Language is a creation of the mind. This mental capacity makes one human. Linguistic theory aims to understand this mental capacity. The questions that arise are, ‘what is the knowledge of language?’ and ‘how is the mind structured to acquire knowledge of language?’. We cannot directly examine the mind so we need to find the indirect methods to investigate. The best methodology is to examine closely selected sentences and phrases that native speakers of a language judge to be possible, impossible, and marginal.
The speaker has the knowledge of language in his mind and he uses this knowledge to produce or understand the language. What people say can be distorted. For example, people make mistakes when they are afraid, drunk or tired. These mistakes cannot be considered as part of knowledge.