 A natural response to this worry is to propose that the earlier set of states C will be sufficiently coherent to form a natural event, while the later set of states D will be a heterogeneous collection, having nothing in common except for the fact that they all evolved states in C. Such a heterogeneous collection of states would not qualify as an event—see, e.g., Lewis (1986) for discussion of what qualifies as a genuine event. Hence, there is an earlier event that screens off A and B, but no later event that does so. However, the framework of statistical mechanics gives us no principled reason for thinking that in all such cases, the earlier screening off event will be coherent enough to be a proper event, while the later screener will not be. Moreover, Arntzenius (1990) offers a counterexample to one way of making this proposal precise.