 The second assumption, that the perfectly natural properties and relations are sufficient to characterise each possible world, has been rejected by Sider (1995) and Schaffer (2004). Both claim that it is metaphysically possible for there to be endless sequences of more and more natural properties, without any set of perfectly natural properties out of which all the other properties can be defined. In response to this problem, Sider (1993b) has proposed a modification of Lewis’s account which he claims is consistent with there being such endless sequences of more and more natural properties. Sider (2011) and Marshall (2016b) have recently argued in favour of the second assumption, with Sider’s argument appealing to his Humean theory of metaphysical possibility.