 Quine is an example of a philosopher who accepts, to some extent, natural classifications into kinds. He holds (1969) that kinds are sets, so he does think that there are entities that are kinds, and to that extent he is a realist. That said, his realism is minimal, since sets are ubiquitous entities. Any coherent classificatory principle will determine a set of things thus classified, whether natural or artificial. Thus the ontology of natural kinds does not of itself explain there being natural kinds. Quine regards the notions of  kind and similarity as “substantially one notion” (1969, 119) but denies that it is straightforward to give a definition of one in terms of the other. He does consider defining relative similarity thus: a is more similar to b than to c iff a and b share more properties than a and c do. But this will work only if properties are less abundant than sets are (since any two things are jointly members of any number of sets). And Quine regards such a notion of property as no clearer than the notion of kind.