Grice's Cooperative Principle and the Conversational Maxims (1975)

Maxims of quantity

1. Make your contribution as informative as required.
2. Do not make your contribution more informative than is required.

Maxims of quality

1. Do not say what your believe to be false.
2. Do not say that for which you lack adequate evidence.

Maxim of relation

1. Be relevant.

Maxims of manner

1. Avoid obscurity of expression.
2. Avoid ambiguity.
3. Be brief.
4. Be orderly.

But not all discourse is `Gricean':

deceit, long-windedness, irrelevance, obscurity, taciturnity are all, for good or ill, part of natural communication ...