 Some properties are perfectly natural. Other properties are less natural, but not all unnatural properties are alike. Green things are a diverse and heterogeneous bunch, but they are more alike than the grue things are. And the grue things are more alike than some other even more disjunctive bunches. So as well as positing perfectly natural properties, Lewis posits a relation of more and less natural on properties. He suggests that we just need to take the perfectly natural as primitive, and we can define the naturalness of other properties in terms of it. The idea is that the naturalness of a property is a function of the complexity of that property’s definition in terms of perfectly natural properties. It isn’t at all obvious that this suggestion will capture the intuitive idea, and Lewis does not defend it at any length.