 Remember that laying out the foundations of one’s ontology requires two things: to say what, fundamentally, there is; and to say what it is like, presumably by stating some facts about the fundamental entities. But not just any facts matter. For example, it may be true of some of the fundamental entities that they coexist with at least one pig; but saying so does nothing to help articulate the fundamental structure of reality. To do that, Lewis thinks, one needs a distinction among the properties and relations: some are special, in that it is their pattern of instantiation among the fundamental entities that constitutes the fundamental structure of reality—the “joints” along which nature is to be ultimately carved. These special properties and relations are the “perfectly natural” ones.