 This definition guarantees that causation will be transitive: if C causes D, and D causes E, then C causes E. This modification is useful for addressing certain types of preemption. Consider the example from the previous section, where Suzy throws her rock, preempting Billy. We can interpolate an event D between Suzy’s throw, C, and the bottle’s shattering E. Let D be the presence of Suzy’s rock on its actual trajectory, at some time after Billy has already failed to throw. If Suzy hadn’t thrown, D would have been much less likely. And if D hadn’t occurred, E would have been much less probable. Since D occurs after Billy has already declined to throw, if D hadn’t occurred, there would not have been any rock on a trajectory toward the bottle. Thus there is a chain of causal dependence from C to D to E. 