Van Fraassen's pragmatic view of explanation is that an explanation is a particular type of answer to a why-question, i.e., an answer that provides relevant information that "favors" the event to be explained over its alternatives. For van Fraassen, these features are determined by the context in which the why-question is asked.

The Basic Elements Of The Pragmatic View Of Explanation

According to van Fraassen, a why-question consists of (1) a presupposition (Why X), (2) a contrast class (Why X rather than Y, Z, and so on), and (3) an implicitly understood criterion of relevance. Information given in response to a particular why-question constitutes an explanation of the presupposition if the information is relevant and "favors" the presupposition over the alternatives in its contrast class. (Explain and give examples.)

Both the contrast class and the criterion of relevance are contextually determined, based on interests of those involved. Subjective interests define what would count as an explanation in that context, but then it's an objective matter whether that information really favors the presupposition over the alternatives in its contrast class. (Explain and give examples.)

Contrasts Between The Pragmatic And Causal Views Of Explanation

1. Any type of information can be counted as relevant (of course, it's a scientific explanation if only information provided by science counts; however, there might be different kinds of scientific explanation; not any old information will do).
2. Context (interests) determines when something counts as an explanation, vs. when we would find an explanation interesting or salient. (According to Lewis, what makes it an explanation is that it gives information about the causal history leading up to a given event; whether we find that explanatory information interesting or salient is another matter.)
3. Distinction: Pragmatic theory of explanation vs. theory of the pragmatics of explanation. On the pragmatic view, God could never have a "complete" explanation of an event, unless he had interests. (A mere description of the causal history leading up to an event--even a complete one--is not an explanation of any sort according to the pragmatic view.)

On the pragmatic view, asymmetries only exist because of the context; thus, they can be reversed with a change in context. That is what van Fraassen's Tower example is supposed to illustrate. (Recount the Tower example.) Lewis' Objection to the Tower example: What is really doing the explaining is the intention of the Prince, and that's a cause of the flagpole being that particular height. Discuss: Can you think of a story in which the redshift would explain the galaxies moving away? Where human intention is not possible, it seems difficult; this would seem to confirm Lewis' diagnosis of the Tower story.