According to David Lewis, the fundamental laws of nature are those propositions that collectively optimize a balance of the twin desiderata of informativeness and simplicity. (You can get maximal informativeness by listing all the facts about the world, at the expense of maximal complexity of description, and you can get maximal simplicity by saying nothing, at the expense of minimal informativeness.) And laws are what follows from the fundamental laws.
But laws are explanatory. While Lewis's laws aren't.
Imagine a finite world w1 which contains a powerful contingent magical being who creates a very large number N of golden boxes, and places a chocolate in each one, to fulfill some aesthetic goal—chocolate goes well with gold. No other explanation of the chocolate content of the boxes exists at w1.
We can ask at w1 why the third golden box contains a chocolate. And the answer is that the magical being put a chocolate in every golden box.
Now imagine a world w2 just like w1 but where the magical being doesn't exist and the boxes and chocolates come into existence ex nihilo. Nothing gets added to w2 that wasn't there in w1. Plainly at w2 there just is no explanation of why the third golden box contains a chocolate.
If the number of boxes N is large enough, the proposition that every box contains a chocolate will be informative enough that it'll be included in the system that maximizes informativeness and simplicity (as N increases, the informativeness of the proposition increases but its simplicity stays constant).
But then if Lewis's account of laws is correct, then at w2 it will be a law that all golden boxes contain chocolates. But laws are explanatory. So at w2 we'll be able to explain why the third golden box contains a chocolate. But we said that at w2 there is no explanation of this!
So the Lewisian account of laws is wrong.
Options: (1) Deny that laws are explanatory. (2) Abandon the Lewisian account of laws. (3) Deny that it is possible to have uncaused chocolates.
I think that (3) by itself won't do, because we can run the argument on a counterpossible. And (1) is unattractive. That leaves (2).
In summary, I think "Lewisian laws" aren't explanatory, and hence aren't laws.