Keywords: deon91

In this paper we shall give a short (and incomplete) historic overview of the branche of modal logic that is concerned with (reasoning about) norms and normative behaviour, viz. deontic logic. Typically, deontc logic has operators for deontic / normative modalities such as prohibition, permission and obligation. The paper is meant as an introduction to the more advanced papers presented in DEON'91

Keywords: deon91

Deontic logic is the logic that deals with actual as well as ideal behaviour of systems. In this paper, we survey a number of applications of deontic logic in computer science that have arisen in the eighties, and give a systematic framework in which these applications can be classified. Many applications move in the direction of programming a computer in deontic logic to make the computer prohibit, permit or obligate people to do something. We discuss conditions under which this possibility is realistic and conditions under which it would be admissible to do so.

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Only two of the main philosophical problems of deontic logic will be dealt with here. 1) First the question whether deontic sentences have or lack truth values. 2) Second the question whether it is needed to have formalisms for conditional defeasible norms (prima facie obligations and permissions) and the impact of the question on the general subject of the admissibility of non-monotonic logics. Finally I comment on the problem of how the preceding philosophical observations have incidence on the practical computational problem of knowledge representation of norms in the field of law. To discuss the philosophical questions (question 1) and 2)) two deontic logics are presented by means of a syntactic and a corresponding possible worlds semantic approach backed with their intuitive justifications. One of the logics is designed to give expression to the conceptual import of normative propositions, i.e. the meaning of deontic sentences used to assert the existence or nonexistence of norms. The other is a logic for norms, i.e., the meaning of deontic sentences used to regulate and evaluate human behavior. It is hold: a) that normative propositions have but norms do not have truth values, b) that their corresponding logics are different unless the system created by the competent authorities is consistent and complete (a case in which both logics become isomorphic). The notion of defeasible (prima facie) conditional norm requires the rejection of the rule of strengthening the antecedent (the mirror image of the nonmonotonicity of a consequence relation) and the abandonement of the rule of detachment which makes impossible to apply the norm in practical everyday situations. The two notions that characterize a defeasible conditional, i.e. a) a classical (monotonic) notion of consequence and b) a theory of change (expansion, contraction and revision) of premisses should not be mixed up in the way it has been done in order to present defeasible conditionals and nonmonotonic logics on pain of concealing important distinctions and functions. For the practical computational problem of representing the law many of the theoretical distinctions discussed above may be left aside for more often than not they are not relevant to the problem at hand. Nevertheless it is fundamental to have a clear idea of the nature of the distinctions left aside.

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Normative concepts have been formalized in deontic logic (i) as operators which turn certain descriptive propositions (for example, propositions about actions) into norms or normative propositions, or (ii) as operators which turn action terms or predicates (act-descriptions) into normative propositions. The so-called standard deontic logic represents the former approach, whereas G. H. von Wright's first system, Hector-Neri Castaneda's deontic logic, and the systems of deontic logic based on dynamic logic follow the second approach. This paper compares the merits of the two approaches, with particular attention to Hector-Neri Castaneda's work.

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The normative concepts which deontic logic studies have significant applications in computing. The intention may be to deal with seemingly different issues, such as causality and sequencing in multi-component (agent) systems. Or the intention may be to deal with more obviously normative issues such as fault tolerance or security. Further, the formalization of normative concepts in the setting of the new logics grown in computing is an interesting development which may contribute significantly to deontic logic itself.

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No abstract given.

Keywords: deon91

The A-Hohfeld language is presented as a set of definitions; it can be used to precisely express legal norms. The usefulness of the A-Hohfeld language is illustrated in articulating 2560 alternative structural interpretations of the four-sentence 1982 Library Regulations of Imperial College and constructing an Interpretation-Assistance legal expert system for these regulations by means of the general-purpose Interpretation-Assistance legal expert system builder called MINT. The logical basis for A-Hohfeld is included as an appendix.

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Today, absolute deontic logic with a unique operator O (“it is obligatory that") and its dual P(“it is permitted that") is out-of-date. I propose to improve deontic logic with using operators indexed on two kinds of individuals: authorities and addressees. First, each of them are dealt with directly from intuition, and axiomatical systems, with their corresponding semantics, are built. At this stage, as in the whole article, an essential procedure consists in working on sets of individuals, not merely on individuals. Then I introduce an elementary logic of action, so that emanating a norm and being touched by a norm are respectively considered as making an absolute norm and being normed to do something. New systems, more complicated, result from this analysis

Keywords: deon91

A variant of Belbap's stit-semantics due to Horty and von Kutschera is used to provide a semantics of obligation. This semantics is used to discuss conditional obligation as well as some of the paradoxes of deontic logic. The paper argues for the importance of an analysis of agency for deontic logic.

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Stit theory (a logic of seeing-to-it that) is applied to cases involving many agents. First treated are complex nestings of stits involving distinct agents. The discussion is driven by the logical impossibility of “a sees to it that b sees to it that Q" in the technical sense, even though that seems to make sense in everyday language. Of special utility are the concepts of “forced choice", of the creation of deontic states, and of probabilities. Second, joint agency, both plain and strict (every participant is essential) is given a rigorous treatment. A central theorem is that strict joint agency is itself agentive.

Keywords: deon91

With respect to confidentiality, a computer security policy defines what information stored in a computer users have the permission to know. We propose to express these policies with an epistemic and deontic logic. In this context, confidentiality is defined by the formula K_A φ->R_A φ that could be read “if A knows φ then A should have the permission to know φ". We provide a new possible-worlds semantics for the R_A operator that depends on the security policy to be modeled. Finally, we express within our framework three examples of security policies.

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This paper is an attempt to clear the following charge leveled against preference logics: preference logics rest upon the mistaken belief that concept construction can satisfactorily be carried out in isolation from theory construction [15]. We construct a logic of preference that is fundamental in the sense that it does not commit itself to any allegedly obvious or intuitive -and in actuality, theory specific- preference principles. A unique feature of our construction is that preferences orderings are placed upon possible worlds. While this has been done before in the work of [16] [10], among others, we do not derive a binary preference relation -from these orders- that acts on individual propositions. Instead, we provide the syntactic means to impose the preference orderings among worlds. Thus, unlike [10] [11], we do not need to assume a priori that our preference orderings be transitive. Such properties can be axiomatized. The close connections between preferences and obligations, in particular, their normative nature, them, allows us to derive a deontic logic that is free of the paradoxes of standard deontic logic. It is interesting to note here, that this work arose in an attempt to provide a logical characterization of document description and layout. Layout directives can be succintly represented as preference criteria.

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Fault tolerant systems are, like most complex systems, structured in layers. In a fault tolerant layered system, a malfunction on lower level layer occurs at a higher level as an imperfection, that may prohibit the upper layer from exhibiting its preferred behaviour. The behaviour of a system in such less than perfect circumstances should not be left unspecified. Deontic logic offers the possibility to specify layered fault tolerant systems in a natural way. More specifically, dyadic modalities are used to specify the preferred behaviours of a fault tolerant system in different conditions. The use of dyadic rather than monadic modalities is also discussed in the context of a particular problem that arises when specifying fault tolerant systems and which is referred to as the `lazy programmer' paradox. The `lazy programmer' paradox is illustrated by a few `toy' examples. The application of deontic logic as a specification language for fault tolerant systems is illustrated by the specification of a non-trivial example.

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In this paper we propose to use deontic logic for expressing and reasoning on specifications elaborated during the initial requirements engineering phase of the software lifecycle. During this phase, we need to capture descriptions of `composite systems' (i.e. systems including manual procedures, hardware devices and software components) and to be able to check that the different interactions taking place between the components are valid with respect to some global goals assigned to the system considered as a whole. To support this activity, we introduce a formal real-time temporal language augmented with some deontic capabilities.

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In this paper a theory about the consistency of rules is developed. The paper starts with initial circumscription of rule consistency, which is gradually refined, until it takes account of the differences between alethic and deontic consistency, and the differences between consistency from distinct points o view.

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No abstract given.

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We outline the approach to the analysis of deontic conditionals taken in our earlier work, compare it briefly with two main trends within dyadic deontic logic, and then discuss problems associated with the augmentation principle and the factual detachment principle. We then modify our previous system, using a classical but not normal (in the sense of Chellas) deontic modality to provide the basis for an alternative analysis of deontic conditionals. This new analysis validates neither the factual detachment nor the augmentation principles. However, influenced by the approach of James Delgrande to default reasoning, we show how a restricted form of factual detachment may be accomodated within our revised system. Finally, we mention a quite different kind of apprach, in which conditionship is seen not as an operation but as a relation. This paper is a report of work in progress.

Keywords: deon91

Two topics dominate this paper: the role of deontic logic in knowledge representation in the legal domain; and the role of deontic logic in the specification of computer systems. In both cases, we take the library regulations at Imperial College as an illustrative example. The essential point for which we then argue is that deontic logic -in some form or other- needs to be taken seriously as soon as one is forced to make explicit the distinction between what ought to be the case and what is the case, or so we also say, between the ideal and the actual. In conclusion we touch on the role of deontic logic in the development of the theory of normative positions -a theory which is of relevance both to the legal knowledge representation and computer system specification. Our overall aim in this paper is to sketch the beginnings of a methodology for the use of deontic logic in the analysis and representation of normative systems.

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The logical content of integrity constraints can be expressed by means of predicate formulas, but this obscures the difference between data, queries and constraints. Kripke semantics provides a natural solution. Integrity constraints do not express facts about the actual instance, but describe the acceptable instances of the database scheme, among which we hope to find the actual one. A new deontic operator X, denoting `what should hold', is presented to describe the logic of integrity constraints in a relational database. The suitability of the operator to model integrity is shown by its application in the context of databases with nulls.

Keywords: deon91

This paper describes some aspects of management policies which could be modeled as objects in a distributed computer system. The motivation for this is to enable their automation, but to do this safely it is necessary to describe them formally in order to be able to reason about them. The aim of the paper is to describe distributed system management policies and then raise the question whether deontic logic might be a suitable vehicle for their formal description. Policies, the plans of an organization to meet its goals, are persistent entities which are intended to influence actions. A distinction is made between policies that motivate actions and those which authorize them. Policies have several characteristics: they are directed to subjects; they are typically organized in hierarchies; and a number of potentially conflicting policies may apply to a single possible action, so that they require to have a preference ordering. We describe an ad hoc formal approach to modeling policy, using the Z specification language, and then examine some existing approaches using deontic logic. Finally we set out a shopping list of the desirable characteristics of a logic for modeling management policies.

Keywords: deon91

There are acknowledge problems in specifying security requirements for complex information processing systems. Even the “simple" notion of access rights is not well understood. In this paper we develop a normative framework for establishing and specifying access rights to a file. It will be suggested that there are 26 different relationships, that there is a practical and simple procedure for determining which relationship each agent stands in to a file, and that this approach enables us to carry out useful checks on the specification. The need for, and the use of, these ideas is illustrated by means of an example from the processing of medical health records.

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This paper presents a formal theory about nontrivial reasoning with inconsistent but hierarchically structured normative information. The theory is based on the observation that the way legal rules are made subject to orderings gives them a defeasible character. For this reason approaches based on standard logic, such as belief revision, are inappropriate as a formal framework for reasoning with ordered norms. Consequently, a nonmonotonic logic, default logic, is chosen as the underlying formalism for a definition of a hierarchically preferred argument. On the basis of this definition two alternative notions of nontrivial consequence from inconsistent knowledge are defined and compared with each other. One application of the formal framework is an alternative analysis of the deontic concepts “conflict of duties" and “defeasible obligation".

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We show how the normative behaviour of a system can be specified by hierarchies of constraints, in a way that allows the specifier to reason about the system's behaviour when it falls outside those constraints. We call such hierarchies structured theory presentations and show that they arise naturally from structured specifications. Structured theory presentations are an alternative to deontic logic for specifying norms.

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Our interests are in the application of deontic logic for the modeling of regulations in commercial law and other social institutions. We provide a first-order framework of deontic reasoning that can model and compute social regulations and rules. This effort is practically important due to the ubiquity and complexity of social regulations and norms. Computer-mediated modeling norms may reduce the overhead of managing complex social norms and avoid inefficiencies and social inequity resulting from complex and ill-maintained social norms. In order to achieve the goal, we apply defeasible reasoning, a Horn-clausal form logic programming approach, and capture deontic concepts in first-order representations. A rule-based expert system of defeasible reasoning is currently under development. The proposed formalism may be applied for the modeling of normative systems such as bureaucratic regulations and legal reasoning.

Keywords: deon91

The obligations issued by contracts may be seen as specification of legally desirable behaviours. It was on the basis of this idea that the concept of obligation in the Civil Law Systems of Contracts Law was explored. We can interpret these kind of obligations as obligations to fulfil promises by behaving in a certain way. In the presented deontic logics, the obligation operators refer directly to some of the mentioned elements of the General Obligation Theory (Contracts Law), namely what should be done for the fulfillment of a duty and the circumstantial and temporal aspects which are relevant to determine the circumstances needed to fulfil an obligation. The basic semantic concept of the proposed logic is the notion of behaviour: a sequence of atomic actions executed from a state of affairs. Different deontic operators are discussed and their relationships are analyzed from the point of view of their fulfilment and or/violation.

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Deontic rules play an important role in conceptual modeling. It has been suggested that deontic logic is therefore a necessary part of a modelling language. However, deontic rules typically concern the interaction of different agents in the domain. It is shown that this interaction can also be modelled, and more transparently, in the form of communication structures, based on illocutionary logic. These communication structures can be organized in an object-oriented way by means of “contracts". This allows for sharing of general rules by means of inheritance. Deontic operators can be defined in terms of the illocutionary primitives, so that a deontic logic can be given as well, on the top of the illocutionary logic.

Keywords: deon91