 It is not enough merely to appeal to such a distinction; for metaphysics to do its job properly, it must also provide an account. Now, one way to proceed would be to provide a theory of what properties and relations are, in which it is stipulated that all such things are to count as “perfectly natural”. On such an approach, while there may well be a property corresponding to the predicate “has mass 5 kg” (for example), there will almost certainly be no property corresponding to the predicate “is green” (let alone that familiar gerrymander, “is grue”). Lewis favors a different approach. Given his commitment to set theory, he already believes in things that, by his lights, deserve to be called the property of being green, and indeed the property of being grue: these are merely certain sets—sets of actual and possible objects. (See the section on Lewis’s modal metaphysics in the entry on  David Lewis,  and the supplement on  The Natural/Non-natural Distinction.)  The question for him, then, is how to distinguish among these sets those that are perfectly natural. Here I will present Almost-Lewis as being, almost like Lewis, agnostic as between four broad alternatives. (Almost, because Lewis eventually decided that the first alternative, according to which natural properties and relations are Aristotelian universals, is unworkable; see his 1986f for the reasons.)