 Given the definition of causation in terms of causal chains, Lewis is able to distinguish preempting actual causes (such as a) from preempted potential causes (such as b). There is a causal chain running from a to the dictator’s death, but no such chain running from b to the dictator’s death. Take, for example, as an intermediary event occurring between a and the dictator’s death, the bullet from A’s gun speeding through the air in mid-trajectory. The speeding bullet causally depends on a, since that particular bullet would not have been in mid-trajectory had A not taken aim; and the dictator’s death causally depends on the speeding bullet, since by the time the bullet is in mid-trajectory B has refrained from firing so that the dictator would not have died without the presence of the speeding bullet. (Recall that we are not allowed to ‘backtrack’: it is not true that if the bullet had not been mid-trajectory A would not have taken aim, and hence it is not true that had the bullet not been mid-trajectory B would have fired after all.) Hence, we have a causal chain, and so causation. But no corresponding intermediary can be found between b and the dictator’s death; hence b does not count as causes of the death.