Gricean Maxims

The philosopher Paul Grice proposed four conversational maxims that arise from the pragmatics of natural language. These maxims are:

Maxim of Quality: Truth

Do not say what you believe to be false.
Do not say that for which you lack adequate evidence.
Maxim of Quantity: Information

Make your contribution as informative as is required for the current purposes of the exchange.
Do not make your contribution more informative than is required.
Maxim of Relation: Relevance

Be relevant.
Maxim of Manner: Clarity

Avoid obscurity of expression. ("Eschew obfuscation")
Avoid ambiguity.
Be brief ("avoid unnecessary prolixity").
Be orderly.

http://en.wikipedia.org/wiki/Gricean_maxims