 Those who take naturalness to correlate with epistemic value might consider trying to define naturalness in value-theoretic terms. McDaniel  (2017b)  attempts something like this, although he appeals to the concept of obligation rather than value. He proposes that for a property to be “fundamental” (i.e. natural) is for there to be a prima facie obligation to form true beliefs about it. One challenge facing views along these lines is that belief is an attitude to propositions, whereas naturalness applies also to properties and relations. McDaniel addresses this challenge using a theory of propositions as structured entities: for being F to be more natural (to a certain degree) than being G is for it to be the case that whenever \(p\) and \(q\) are two true propositions differing only in that being F appears in a position in the structure of \(p\) where being G appears in the structure of \(q\), there is a stronger (by a certain degree) prima facie obligation to believe \(p\) than to believe \(q\). Note that if this scheme worked, it could also used to recover naturalness comparisons for properties and relations from naturalness comparisons for propositions, independent of the connection to obligation. But it is certainly not uncontroversial than this can be done, even setting aside concerns about the consistency of the required fine-grained theory of propositions. One might think that the naturalness of a proposition turns on factors like the diversity of its “constituents” as well as the sum of their individual degrees of naturalness, so that substituting a less natural constituent for a more natural one might sometimes increase the naturalness of a proposition by decreasing its diversity. For example, the proposition that every grue gemstone is surrounded by grue rock is not obviously less natural, or less valuable to believe if true, than the proposition that every grue gemstone is surrounded by green rock.