 Lewis’s solution turns on a metaphysical posit. Some properties, he says, are more natural than others. The natural properties are those that, to use an ancient phrase, carve nature at the joints. They make for objective resemblance amongst the objects that have them, and objective dissimilarity between things that have them and those that lack them. The natural properties, but not in general the unnatural properties, are relevant to the causal powers of things. Although science is in the business of discovering which natural properties are instantiated, when Lewis talks about natural properties he doesn’t mean properties given a special role by nature. It is not a contingent matter which properties are natural, because it isn’t a contingent matter which properties make for objective similarity.