 As Lewis notes (1973b), causal dependence between actual events is sufficient for causation, but not necessary: it is possible to have causation without causal dependence. A standard case of ‘pre-emption’ will illustrate this. Suppose that two shooters conspire to assassinate a hated dictator, agreeing that one or other will shoot the dictator on a public occasion. Acting side by side, assassins A and B find a good vantage point, and, when the dictator appears, both take aim (events a and b respectively). A pulls her trigger and fires a shot that hits its mark, but B desists from firing when he sees A pull her trigger. Here assassin A’s actions (such as her taking aim) are causes of the dictator’s death, while B’s actions (such as his taking aim) are merely preempted potential causes. (Lewis distinguishes such cases of preemption from cases of symmetrical overdetermination in which two processes terminate in the effect, with neither process preempting the other. Lewis believes that these cases are not suitable test cases for a theory of causation since they do not elicit clear judgements.) The problem raised by this example of preemption is that both actions are on a par from the point of view of causal dependence: had neither A nor B acted, then the dictator would not have died; and if either had acted without the other, the dictator would have died.