Requests for explanation advanced in the form of why-questions contain presuppositions. Most obviously,

1. (1) Why is it the case that P?

presupposes that the proposition P is true. In addition, when it is recognised that explanation is a contrastive activity (Drestske 1973, van Fraassen 1980, Garfinkel 1981), an additional source of explanatory presupposition becomes evident. Suppose we seek to answer (1) by showing why P, rather than some contrasting alternative C, is true. The resulting question

1. (2) Why is it the case that P rather than C?

presupposes that P and not-C are both true.

Why-questions with the structure given in (2) make explicit an often tacit relational element in the activity of explaining a proposition. A given proposition may be embedded in different sets of contrasting alternatives, thereby giving rise to different explanatory problems. We may, to use Garfinkel's (1981) example, provide different readings of the question ‘why did Willi Sutton rob banks?’ One device for bringing out these alternative formulations is in the use of emphasis (either via spoken stress or written italics/underlining). We may wish to explain why Sutton robbed banks, why he robbed banks, or why he (Sutton) robbed banks. These alternatives may be fleshed out in the form of three different questions conforming to pattern (2):

1. (3a) Why did Sutton rob banks rather than deposit money into them?

2. (3b) Why did Sutton rob banks rather than candy stores?

3. (3c) Why did Sutton, rather than one of his accomplices, rob banks?

Besides the use of emphasis and formulations conforming to pattern (2), there is another way of posing explanatory questions in which the presuppositions are made evident.