1 What is a Moral Law? Gideon Rosen 1. The grounding problem in metaethics This paper takes up a problem that arises for anyone who believes in normative facts, by which I mean: facts about the normative properties things possess and the normative relations in which they stand. Everyone who believes in normative facts so conceived also believes this: Whenever a particular thing A possesses a normative property F (or stands in a normative relation R to other things), the fact that A is F is always at least partly grounded in, or explained by, A’s non-normative features. Particular moral facts are never completely inexplicable. If an act is wrong, there is always something to say about why it’s wrong, or what makes it wrong. Of course an act can have one normative feature because it has another. An action can be wrong because it’s cruel. But if we then ask why the act is cruel there will always be something to say, at least in principle. And since this can’t go on for ever (or so it’s natural to suppose), the explanation of any particular normative fact will always advert at some point to non- normative features of the act in question. The main metaphysical challenge for realists abut the normative is to characterize this explanatory connection between the particular normative facts and the non-normative facts that ‘underlie’ them. The most straightforward answer is ethical naturalism, which I take to be the view that every particular normative fact [Fa] is metaphysically grounded without remainder in facts whose constituents are 100% non-normative. 1,2 If Osmond lies 1 I take facts to be structured entities built from objects, properties, relations and so on. For present purposes we can think of them as true structured propositions. I write ‘[Fa]’ for the fact that a is F, a fact whose sole constituents are a and F.