We need a compact and general way to describe language:
How can a finite grammar and parser describe an infinite variety of possible sentences?
Unfortunately, this is not easy to achieve.
But the English ... having such varieties of incertitudes, changes, and Idioms, it cannot be in the compas of human brain to compile an exact regular Syntaxis thereof.[James Howell, A New English Grammar, Prescribing as certain Rules as the Language will bear, for Forreners to learn English, London, 1662.]