In his book, Theory of Knowledge, Roderick Chisholm distinguishes very precisely between, amongst others, what he calls acceptable, reasonable and evident propositions. A proposition is acceptable if with-holding it is not more reasonable than believing it; a proposition is reasonable if believing it is more reasonable than with-holding it; and a proposition is evident for a person provided it is reasonable for him, and there is no other proposition such that the latter is more reasonable than the former. The notion of being more reasonable is obviously pivotal, and Chisholm explains what it is for an attitude to be more reasonable than another in the following words: “If the person in question were a rational being, if his concerns were purely intellectual, and if he were to choose between the two attitudes, then he would choose the more reasonable in preference to the less reasonable”. The wording of this conditional suggests that Chisholm does not think a person is necessarily rational, so it is imperative that we ask: When is a person rational?