Cooperation and Implicature
It is generally assumed that speakers and listeners involved in a conversation are cooperating with each other. For instance, in order to accept a speaker's presuppositions, the listeners necessarily need to assume that a speaker who says "my wife", is really married. Thus, people having a conversation are generally assumed to being honest and conveying relevant information.
Source of the picture: celebritysandwich.com
Consider the following scene: In the cafeteria of a university, one student asks another how she likes the sandwich she just started eating. The addressed student replies:
Merely looking at the sentence from a logical perspective reveals that it does not have a communicative value since it expresses a tautology (like "new innovation", "free gift"). Yet, when used in conversation we assume that the speaker intends to express more than is actually said. Thus, the student who received the tautologous answer has to assume that her fellow student is being cooperative and intends to communicate something and then needs to work out the additional conveyed meaning, called implicature.
Exercise: Note a number of possible implicatures in this context.
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Implicatures are one of the primary examples that more is being communicated than is said. However, in order to interpret implicatures, like in the example above, we must assume that some basic cooperative principles are in operation.