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. An example is the question Have you talked to Hans? which presupposes Hans exists. A presupposition is information that is linguistically presented as being mutually known or assumed by the speaker and addressee. 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). A presupposition remains mutually known by the speaker and addressee 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:
Both the sentence and its negation are false; or Strawson's approach: Both "my wife is pregnant" and "my wife is not pregnant" use a wrong presupposition (i.e. that there exists a referent which can be described with the noun phrase my wife) and therefore can not be assigned truth values. Bertrand Russell tries to solve this dilemma with two interpretations of the negated sentence:
"There exists exactly one person, who is my wife and who is not pregnant" "There does not exist exactly one person, who is my wife and who is pregnant." 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.
John thinks that my wife is beautiful. John said that my wife is beautiful. 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:
If I have a wife, then my wife is blonde. Here, the presupposition (that I have a wife) triggered by the expression my wife is blonde, 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.
If it's already 4am, then my wife is probably angry. 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 represents 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.
John saw the man with two heads. »there exists a man with two heads.
Factive verbs
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