 Van Fraassen begins his chapter on explanation with a brief story that provides a good point of entry into how he intends his account to work. Recall from  Section 2.5  that a well-known counterexample to the DN model involves the claim that one can explain the length S of the shadow cast by a flagpole in terms of the height H of the flagpole but that (supposedly) one cannot explain H in terms of S, despite the fact that one can construct a DN derivation from S to H. This is commonly taken to show that the DN model has left out some factor having to do with the directional or asymmetric features of explanation—e.g., perhaps an asymmetry in the relation between cause and effect that ought to be incorporated into one’s model of explanation. In van Fraassen’s story, a straightforward causal explanation of the usual sort of S in terms of H (although the object in question is a tower rather than a flagpole) is first offered. Then a second explanation, according to which the height of the tower is “explained” by the fact that it was designed to cast a shadow of a certain length is advanced. Presumably the moral we are to draw is that as the context and perhaps the relevance relation R are varied, both