 The reasoning about chains helped Lewis sidestep a problem that many thought unavoidable for a counterfactual theory of causation, namely the problem of pre-empting causes. Imagine that Suzy throws a rock, the rock hits a window and the window shatters. Suzy’s throw caused the window to shatter. But there is a backup thrower—Billy. Had Suzy not thrown, Billy would have thrown another rock and broken the window. So the window breaking is not counterfactually dependent on Suzy’s throw. Lewis’s solution was to posit an event of the rock flying towards the window. Had Suzy not thrown, the rock would not have been flying towards the window. And had the rock not been flying towards the window, the window would have not shattered. Lewis’s thought here is that it is Suzy’s throwing that causes Billy to not throw; once she has thrown Billy is out of the picture and the window’s shattering depends only on what Suzy’s rock does. So we avoid this problem of pre-empters.