- Do you want to do it again?
- Presupposition: that you have done it already, at least once.
- Jane no longer writes fiction.
- Presupposition: that Jane once wrote fiction.
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 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:
- 1. Both the sentence and its negation are false; or
- 2. 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.
- 1. "There exists exactly one person, who is my wife and who is not pregnant"
- 2. "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. We've seen that 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.
- 1. John thinks that my wife is beautiful.
- 2. 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 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.
- 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.