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)

Appears to give the first model-theoretic account of both "must" and "ought" (without conflating them with one another). Some key pre-theoretic semantic and pragmatic phenomena that support a negative answer to the main title question are identified and a conclusion of some significance is drawn: a pervasive bipartisan presupposition of twentieth century ethical theory and deontic logic is false. Next, an intuitive model-theoretic framework for "must" and "ought" is hypothesized. It is then shown how this hypothesis helps to explain and (...) predict all the pre-theoretic phenomena previously observed. Next, I show that the framework hypothesized possesses additional expressive and explanatory power (e.g. derivatively predicting the existence of supererogatory and permissibly suboptimal alternatives), thus adding further confirmation that it is on the right track. (shrink)

There are a number of concepts of common-sense morality, what one must do, what one ought to do, the supererogatory, the minimum that duty allows, the morally optional and the morally indifferent, that philosophers have been hard-pressed to represent in an integrated conceptual framework. Indeed, many philosophers have despaired at the attempt and concluded that only a fragment of these concepts belong to that fundamental sphere of morality that is the central focus of the ethicist. For example, the traditional scheme, (...) with its triad of the obligatory, the forbidden and the permissible, pigeonholes all actions into three mutually exclusive and exhaustive classes: those which are obligatory, those which are forbidden and those which are optional. Hence, at best, it can represent exactly two of the six aforementioned concepts. For from the standpoint of this scheme, what one must do and what one ought to do can't be distinguished and hence they can't both be represented. Although the morally optional can be represented, the supererogatory, one of its subclasses, cannot be represented. Furthermore, the morally indifferent, another subclass of the optional--one which is obviously disjoint from the supererogatory--cannot be represented. Finally, the minimum that duty allows finds no distinctive place in the traditional scheme. Thus, on the face of it, the traditional scheme is radically incomplete. ;I present, motivate and defend a new conceptual scheme for common-sense morality in which these concepts are represented and systematically integrated. An intuitively motivated semantic framework underpinning this conceptual scheme is also presented. Such a scheme, along with the associated semantic framework, is motivated by reflecting on the supererogationist's objections to utilitarianism and to the traditional scheme. But in addition, a new integrated network of linguistic motivations for this conceptual scheme is uncovered, one which is completely independent of supererogationistic considerations. Hence, these two separate sources of evidence for the centrality of this new scheme to our pre-theoretic thinking corroborate one another--thus jointly boosting the evidence beyond the mere sum of their separate evidential values. (shrink)