Abstract

The thesis develops a formal semantics for present necessity deontic modals, such as should, ought to, must, and past necessity deontic modals, such as should have, ought to have, had to. Contrary to the traditional approaches in deontic logic, we concentrate on the prescriptive use of such modals. In analyzing the different behavior of present and past necessity deontic modals in everyday discourse practice, we focus on the contrast that arises when the proposition embedded under those modals is eventive and non-progressive. We distinguish between a “counterfactual” and an “open-past” reading of past necessity deontic modals, and argue that the semantic differences between present and (the two readings of) past necessity deontic modals are ultimately due to two factors: (i) when the embedded proposition is evaluated, and (ii) whether the embedded proposition is an open possibility according to the common ground of the participants in the conversation. We develop a framework from the update semantics’ tradition. The approach we propose, called Temporal Deontic Update Semantics (TDUS), permits us to take into account the two factors (i) and (ii), and to model present and past necessity deontic modals by means of a single deontic operator: Oblige. We conclude our thesis by taking into account some more classical topics in the literature of deontic logic, such as conditional obligations and deontic paradoxes. We show that our considerations regarding present and past necessity deontic modals are also relevant to a solution of the paradoxes