When we use a referring expression like this, he or Shakespeare, we usually assume that our listeners can recognize which referent is intended. In a more general way, we design our linguistic messages on the basis of large-scale assumptions about what our listeners already know. Some of these assumptions may be mistaken, of course, but mostly they’re appropriate. What a speaker (or writer) assumes is true or known by a listener (or reader) can be described as a presupposition.

If someone tells you Your brother is waiting outside, there is an obvious presupposition that you have a brother. If you are asked Why did you arrive late?, there is a presupposition that you did arrive late. And if you are asked the question When did you stop smoking?, there are at least two presuppositions involved. In asking this question, the speaker presupposes that you used to smoke and that you no longer do so. Questions like this, with built-in presuppositions, are very useful devices for interrogators or trial lawyers. If the defendant is asked by the prosecutor, Okay, Mr. Buckingham, how fast were you going when you ran the red light?, there is a presupposition that Mr. Buckingham did in fact run the red light. If he simply answers the How fast part of the question, by giving a speed, he is behaving as if the presupposition is correct.

One of the tests used to check for the presuppositions underlying sentences involves negating a sentence with a particular presupposition and checking if the presupposition remains true. Whether you say My car is a wreck or the negative version My car is not a wreck, the underlying presupposition (I have a car) remains true despite the fact that the two sentences have opposite meanings. This is called the “constancy under negation” test for identifying a presupposition. If someone says, I used to regret marrying him, but I don’t regret marrying him now, the presupposition (I married him) remains constant even though the verb regret changes from affirmative to negative.