 Many philosophers and legal theorists have been interested in the relation of actual causation. This concerns the assignment of causal responsibility for an event, based on how events actually play out. For example, suppose that Billy and Suzy each throw a rock at a bottle, and that each has a certain probability of hitting and breaking it. As it happens, Suzy’s rock hits the bottle, and Billy’s doesn’t. As things actually happened, we would say that Suzy’s throw caused the bottle to shatter, while Billy’s didn’t. Nonetheless, Billy’s throw increased the probability that the bottle would shatter, and it would be identified as a cause by the theories described in sections  2  and  3.  Billy’s throw had a tendency to shatter the bottle; it was a potential cause of the bottle shattering; it was the sort of thing that generally causes shattering; but it did not actually cause the bottle to shatter. 