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Deontic Modality in Rationality and Reasoning Lay Summary Alessandra Marra The present dissertation investigates certain facets of the logical structure of oughts – where “ought” is used as a noun, roughly meaning obligation. I do so by following two lines of inquiry. The first part of the thesis places oughts in the context of practical rationality. The second part of the thesis concerns the inference rules governing arguments about oughts, and specifically the inference rule of Reasoning by Cases. These two lines of inquiry, together, aim to expound upon oughts in rationality and reasoning. The methodology used in this dissertation is the one of philosophical logic, in which logical, qualitative models are developed to support and foster conceptual analysis. The dissertation consists of four main chapters. The first two chapters are devoted to the role of oughts in practical rationality. I focus on the so-called Enkratic principle of rationality, which – in its most general formulation – requires that if an agent believes sincerely and with conviction that she ought to do X, then she intends to X. I develop a logical framework to investigate the (static and dynamic) relation between those oughts believed by the agent and her intentions. It is shown that, under certain minimal assumptions, the Enkratic principle of rationality is a principle of limited validity. The following two chapters of the dissertation constitute a study of the classical inference rule of Reasoning by Cases, which – in its simplest form – moves from the premises “A or B”, “if A then C” and “if B then C” to the conclusion “C”. Recent literature has called the validity of Reasoning by Cases into question, with the most influential counterexample being the so-called Miners’ Puzzle – an instance of Reasoning by Cases where “C” involves oughts. I provide a unifying explanation of why the Miners’ Puzzle emerges. It is shown that, within specific boundaries, Reasoning by Cases is a valid inference rule.

Van der Sandt’s (1992) anaphoric account of presupposition is generally considered to be the theory which makes the best empirical predictions about presupposition projection (see e.g. Beaver 1997:983). The main insight is that there is an interesting correspondence between the behavior of anaphoric pronouns in discourse and the projection of presuppositions in complex sentences. Van der Sandt proposes to ‘resolve’ presuppositions just like anaphoric pronouns are resolved in Discourse Representation Theory (DRT, Kamp & Reyle, 1993). Van der Sandt contends that there is also an important difference between pronouns and presuppositions: when there is no antecedent for an anaphoric pronoun, the sentence containing the pronoun cannot be interpreted. However, when there is no antecedent for a presupposition — and the presupposition has sufficient descriptive content — then the presupposition can be accommodated and, as it were, create its own antecedent. This combination of resolution and accommodation constitutes the empirical strength of van der Sandt’s approach.