Don't give more information than necessary.
Don't say things for which you lack evidence.
Don't say things you believe to be false.
Avoid obscurity of expression.
I'm not certain that's all of them, but it's enough for now. The conversational principles are not exactly rules, they're just tendencies that most speakers will tend to follow. And, in general, they're so universally recognized that when a speaker breaks one intentionally ("flouts it," as semanticists would say), it causes certain predictable implications (or "implicatures"). For example, consider the following conversation:
A: Don't you hate that Mrs. Smith? Isn't she a cheese-reeking old bag?Since B's comment isn't relevant, it breaks one of the conversational principles (which tells speakers to Be Relevant), and implies something completely unstated in the dialogue: that B, for whatever reason, doesn't want to talk about Mrs. Smith.
B: Uh... so, how 'bout those Mets?
The more interesting thing about the conversational principles is that, while speakers (and listeners) usually have no knowledge of them, there are certain cases in which we consider them non-operational. The implicatures usually generated by flouting the principles do not arise if the speaker is judged to be linguistically incompetent (i.e., a child, insane, a non-native speaker, sleep-talking, etc.)
And, by and large, the exclusion of those considered incompetent is useful. But, subtler and more common exclusions occur all the time, based only on the way in which a speaker uses the language. Many such instances have already appeared in our previous posts: teenagers who overuse "like" are deemed unworthy and stupid; individual poems and poets are disregarded because they do not rely on traditional rhyme and meter; bloggers are accused of corrupting the essay form; even "incorrect" spelling is used to discredit arguments.
Each of these critiques share a common feature: they are each based on a distinction between "correct" and "incorrect" uses of language. And I think in each case we (and some of our readers) have argued that, in language, there is only convention, and no correctness. In each case, understandability isn't threatened, but conventional norms are. Why protect the norms if breaking them doesn't interfere with comprehension?
How we judge each other's competence: level of education, normalcy, maturity, right-mindedness, also determines the very rules by which we listen to each other. Why respect the rules of language more than the people who use and change them?