 Within each of the candidate families, one can formulate certain laws that might plausibly be claimed to be metaphysically necessary. For example, it is tempting to suppose that it would be metaphysically (rather than merely nomically) impossible for an object to be both one gram in mass and two grams in mass, or for one object to be both equal in mass to another and twice as massive as it. But if we combine all these judgments with combinatorialism, we will have to deny that any of the families we have considered consists entirely of perfectly natural  properties.[40]  This is not out of the question: one can imagine many ways in which mass-theoretic properties might supervene on perfectly natural properties we do not yet have words for, in a manner consistent with the relevant modal judgments. However, precisely because there are so many ways this might be achieved, this response threatens to lead to a sort of scepticism about our ability to know what the perfectly natural properties are, or how the properties we have words for supervene on them. Combinatorialists thus have reason to be open to the idea that some of the relevant laws might turn out, surprisingly, to be metaphysically contingent (though they may still be nomologcally necessary). 