The aim of this paper is twofold: (i) to introduce the framework of update semantics and to explain what kind of semantic phenomena may successfully be analysed in it: (ii) to give a detailed analysis of one such phenomenon: default reasoning.

Sir David Ross introduced prima facie duties, or acts with a tendency to be duties proper. He also spoke of general prima facie principles, wwhich attribute to acts having some feature the tendency to be a duty proper. Like Utilitarians from Mill to Hare, he saw a role for such principles in the epistemology of duty: in the process by means of which, in any given situation, a moral code can help us to find out what we ought to do.After (...) formalizing general prima facie principles as universally quantified conditionals I will show how seeming duties can be detached from them. There will be examples involving lies, burnt offerings and the question of whether to have a napkin on your lap while eating asparagus. They will illustrate the defeasibility of this detachment, how it can lead into dilemmas, and how general prima facie principles are overridden by more specific ones. (shrink)

We discuss similarities and residual differences, within the general semantic framework of minimality, between defeasible inference, belief revision, counterfactual conditionals, updating — and also conditional obligation in deontic logic. Our purpose is not to establish new results, but to bring together existing material to form a clear overall picture.

A new possible world semantics for deontic logic is proposed. Its intuitive basis is that prohibitive predicates (such as "wrong" and "prohibited") have the property of negativity, i.e. that what is worse than something wrong is itself wrong. The logic of prohibitive predicates is built on this property and on preference logic. Prescriptive predicates are defined in terms of prohibitive predicates, according to the wellknown formula "ought" = "wrong that not". In this preference-based deontic logic (PDL), those theorems that give (...) rise to the paradoxes of standard deontic logic (SDL) are not obtained. (E.g., O(p & q) → Op & Oq and Op → O(p v q)) are theorems of SDL but not of PDL.) The more plausible theorems of SDL, however, can be derived in PDL. (shrink)

A textbook on modal logic, intended for readers already acquainted with the elements of formal logic, containing nearly 500 exercises. Brian F. Chellas provides a systematic introduction to the principal ideas and results in contemporary treatments of modality, including theorems on completeness and decidability. Illustrative chapters focus on deontic logic and conditionality. Modality is a rapidly expanding branch of logic, and familiarity with the subject is now regarded as a necessary part of every philosopher's technical equipment. Chellas here offers an (...) up-to-date and reliable guide essential for the student. (shrink)