A Problem about Permission and Possibility

Abstract

This chapter explores the prospects for a unified theory of deontic and (socalled) epistemic modality. A theory of deontic modality needs to solve the puzzles raised by David Lewis in 'A Puzzle About Permission'. In particular, it needs to say what the effect is of making something permissible, and what consequences a permission has in terms of what else is thereby permitted. It is argued that when p is made permissible, then a world w is still impermissible if, antecedently, w was impermissible for a reason not implying p. This model is extended to (so-called) epistemic modality. What should happen to the conversational context when it is accepted that it might be that p? The chapter suggests that three things happen. Most obviously, the common ground of the conversation now includes at least one world where p. Further, the common ground now includes worlds that were previously only ruled out for reasons that entailed ~p. Finally, once it is accepted that it might be that p, this cancels any assertion that ~p, even one that has not been explicitly made in this conversation.