I use a distinction between single-minded and indifferent choice attitudes, modeled across maximally determinate plans of action, as a basis for interpreting deontic claims – about what ought, ought not, and may be done – as expressing a mode of relation between mind and world that gives voice to the exercise of practical rationality. At the same time, I use maximally determinate possible worlds to model descriptive claims in order to understand them as involving a mode of relation between mind (...) and world that manifests our theoretical rationality. The result is of interest to both linguists looking for a formal treatment of deontic modality that captures the role prescriptive mental states play in our lives, and philosophers interested in substantive questions about action-guiding and representational mental states as exercises of practical and theoretical rationality. (shrink)