 Naturalness comes in degrees. But an important part of Lewis’s picture, also dominant in the subsequent literature, is that it has a maximal degree. Some properties are perfectly natural. They are all equally natural, and no properties are more natural than them. Lewis also calls these “fundamental” properties. And the connotations of the word ‘fundamental’ are apt. According to Lewis, there is a good sense in which one would have exhaustively described the world if one said which things (or \(n\)-tuples of things) instantiate each perfectly natural property, and in which one would have failed to exhaustively describe the world if one merely provided this information for some but not all of the perfectly natural properties. As we will see in  §4.4,  Lewis cashes out these thoughts in modal terms: all truths are necessitated by the truths about the distribution of all perfectly natural properties, but not by the truths about the distribution of only some of the perfectly natural properties. Others, like Sider  (2011),  have explored quite different ways of cashing them out.