In linguistics and philosophy, a presupposition is an implicit assumption about the world or background belief relating to an utterance whose truth is taken for granted in discourse.
This may be required for the utterance to be considered appropriate in context, but it is not uncommon for new information to be encoded in presuppositions without disrupting the flow of conversation (see accommodation below).
John might be mistaken about his belief that I have a wife, or he might be deliberately trying to misinform his audience, and this has an effect on the meaning of the second sentence, but, perhaps surprisingly, not on the first one.
A significant amount of current work in semantics and pragmatics is devoted to a proper understanding of when and how presuppositions project.
[4] The following is a selection of presuppositional triggers following Stephen C. Levinson's classic textbook on Pragmatics, which in turn draws on a list produced by Lauri Karttunen.
The description is said to be proper when the phrase applies to exactly one object, and conversely, it is said to be improper when either there exist more than one potential referents, as in "the senator from Ohio", or none at all, as in "the king of France".
In conventional speech, definite descriptions are implicitly assumed to be proper, hence such phrases trigger the presupposition that the referent is unique and existent.
A linguistic question thus arises regarding the usage of such phrases: does a person who states "John knows X" implicitly claim the truth of X?
Steven Pinker explored this question in a popular science format in a 2007 book on language and cognition, using a widely publicized example from a speech by a U.S.
[6] A 2003 speech by George W. Bush included the line, "British Intelligence has learned that Saddam Hussein recently sought significant quantities of uranium from Africa.
There is however a strong alternative view that the factivity thesis, the proposition that relational predicates having to do with knowledge, such as knows, learn, remembers, and realized, presuppose the factual truth of their object, is incorrect.
[9] Some further change of state verbs: start; finish; carry on; cease; take (as in X took Y from Z » Y was at/in/with Z); leave; enter; come; go; arrive; etc.
What is needed for the sentence to be felicitous is really that somebody relevant to the interlocutors had dinner in New York last night, and that this has been mentioned in the previous discourse, or that this information can be recovered from it.
[11] Through the analysis of written texts and verbal speech, Teun A. van Dijk (2003) says CDA studies power imbalances existing in both the conversational and political spectrum.
[11] With the purpose of first identifying and then tackling inequality in society, van Dijk describes CDA as a nonconformist piece of work.