 The traditional realism versus nominalism debate is an ontological debate over whether in order to account for properties we need to invoke a class of entity, universals. There is an analogous debate as regards natural kinds, whether in order to account for our natural kind talk and our natural classifications, we need a special sort of entity in our ontology. The natural kind nominalist may accept that there are genuinely natural classifications, but will reject the idea that we should invoke any object as a consequence. For example, the nominalist might take the world to be made up of individuals, which can be classified as kinds in a natural way. But, the nominalist claims, there is no kind of entity beyond the individual instances of each kind. The (strong) realist maintains that we cannot explain the distinction between natural and non-natural classifications without appealing to certain entities, the natural kinds. The naturalism versus conventionalism debate discussed in the preceding section asked whether there is a genuine metaphysical difference between natural classifications and non-natural classifications; it is not obvious that the naturalist, who maintains that there is a metaphysical difference, is obliged to understand that difference in terms of an ontological category of entities that are natural kinds. The analogue in the debate over properties is whether there is a metaphysical distinction between natural properties and non-natural properties, between, in David Lewis’s terminology, sparse and abundant properties (Lewis, 1986). There is no immediate contradiction in the position of a nominalist who rejects universals, but who maintains that there is a metaphysical difference between natural properties and other properties. The realist in both cases argues that, ultimately, naturalism (about properties and about classification) requires a commitment to entities (universals and natural kinds respectively).