 A why-question, according to van Fraassen (1980, 144–145) presupposes (i) that its topic is true, (ii) that, in its contrast class, only its topic is true, and (iii) that at least one proposition bearing the explanatory relevance relation to the topic/contrast-class pair is true. When the first or second presupposition fails (because the contextually determined body of background knowledge in play does not entail both (i) and (ii)), the why-question does not arise. When the third presupposition fails, the why-question has no answer even if it arises. For example, suppose that paresis indeterministically strikes some people who have untreated syphilis. Then, if ten people have untreated syphilis, and exactly one of them, John, goes on to contract paresis, there may be no answer to the question ‘Why did John, in contrast to the other nine, contract paresis?’ Since paresis develops indeterministically from syphilis, nothing favors John (in contrast to the other nine syphilis patients) as being likely to develop paresis. On the other hand, if Bill and Sarah never had syphilis, the question ‘Why did John, in contrast to Bill and Sarah, develop paresis?’ does have an answer: ‘John developed paresis, in contrast to Bill and Sarah, because John had syphilis but Bill and Sarah did not.’ In this case, as in the first, the why-question requests causal factors that led to John’s getting paresis while the others mentioned in the contrast class did not. In both cases, then, the same relevance relation \(R\) is in play because the same kind of information is requested, namely causal factors leading to the truth of the topic in contrast to the other members of the contrast class. If there are no such causal factors, as in the first version of the paresis case, the question is to be rejected. If, as in the second version of the paresis case, there are such factors, so that at least one proposition bears the relevance relation to the topic/contrast-class pair, then a candidate answer \(A\) is evaluated according to three criteria: how acceptable or likely \(A\) is, the degree to which \(A\) favors \(P\) over other members of \(X\), and whether \(A\) is made irrelevant by other answers.