Full Idea
If a counterfactual holds in a possible world, that is presumably because a law holds in that world, which means there could be basic causal laws that lack all instances. But then causal laws cannot be totally supervenient on the history of the universe.
Gist of Idea
In counterfactual worlds there are laws with no instances, so laws aren't supervenient on actuality
Source
Michael Tooley (Causation and Supervenience [2003], 4.1.2)
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.395
A Reaction
A nice argument, which sounds like trouble for Lewis. One could deny that the laws have to hold in the counterfactual worlds, but then we wouldn't be able to conceive them.