Sven Ove Hansson

Situationist deontic logic is a model of that fraction of normative discourse which refers to only one situation and one set of alternatives. As we can see from a whole series of well-known paradoxes, standard deontic logic (SDL) is seriously mistaken even at the situationist level. In this paper it is shown how a more realistic deontic logic can be based on the assumption that prescriptive predicates satisfy the property of contranegativity. A satisfactory account of situation-specific norms is a necessary prerequisite for a successful treatment of more complex normative structures

The basic assumption of repertoire contraction is that only some of the logically closed subsets of the original belief set are viable as contraction outcomes. Contraction takes the form of choosing directly among these viable outcomes, rather than among cognitively more far-fetched objects such as possible worlds or maximal consistent subsets of the original belief set. In this first investigation of repertoire contraction, postulates for various variants of the operation are introduced. Necessary and sufficient conditions are given for when repertoire contraction coincides with AGM contraction or with operations generated by AGM-style contraction on a belief base. A close connection is shown to hold between repertoire contraction and specified meet contraction

An operator of belief change is reconstructible as another such operator if and only if any outcome that can be obtained with the former can also be obtained with the latter. Two operators are mutually reconstructible if they generate exactly the same set of outcomes. The relations of reconstructibility among fifteen operators of contraction, including the common AGM contraction operators, are completely characterized. Furthermore, the additional such relations are characterized that arise if all belief sets are required to be finite-based or if the language is finite. These results are significant for the choice of a formal model to represent how real-world agents change their beliefs. In particular, if two contraction operators are mutually reconstructible, then that limits the types of considerations that can legitimately be invoked in favour of one over the other

By replacement is meant an operation that replaces one sentence by another in a belief set. Replacement can be used as a kind of Sheffer stroke for belief change, since contraction, revision, and expansion can all be defined in terms of it. Replacement can also be defined either in terms of contraction or in terms of revision. Close connections are shown to hold between axioms for replacement and axioms for contraction and revision. Partial meet replacement is axiomatically characterized. It is shown that this operation can have outcomes that are not obtainable through either partial meet contraction or partial meet revision.

Two types of measures of probabilistic uncertainty are introduced and investigated. Dispersion measures report how diffused the agent’s second-order probability distribution is over the range of first-order probabilities. Robustness measures reflect the extent to which the agent’s assessment of the prior (objective) probability of an event is perturbed by information about whether or not the event actually took place. The properties of both types of measures are investigated. The most obvious type of robustness measure is shown to coincide with one of the major candidates for a dispersion measure, the mean square deviation measure.

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

The prescriptive has both an objective and a subjective interpretation. In the objective sense, what one ought to do depends on what is actually true. In the subjective sense it depends on what one believes to be true. Ordinary usage seems to vacillate between these two interpretations. An example (the indecisive terrorist) is used to show that a subjective ought statement can have a determinate truth-value in situations where the corresponding objective ought statement has no truth-value, not even an unknowable one. Therefore the subjective ought is not definable in terms of the objective ought. However, definability in the other direction is not excluded by this argument

In order to reduce the death toll of road traffic it is necessary to focus on how vehicles and roads can be improved. Like other dangerous machines, motor vehicles should be equipped with safety devices that prevent mistakes by the operator from leading to serious consequences. Speed limiters that prevent driving at illegal speeds would save many lives, and so would alcohol interlocks. Meshi Ori's proposal that most cars should be replaced by motorcycles would not lead to the moral improvement that he hopes for. We need motor vehicles with better, not worse, protection of the driver and passengers than the cars presently in use

Abstract: This article offers a perspective on the role of philosophy in relation to other academic disciplines and to society in general. Among the issues treated are the delimitation of philosophy, whether it is a science, its role in the community of knowledge disciplines, its losses of subject matter to other disciplines, how it is influenced by social changes and by progress in other disciplines, and its role in interdisciplinary work. It is concluded that philosophy has an important mission in promoting clarity, precision, and open-mindedness in academic research and in society at large.

Although choice and preference are distinct categories, it may in some contexts be a useful idealization to treat choices as fully determined by preferences. In order to construct a general model of such preference-based choice, a method to derive choices from preferences is needed that yields reasonable outcomes for all preference relations, even those that are incomplete and contain cycles. A generalized choice function is introduced for this purpose. It is axiomatically characterized and is shown to compare favourably with alternative constructions.