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 la