 For example, consider ‘Why do birds in the northern hemisphere go south for the winter, whereas mammals and reptiles do not?’ In this case, \(P\) is the proposition that birds in the northern hemisphere go south for the winter, and \(X\) is the set containing \(P\) along with the proposition that mammals in the northern hemisphere go south for the winter and the proposition that reptiles in the northern hemisphere go south for the winter. The contrast class parameter allows one to distinguish different why-questions that have the same topic. Thus one can ask why northern hemisphere birds (rather than mammals or reptiles) go south for the winter, and this is different from asking why northern hemisphere birds go south (rather than north or west) for the winter. Until one specifies a contrast class, van Fraassen argues, a particular why-question has not been identified or posed. Like van Fraassen, Garfinkel (1981) advances a view on which explanatory contrast takes center stage, but we will focus here on the details of van Fraassen’s account. See Temple 1988 for a comparison of van Fraassen’s and Garfinkel’s respective treatments of explanatory contrast.