguistics in this sense is concerned with an idealized model of the grammar a speaker has in his/her head. In science, idealizations are very common. In physics, for example, models of gravity abstract away from the actual falling of an apple to the ground (which is influenced by all kinds of side factors irrelevant to the theory), and focus on the laws behind this phenomenon. Similarly, linguistics is concerned with the "laws behind" the concrete use of language in a particular situation. Secondly, the speech community is homogenous - and this is an idealization as well. Strictly speaking, there are as many different languages as there are humans. Again, linguistics abstracts away from many of these differences and presumes that a speech community shares many important features of a languages (the speech communication is "homogenous"). So linguists make a choice in saying that there are differences between speakers that can be abstracted away from since they don't reveal the "laws behind" the specific use of language. Thirdly, actual utterances of speakers are assumed to have an underlying system which consists of rules and general principles. The system constitutes our knowledge of language (competence), and the actual utterances are the way we make use of that knowledge (performance). So, for example, the syntactic category Verb of the word run is abstract. It is not something that we hear or see, but it is something that linguists investigate and that speakers have knowledge of when they use the word in a particular context. The same goes, of course, for more complex and intricate rules and principles.
The General Ideas of early Generative Grammar
There is an excellent Chomsky-interview comprising 5 parts which was broadcasted on BBC. Although it is old, it is a very interesting overview of some of the ideas of the generative enterprise.