 Quine’s conception of natural kinds is liberal. Not all sets are natural kinds, but any set whose members share a natural property are a natural kind. So the set of white objects is at least a candidate natural kind and Quine asserts that positively charged objects form a natural kind. One might wonder whether an up quark, an uranium nucleus, a hydronium ion, a charged water droplet, and a balloon that has been rubbed on someone’s jumper all belong to one kind. The literature on natural kinds (cf. Mill 1884) has tended to adopt a more restricted conception of kinds, although articulating that conception is not straightforward. This question is related to the link that Quine draws between kinds and what it is that confirms induction. If one adopts the narrower conception of kinds, then one will hold that a law such as ‘positively charged objects repel one another’ does not concern kinds at all, but nonetheless may be confirmed by instances of objects that are positively charged repelling one another. So the real answer to “what tends to confirm an induction?” is not “natural kinds and their instances” but is “natural properties and their instances”. A further reason for taking this view is that inductions often concern relations (such as the repulsion between two objects). But we do not suppose that objects that enter into a natural relation form a natural kind, nor that there is a natural kind of ordered sequences of objects satisfying a given natural relation.