Presupposition is what the speaker assumes to be the case prior to making an utterance. Entailment, which is not a pragmatic concept, is what logically follows from what is asserted in the utterance. Speakers have presuppositions while sentences, not speakers, have entailments. Take a look at the example below:
Jane’s brother bought two apartments.
This sentence presupposes that Jane exists and that she has a brother. The speaker may also hold the more specific presupposition that she has only a brother and her brother has a lot of money. All these presuppositions are held by the speaker and all of them can be wrong.
In pragmatics entailment is the relationship between two sentences where the truth of one (A) requires the truth of the other (B). For example, the sentence (A) The president was assassinated. entails (B) The president is dead.
The concept of presupposition is often treated as the relationship between two propositions. In the case below, we have a sentence that contains a proposition (p) and another proposition (q), which is easily presupposed by any listener. However, the speaker can produce a sentence by denying the proposition (p), obtaining as a result the same presupposition (q).
Debora’s cat is cute. (p)
Debora has a cat. (q)
When I say that Debora’ s cat is cute, this sentence presupposes that Debora has a cat. In
Debora’ s cat is not cute. (NOT p)
the same thing holds true, that is, it presupposes that she has a cat. This property of presupposition is generally described as constancy under negation. Basically, it means that the presupposition of a statement will remain constant (i.e. still true) even when that statement is negated.
Types of Presupposition
In the analysis of how speakers’ assumptions are typically expressed, presupposition has been associated with the use of a large number of words, phrases and structures. These linguistic forms are considered here as indicators of potential presupposition, which can only become actual presupposition in contexts with speakers. The types of presupposition are:
1-Existential presupposition: it is the assumption of the existence of the entities named by the speaker.
For example, when a speaker says "Tom’s car is new", we can presuppose that Tom exists and that he has a car.
2-Factive presupposition: it is the assumption that something is true due to the presence of some verbs such as "know" and "realize" and of phrases involving glad, for example. Thus, when a speaker says that she didn’t realize someone was ill, we can presuppose that someone is ill. Also, when she says "I’m glad it’s over”, we can presuppose that it’s over.
3-Lexical presupposition: it is the assumption that, in using one word, the speaker can act as if another meaning (word) will be understood. For instance:
Andrew stopped running. (>>He used to run.)
You are late again. (>> You were late before.)
In this case, the use of the expressions "stop" and "again" are taken to presuppose another (unstated) concept.
4-Structural presupposition: it is the assumption associated with the use of certain words and phrases. For example, wh-question in English are conventionally interpreted with the presupposition that the information after the wh-form (e.g. when and where) is already known to be the case.
When did she travel to the USA? ( >> she traveled)
Where did you buy the book? (>> you bought the book)
The listener perceives that the information presented is necessarily true rather than just the presupposition of the person asking the question.
5- Non- factive presupposition: it is an assumption that something is not true. For example, verbs like "dream", "imagine" and "pretend" are used with the presupposition that what follows is not true.
I dreamed that I was rich. (>> I am not rich)
We imagined that we...