 The argument from counterfactual robustness (Swoyer 1982; Fales 1990, 1993; also see Lange 2004 for discussion). Natural laws are often believed to differ from accidental generalizations by their counterfactual robustness (counterfactual-supporting power). Setting aside some minor complications (see Lewis 1979a), we can describe this feature as follows: if it is a law that all \(F\)s are \(G\), then this generalization would still have been true if there had been more \(F\)s than there actually are, or if some \(F\)s had found themselves in conditions different from the ones that actually obtain. For example, it would still have been true that nothing moves faster than light if there had been more objects than there actually are, or if some bodies had been moving in a different direction. Contrast this with No emerald has ever decorated a royal crown. That may be true, but it is not very robust. It would have been false if some rulers of the past had made different fashion choices. Some necessitarians have argued that contingentism about the laws cannot provide a plausible explanation of the special counterfactual robustness of the laws. Note that a counterfactual “if it had been the case that \(P\), then it would have been the case that \(Q\)” is usually taken to be true if \(Q\) is true in those metaphysically possible \(P\)-worlds that are closest to actuality. On this view, the special counterfactual robustness of the law All \(F\)s are \(G\) amounts, roughly speaking, to this: of all the metaphysically possible worlds that contain some additional \(F\)s, or where some actual \(F\)s are in somewhat different circumstances, the ones where the actual law holds are closer than the rest. If the laws hold in some metaphysically possible worlds but not in others, then the reason why the former are closer than the latter must be that the rules we are using for deciding which worlds count as the closest say so. But which such rules we use is a matter of convention. The counterfactual-supporting power of the laws does not seem to be a purely conventional matter, however. Necessitarianism, the argument continues, offers a better explanation: the laws hold in the closest possible worlds simply because they hold in all metaphysically possible worlds. Conventions don’t come into it. The contingentist may reply that, even though the counterfactual robustness of the laws is grounded in a convention, that convention is not arbitrary. Instead, it may have its rationale in certain features of the laws that make them different from accidental generalizations in an objectively important way (Sidelle 2002), e.g., the fact that laws relate to pervasive and conspicuous patterns in the history of the world, or the fact that the truth of a law (unlike the truth of an accidental generalization) is explanatorily independent of matters of particular fact (Kment 2015b, 11–12).