 For van Fraassen, a why-question \(Q\) can be identified with a triple \(\langle P,X,R\rangle\), where \(P\) is a true proposition (the topic of the question); \(X\) is a set of propositions to which \(P\) belongs and of which \(P\) is the only member that is true (the contrast class of \(Q)\); and \(R\) is a contextually determined relation of explanatory relevance, which holds between a proposition and the topic/contrast-class pair \(\langle P,X\rangle\). The standard linguistic expression of \(Q\) is: