 Where van Fraassen’s theory places few restrictions on the contrast class or foils of a why-question, some authors have argued that the set of possible foils is constrained in various ways. One example is Sober (1986), who contends that the presuppositions of ‘Why \(P\) rather than \(Q\)?’ include a two-part common cause presupposition, namely that (1) the truth of \(P\) and the falsity of \(Q\) trace back to a common cause, and (2) the common cause discriminates \(P\) from \(Q\) in the sense that it makes \(P\) more likely than \(Q\). Both presuppositions fail for Sober’s (1986, 145) example of the unanswerable question, ‘Why is Kodaly a Hungarian rather than a vegetarian?’ Another author who argues for additional constraints on foils is Lipton (1990), who aims in part to improve on Lewis’s (1986) account of causal explanation. Lipton contends that a causal answer to ‘Why \(P\) rather than \(Q\)?’ must cite a cause of \(P\) and the absence of a corresponding event in the history of \(\neg Q\), i.e., a causal difference between \(P\) and \(\neg Q\). Lipton (1990, 256) calls this The Difference Condition. The central requirement for a sensible contrastive question is that fact and (negated) foil have a similar causal history against which the differences stand out (Lipton 1990, 258). According to Barnes (1994), Lipton is correct that fact and (negated) foil must have a similar causal history, but Barnes goes further and claims that a why-question presupposes that the fact and foil can be viewed as culminating outcomes of some single type of natural causal process (Barnes 1994, 50).