There is no causal dependence between Suzy's throw and the shattering of the bottle, because even if Suzy had not thrown her rock, the bottle would have shattered due to Billy's throw.

Lewis’s theory cannot explain the judgement that Suzy’s throw caused the shattering of the bottle. For there is no causal dependence between Suzy’s throw and the shattering, since even if Suzy had not thrown her rock, the bottle would have shattered due to Billy’s throw. Nor is there a chain of stepwise dependences running cause to effect, because there is no event intermediate between Suzy’s throw and the shattering that links them up into a chain of dependences. Take, for instance, Suzy’s rock