Counterpossibles and Similarity

Abstract Several themes of David Lewis’s theory of counterfactuals, especially their sensitivity to context, pave the way for a viable theory of non-trivial counter possible. If Lewis was successful in defending his account against the early objections, a semantics of counter possible can be defended from similar objections in the same way. The resulting theory will be extended to address ‘might’ counterfactuals and questions about the relative ‘nearness’ of impossible worlds. David Letterman once asked his audience, ‘Based on what you know about him in history books, what do you think Abraham Lincoln would be doing if he were alive today? One, writing his memoirs of the Civil War. Two, advising the President. Three, desperately clawing at the inside of his coffin.’ We are being asked to evaluate the counterfactual David Lewis has had some important insights into how we are to understand counterfactuals of this sort, and these insights help us see how to think about counter possibles, i.e., counterfactuals with impossible antecedents, such as if Hitler had time-travelled into the past and killed himself as an infant, then World War II would not have occurred.A Lewisian semantics for counterfactuals can be extended in such a way that counterpossibles are treated as false or as non-trivially true. The context-sensitivity of counterfactuals and a distinction between certain types of contextual influence, both highlighted by Lewis, take on an even greater importance in the case of counterpossibles. Though Lewis would endorse neither the semantics to be proposed nor its underlying ontology, the proposed account is Lewisian in spirit. In fact, the primary objection to the extended semantics parallels many of the early objections to Lewis’s original semantics. If Lewis was successful in defending his account against these, a semantics of counterpossibles can be defended from similar objections in the same way.