Trans-world causation?

According to Lewis, causal claims must be analysed in terms of counterfactual conditionals, and these in turn are understood in terms of relations of comparative similarity among single concrete possible worlds. Lewis also claims that there is no trans-world causation because there is no way to make sense of trans-world counterfactuals without automatically making them come out to be false. In this paper I argue against this claim. I show how to make sense of trans-world counterfactuals in a non-trivial way that can make them come out to be true, by appealing to relations of comparative similarity among concrete possible worlds (i.e., assuming modal realism). I argue that either merely making such sense of a relevant counterfactual is not enough to have causation, or that Lewis’ modal realism must be given up.