Peter Simons - The Importance of Relations in Delimiting Natural Classes

Folk taxonomies, those operative in natural language, and scientific taxonomies, those which are products of mature science, group individuals into classes which are far from the arbitrary collections admitted in set theory. Traditional classification theory stressed the role of properties, especially essential properties, in defining these natural classes. This paper urges greater recognition of the crucial importance of relations in delimiting natural classes. Far from being ontologically posterior to properties, relations are frequently prior to properties or to natural collections in that the delimitative identity of these collections depends on the obtaining of certain relations among the members of these classes. Class delimitation as an ontological affair is distinguished from the semantico-epistemological discrimination of classes. Examples are given where natural classes are delimited non-trivially by relations. Many nominalist or other parsimonious accounts of natural classes rely on a relation of similarity as the delimiting factor. This is denied. Not only is there no ontological relation of similarity, whether overall or aspect-specific; similarity is incapable of playing the delimitative role, even when (as in bacteriology or astronomy) individuals are classified according to phenotypical features. In such cases not similarity but other relations are playing the demilimiting role. Examination of real-life cases of natural collective particulars – dialects, species and higher biological taxa, and other historical particulars – shows that the logicians’ ideal of delimitation by a single equivalence relation is far removed from ontological reality. An additional complication is the vagueness inherent in most real taxonomies. The ontologist must learn to live with this inelegant situation, for that is how the world organizes itself.