Presupposition

In the branch of linguistics known as pragmatics, a presupposition (or PSP) is an implicit assumption about the world or background belief relating to an utterance whose truth is taken for granted in discourse. Examples of presuppositions include:
A presupposition must be mutually known or assumed by the speaker and addressee for the utterance to be considered appropriate in context. It will generally remain a necessary assumption whether the utterance is placed in the form of an assertion, denial, or question, and can be associated with a specific lexical item or grammatical feature (presupposition trigger) in the utterance.
Crucially, negation of an expression does not change its presuppositions: "I want to do it again" and "I don't want to do it again" both presuppose that the subject has done it already one or more times; "My wife is pregnant" and "My wife is not pregnant" both presuppose that the subject has a wife. In this respect, presupposition is distinguished from entailment and implicature. For example, "The president was assassinated" entails that "The president is dead", but if the expression is negated, the entailment is not necessarily true.
Negation of a sentence containing a presupposition.
If presuppositions of a sentence are not consistent with the actual state of affairs, then one of two approaches can be taken. Given the sentences "My wife is pregnant" and "My wife is not pregnant" when one has no wife, then either:
Bertrand Russell tries to solve this dilemma with two interpretations of the negated sentence:
For the first phrase, Russell would claim that it is false, whereas the second would be true according to him.
Projection of presuppositions.
A presupposition of a part of an utterance is sometimes also a presupposition of the whole utterance, and sometimes not. For instance, the phrase "my wife" triggers the presupposition that I have a wife. The first sentence below carries that presupposition, even though the phrase occurs inside an embedded clause. In the second sentence, however, it does not. 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.
Thus, this seems to be a property of the main verbs of the sentences, "think" and "say", respectively. After work by Lauri Karttunen, verbs that allow presuppositions to "pass up" to the whole sentence ("project") are called holes, and verbs that block such passing up, or "projection" of presuppositions are called plugs. Some linguistic environments are intermediate between plugs and holes: They block some presuppositions and allow others to project. These are called filters. An example of such an environment are indicative conditionals ("If-then" clauses). A conditional sentence contains an "antecedent" and a "consequent". The antecedent is the part preceded by the word "if," and the consequent is the part that is (or could be) preceded by "then." If the consequent contains a presupposition trigger, and the triggered presupposition is explicitly stated in the antecedent of the conditional, then the presupposition is blocked. Otherwise, it is allowed to project up to the entire conditional. Here is an example:
Here, the presupposition triggered by the expression "my wife" (that I have a wife) is blocked, because it is stated in the antecedent of the conditional: That sentence doesn't imply that I have a wife. In the following example, it is not stated in the antecedent, so it is allowed to project, i.e. the sentence "does" imply that I have a wife.
Hence, conditional sentences act as "filters" for presuppositions that are triggered by expressions in their consequent.
A significant amount of current work in semantics and pragmatics is devoted to a proper understanding of when and how presuppositions project.
Presupposition triggers.
A presupposition trigger is a lexical item or linguistic construction which is responsible for the presupposition, and thus "triggers" it. 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. As is customary, the presuppositional triggers themselves are italicized, and the symbol » stands for 'presupposes'.
Definite descriptions.
Definite descriptions are phrases of the form "the X" where X is a noun phrase. 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.
Factive verbs.
In Western epistemology, there is a tradition originating with Plato of defining knowledge as justified true belief. On this definition, for someone to know X, it is required that X be true. 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. president. 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." Over the next few years, it became apparent that this intelligence lead was incorrect. But the way the speech was phrased, using a factive verb, implicitly framed the lead as truth rather than hypothesis. There is however a strong alternative view that "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.
Some further factive predicates: "know; be sorry that; be proud that; be indifferent that; be glad that; be sad that."
Implicative verbs.
Some further implicative predicates: "X happened to V"»X didn't plan or intend to "V"; "X avoided Ving"»X was expected to, or usually did, or ought to "V", etc.
Change of state or continuation of state verbs.
With these presupposition triggers, the current unfolding situation is considered presupposed information.
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.
Iteratives.
These types of triggers presuppose the existence of a previous state of affairs.
Further iteratives: "another time; to come back; restore; repeat; for the nth time."
Temporal clauses.
The situation explained in a clause that begins with a temporal clause constructor is typically considered backgrounded information.
Further temporal clause constructors: "after; during; whenever; as" (as in "As John was getting up, he slipped").
Cleft sentences.
Cleft sentence structures highlight particular aspects of a sentence and consider the surrounding information to be backgrounded knowledge. These sentences are typically not spoken to strangers, but rather to addressees who are aware of the ongoing situation.
Comparisons and contrasts.
Comparisons and contrasts may be marked by stress (or by other prosodic means), by particles like "too", or by comparatives constructions.
Questions.
Questions often presuppose what the assertive part of the question presupposes, but interrogative parts might introduce further presuppositions. There are three different types of questions: yes/no questions, alternative questions and WH-questions.
Accommodation of presuppositions.
A presupposition of a sentence must normally be part of the common ground of the utterance context (the shared knowledge of the interlocutors) in order for the sentence to be felicitous. Sometimes, however, sentences may carry presuppositions that are not part of the common ground and nevertheless be felicitous. For example, I can, upon being introduced to someone, out of the blue explain that "my wife is a dentist," this without my addressee having ever heard, or having any reason to believe that I have a wife. In order to be able to interpret my utterance, the addressee must assume that I have a wife. This process of an addressee assuming that a presupposition is true, even in the absence of explicit information that it is, is usually called presupposition accommodation. We have just seen that presupposition triggers like "my wife" (definite descriptions) allow for such accommodation. In "Presupposition and Anaphora: Remarks on the Formulation of the Projection Problem", the philosopher Saul Kripke noted that some presupposition triggers do not seem to permit such accommodation. An example of that is the presupposition trigger too. This word triggers the presupposition that, roughly, something parallel to what is stated has happened. For example, if pronounced with emphasis on "John", the following sentence triggers the presupposition that somebody other than John had dinner in New York last night.
But that presupposition, as stated, is completely trivial, given what we know about New York. Several million people had dinner in New York last night, and that in itself doesn't satisfy the presupposition of the sentence. 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. Presupposition triggers that disallow accommodation are called anaphoric presupposition triggers.
Presupposition in critical discourse analysis.
Critical discourse analysis (CDA) is a broad study belonging to not one research category. It focuses on identifying presuppositions of an abstract nature from varying perspectives. CDA is considered critical, not only in the sense of being analytical, but also in the ideological sense. 
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. With the purpose of first identifying and then tackling inequality in society, van Dijk describes CDA as a nonconformist piece of work. One notable feature of ideological presuppositions researched in CDA is a concept termed synthetic personalisation
Logical construct.
To describe a "presupposition" in the context of propositional calculus and truth-bearers, Belnap defines "A sentence is a "presupposition" of a question if the truth of the sentence is a necessary condition of the question's having some true answer." Then referring to the semantic theory of truth, interpretations are used to formulate a "presupposition": "Every interpretation which makes the question truly answerable is an interpretation which makes the presupposed sentence true as well."
A sentence that "expresses a presupposition" in a question may be characterized as follows: the question has some true answer if and only if the sentence is true.