There is a well-known argument against irreducibly normative properties that appeals to the following claim about supervenience: for all possible worlds W and W*, if the instantiation of descriptive properties in W and W* is exactly the same, then the instantiation of normative properties in W and W* is also exactly the same. This claim used to be uncontroversial, but recently several philosophers have challenged it. Do these challenges undermine this argument? I argue that they do not, since the negation (...) of this claim about supervenience has consequences that are much more implausible than the negations of key premises in these challenges. (shrink)