Abstract

This paper studies the dynamics of counterfactual supposition using qualitative belief revision techniques; a second paper will provide a quantitative account using probabilities. We individuate four general features of suppositional thinking; define a counterfactual dynamics; show that it displays those features by recapturing the axioms of KGM belief update theory. We then isolate a crucial difference between counterfactual supposition and supposition ‘in the indicative mood’, namely that the former, but not the latter, can make the supposing agent lose certainties. We propose a Dynamic Logic of Counterfactual Supposition (DLCS) with a sound axiomatization. We prove a Ramsey Test for the logic: one believes the counterfactual that if it were that P, it would be that Q, precisely when one comes to believe that Qon the counterfactual supposition of P.