 Lewis addressed this issue in “Redundant Causation”, one of the six postscripts to the reprinting of “Causation” in (1986b). He started by introducing a new concept: quasi-dependence. B quasi-depends on A if and only if there is a process starting with A*, and ending with B* , and B* counterfactually depends on A*, and the process from A* to B* is an intrinsic duplicate of the process from A to B, and the laws governing the process from A* to B* (i.e. the laws of the world in which A* and B* happen) are the same as the laws governing the process from A to B. In short, quasi-dependence is the relation you get if you start with dependence, then add all of the duplicates of dependent processes. Causation is then the ancestral of quasi-dependence. Although the window’s shattering does not depend on Suzy’s throw, it does quasi-depend on it. That’s because there is a world, with the same laws, with a duplicate of Suzy’s throw, but Billy determined not to throw, and in that world the window shatters in just the same way, and depends on Suzy’s throw.