 The doctrine can be factored into two distinct theses. The first is the thesis that, in John Bigelow’s words, “truth supervenes on being”. That is, all the truths about a world  supervene  on the distribution of perfectly natural properties and relations in that world. The second is the thesis that the perfectly natural properties and relations in this world are  intrinsic properties  of point-sized objects, and spatiotemporal relations. Lewis held that the first of these was necessary and a priori. (See, for instance, “Parts of Classes” (1991a), “Reduction of Mind”, “Truthmaking and Difference-making” (2001d).) The second is contingently true if true at all. Indeed, modern physics suggests that it is not true (Maudlin 2007: Ch. 2). Lewis was aware of this. His aim in defending Humean supervenience was to defend, as he put it, its “tenability” (1986a: xi). We will return at the end of this section to the question of why he might have wanted to do this. For now, we will focus on how he went about this project.