Intertranslating Counterfactuals and Updates

. We recall that the Ramsey Rule can be seen as axiomatising the relationship of inverse accessibility relations which exists between the notions of update and counterfactual conditional. We use this fact to translate between postulates for updates and postulates for counterfactuals. Thus, Katsuno/Mendelzon's postulates U1--U8 are translated into counterfactual postulates C1--C8 (theorem 6), and many of the familar counterfactual postulates are translated into postulates for updates (theorem 7). Our conclusions are summarised in table 5. 1 INTRODUCTION It is now known that update, rather than revision, is the correct notion of theory change to fulfil Ramsey's test for the acceptance of counterfactual conditionals [7]. Moreover, in previous work [17] we have shown that Katsuno/Mendelzon's theory of updates is essentially an exercise in multi-modal logic: the update operator is an existential modality. And we have shown that the accessibility relation corresponding to this operator is..