 The central notion of a possible world semantics for counterfactuals is a relation of comparative similarity between worlds (Lewis 1973a). One world is said to be closer to actuality than another if the first resembles the actual world more than the second does. In terms of this similarity relation, the truth condition for the counterfactual “If A were (or had been) the case, C would be (or have been) the case” is stated as follows: