H. P. Grice, answered from the texts and cited to the page.
Conversation is, in the relevant cases, a rational cooperative endeavour in which participants are expected to make their contributions such as is required, at the stage at which it occurs, by the accepted purpose or direction of the talk exchange in which they are engaged. This is the Cooperative Principle, and four categories of maxims attend it: Quantity (be as informative as required, but no more), Quality (do not say what you believe false, or that for which you lack adequate evidence), Relation (be relevant), Manner (be perspicuous — brief, orderly, unambiguous).12
A speaker who flouts a maxim while remaining cooperative generates a conversational implicature: what is conveyed beyond what is said, by the fact that this was said in this context. The framework is offered as an analysis of an idealised case — like frictionless pulleys — not as a description of every actual conversation; the abstraction is what gives it explanatory power.345
Echoing Kant, I call these categories Quantity, Quality, Relation, and Manner. The category of Quantity relates to the quantity of information to be provided, and under it fall the following maxims: 1. Make your contribution as informative as is required (for the current purposes of the exchange). 2. Do not make your contribution more informative than is required.Studies in the Way of Words, pp. 26–27
He may opt out from the operation both of the maxim and of the Cooperative Principle; he may say, indicate, or allow it to become plain that he is unwilling to cooperate in the way the maxim requires. He may say, for example, I cannot say more; my lips are sealed. 3.Studies in the Way of Words, pp. 30–31
But while I have said that he is an Englishman, and said that he is brave, I do not want to say that I have said (in the favored sense) that it follows from his being an Englishman that he is brave, though I have certainly indicated, and so implicated, that this is so. I do not want to say that my utterance of this sentence would be, strictly speaking, false should the conse- quence in question fail to hold.Studies in the Way of Words, p. 26
There is some sort of understanding (which may be explicit but which is often tacit) that, other things being equal, the transaction should continue in appropriate style unless both parties are agreeable that it should terminate. You do not just shove off or start doing something else.Studies in the Way of Words, pp. 29–30
While this may be correct, so far as the generation of implicatures is concerned it seems to play a role not totally different from the other maxims, and it will be convenient, for the present at least, to treat it as a member of the list of maxims.Studies in the Way of Words, p. 28
The canonical 20th-century theory of pragmatics and one of the most-cited pieces of philosophy of language in modern linguistics. Sperber and Wilson's Relevance Theory (1986, 1995) proposes a single Principle of Relevance as replacement of the four maxims, on the grounds that the maxims are not independently motivated cognitive principles. Wayne Davis (1998) argues that the framework fails as both descriptive and explanatory account. Keenan (1976) raised the cultural-variability objection from Malagasy speech behaviour. Critics from ethnography (Hymes), critical discourse analysis (Fairclough), and conversation analysis (Sacks, McHoul) charge that the framework idealises conversation and ignores power asymmetries. Defenders: Horn's and Levinson's neo-Gricean reductions are friendly extensions; Brown and Levinson's politeness theory supplements rather than replaces; Robert Harnish, Geoffrey Leech, and Jenny Thomas defend the framework in pragmatics; Stephen Neale defends the said/implicated distinction in philosophy of language.