 Almost-Lewis’s theses about what fundamental ontology comprises, and how all other facts reduce to facts about it, bears a very close relationship to Lewis’s celebrated thesis of Humean Supervenience (hereafter: “HS”). But they are not the same, and the differences are worth keeping track of. Here is a typical statement of HS (slightly stronger, as we’ll see, than the version Lewis officially endorses): No two possible worlds differ with respect to what is true at them, without differing with respect to the geometrical arrangement of their spacetime points, or with respect to which perfectly natural properties are instantiated at those  points.[6]  (Note that so stated, HS is automatically metaphysically necessary.) Thus, HS is a supervenience claim, logically weaker than Almost-Lewis’s claim of reduction. It is also a claim that—for some good reasons and some bad reasons—Lewis accepts only in a weaker form that is metaphysically contingent. More significantly, it is no part of HS that facts about possible worlds themselves reduce to anything else; whereas both Almost-Lewis and Lewis are explicit in their commitment to this further claim. Having said all this, it will be worth remembering in what follows that Almost-Lewis’s position (which, remember, incorporates Lewis’s modal realism) entails HS. So, any doubts about HS will carry over to Almost-Lewis’s fundamental ontology.