In his Enquiry Concerning Human Understanding (1748) Hume stated briefly another way to view causality. He said that an object is the cause of another when "if the first object had not been, the second never had existed" (1975 ed., p. 146). This view has been articulated into a theory of causality by David Lewis. Lewis (1986) defined causality in terms of the counterfactual dependence of the effect on the cause: the cause is rendered counterfactually necessary for the effect. For instance, to say that the short-circuit caused the fire is to say that if the short-circuit had not happened, the fire would not have ensued. To be more precise, Lewis defined causality by reference to a causal chain of counterfactually dependent events, where a sequence of events (C, E, E, …) is a chain of counterfactual dependence if and only if E counterfactually depends on C, E counterfactually depends on E, and so on. This move is meant to enforce that causation is a transitive relation among events (that is, if C causes E and E causes E, then C causes E). As Lewis put it: "one event is a cause of another if and only if there exists a causal chain leading from the first to second" (p. 167). Statements such as "if C had happened, then E would have happened" are called counterfactual conditionals (another example, "if this sugar cube had been in water, it would have dissolved") for they state what could or could not have happened, under certain circumstances. But it has been notoriously difficult to specify the conditions under which counterfactual conditionals are true or false. Lewis articulated a rather complicated logic of counterfactual conditionals, which was based on the idea that, besides the actual world, there are also other possible worlds, which can be deemed more or less similar to the actual. A chief but not inviolable criterion for judging the similarity among worlds was taken to be whether the same laws of nature govern the worlds under comparison.
Though it is still one of the main contestants, this view of causality faces important difficulties. A chief among them comes from cases of causal overdetermination, where there are two factors each of which is sufficient to bring about the effect, but none of them is necessary, since even if the one was not present, the other factor would ensure the occurrence of the effect. For instance, two rocks are simultaneously thrown at a bottle and they shatter it. They both caused the shattering, but the effect is not counterfactually dependent on either of them, since if the first rock had missed the bottle, the other would have still shattered it. So there is causality without the cause being counterfactually dependent on the effect.