Full Idea
Lewis claims that most events are over-determined by subsequent states of the world, but not by their history. That is, the future of every event contains many independent traces of its occurrence, with little prior indication that it would happen.
Gist of Idea
There are few traces of an event before it happens, but many afterwards
Source
report of David Lewis (Counterfactual Dependence and Time's Arrow [1979]) by Paul Horwich - Lewis's Programme p.209
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.209
A Reaction
Lewis uses this asymmetry to deduce the direction of causation, and hence the direction of time. Most people (including me, I think) would prefer to use the axiomatic direction of time to deduce directions of causation. Lewis was very wicked.