 Subsequent attempts to explain naturalness in terms of resemblance have used richer inputs. For example, Rodriguez-Pereyra  (2002)  defines ‘property class’ in terms of a three-place predicate ‘\(x\) resembles \(y\) to degree \(n\)’. Here \(n\) is, surprisingly, a natural number—the intention is that the degree of resemblance between two objects should turn out to equal the total number of “property classes” that contain  both.[10]  Rodriguez-Pereyra’s conception of degrees of resemblance is unusual, since it conflicts with the intuitive thought that everything resembles itself at least as strongly as anything could resemble anything else. On a more popular approach, the degree of dissimilarity between two objects \(x\) and \(y\) is represented by a non-negative real number \(d(x,y)\) satisfying the following “pseudometric space” axioms: