 Lewis’s account is best seen as an attempt at analysing duplication preservation, rather than identity interiority which we argued in section  2.5  is the dominant notion of intrinsicality in philosophy. Lewis’s account rests heavily on the claim that all perfectly natural properties are intrinsic, and, implicitly, that the perfectly natural properties and relations are sufficient to characterise the world completely. The last assumption is needed because the theory rules out the possibility that there are two things having no proper parts that share all their perfectly natural properties, but differ with respect to some of their intrinsic properties.