Lewis’ Theory of Counterfactuals and Essentialism
Modern logic since the end of the Second World War has undergone many developments.  Two of the most interesting of these are the Kripkian Possible World Semantics and Lewis’ system of Counterfactuals.  The first was developed by Saul Kripke in the 1960s and the second was developed by David Lewis in the 1970s.  In some senses we can say that Lewis’ system of counterfactuals or Counter Factual Semantics (CFS) is built on top of the architecture which Kripke created with his Possible Worlds Semantics (PWS).  But, what is the Kripkian Possible World Semantics itself built on?  The answer it seems is very finely tuned ontology founded on the notion of possible worlds.  This paper will attempt to do the following. First, attempt to draw a distinction between on the one hand conditionals and the other counterfactuals and at the same time attempt to look at some of the historical literature surrounding counterfactuals and their application in various fields like the philosophy of science.  Second, recapitulate Lewis’ system of counterfactual semantics as developed primarily in Lewis’ book Counterfactuals.  Finally this paper will attempt to explore the metaphysical foundations of the possible worlds account argued for by David Lewis in his conception of Modal Realism.
Conditional Counterfactual Possible Worlds Semantics Logic