A Sequent System for Lewis''s Counterfactual Logic VC

This report presents a technical result concerning practical proof systems for counterfactual (also known as conditional) logics. In a 1983 paper, HCM de Swart gave sequent based proof systems for two counterfactual logics: Stanlaker''s VCS and Lewis''s VC. In this report I demonstrate that de Swart''s system for VC is incorrect by giving a counterexample. This counterexample does not effect de Swart''s system for VCS. Then I give a new sequent based proof system for VC together with soundness and completeness proofs. The system I give is closely modelled on de Swart''s/