Marek J. Sergot

A logic-based calculus of events

We outline an approach for reasoning about events and time within a logic programming framework. The notion of event is taken to be more primitive than that of time and both are represented explicitly by means of Horn clauses augmented with negation by failure.The main intended applications are the updating of databases and narrative understanding. In contrast with conventional databases which assume that updates are made in the same order as the corresponding events occur in the real world, the explicit treatment of events allows us to deal with updates which provide new information about the past.Default reasoning on the basis of incomplete information is obtained as a consequence of using negation by failure. Default conclusions are automatically withdrawn if the addition of new information renders them inconsistent.Because events are differentiated from times, we can represent events with unknown times, as well as events which are partially ordered and concurrent.

The British Nationality Act as a logic program

The formalization of legislation and the development of computer systems to assist with legal problem solving provide a rich domain for developing and testing artificial-intelligence technology.

A Formal Characterisation of Institutionalised Power

It is a standard feature of norm-governed institutions that designated agents are empowered to create particular kinds of states of aaairs by means of the performance of speciied types of actions. Frequently, the states of affairs are of a normative kind, in the sense that they pertain to rights and obligations, as for instance when a Head of Department signs a purchase agreement and thereby creates an obligation on his employer to pay for goods received. We use the term institutionalised power to stand for the notion of power we here seek to explicate. Following a lead from jurispru-dential discussions of legal power, we distinguish institutionalised power from permission and practical possibility. We deene a conditional connective intended to capture the consequence relation implicit in statements of the form: according to the constraints operative in institution s, the performance of some act A by agent x counts as a means of creating state of aaairs B. When combined with deontic and action logics, the new connective facilitates the analysis of a number of notions crucial to the understanding of organised interaction in institutions, such as authorisation and delegation. We conclude with some illustrations of the expressive power of the new logical language.

Specifying norm-governed computational societies

Electronic markets, dispute resolution and negotiation protocols are three types of application domains that can be viewed as open agent societies. Key characteristics of such societies are agent heterogeneity, conflicting individual goals and unpredictable behavior. Members of such societies may fail to, or even choose not to, conform to the norms governing their interactions. It has been argued that systems of this type should have a formal, declarative, verifiable, and meaningful semantics. We present a theoretical and computational framework being developed for the executable specification of open agent societies. We adopt an external perspective and view societies as instances of normative systems. In this article, we demonstrate how the framework can be applied to specifying and executing a contract-net protocol. The specification is formalized in two action languages, the C+ language and the Event Calculus, and executed using respective software implementations, the Causal Calculator and the Society Visualizer. We evaluate our executable specification in the light of the presented case study, discussing the strengths and weaknesses of the employed action languages for the specification of open agent societies.

Deontic interpreted systems

We investigate an extension of the formalism of interpreted systems by Halpern and colleagues to model the correct behaviour of agents. The semantical model allows for the representation and reasoning about states of correct and incorrect functioning behaviour of the agents, and of the system as a whole. We axiomatise this semantic class by mapping it into a suitable class of Kripke models. The resulting logic, KD45ni-j, is a stronger version of KD, the system often referred to as Standard Deontic Logic. We extend this formal framework to include the standard epistemic notions defined on interpreted systems, and introduce a new doubly-indexed operator representing the knowledge that an agent would have if it operates under the assumption that a group of agents is functioning correctly. We discuss these issues both theoretically and in terms of applications, and present further directions of work.

Animated specifications of computational societies

E-markets and negotiation protocols are two types of application domains that can be viewed as open computational societies. Key characteristics of such societies are agent heterogeneity, conflicting individual goals and limited trust. The risk that members of such societies will not conform to specifications imposes the need for a framework that will facilitate the designers to determine to what extent it is desirable to deploy their agents in such societies. We address this need by presenting a formal framework for specifying, animating, and ultimately reasoning about and verifying the properties of open computational systems. We view computational systems from an external perspective, aiming to account for the institutional and social aspects of these systems. We identify the key concepts and illustrate how they are used by formalising an example employing the contract net protocol. The framework and associated logical inferences have been implemented as a software platform that provides automated animation of the global states of an open system (society) during its execution. Simulations have demonstrated that the implementation of the framework establishes a foundation for a rich, formal representation of open computational societies.

Dyadic Deontic Logic and Contrary-to-Duty Obligations

One of the main issues in the discussion on standard deontic logic (SDL) is the representation of contrary-to-duty (CTD) obligations. A well-known example is Forrester’s (1984) paradox of the gentle murderer: it is forbidden to kill, but if one kills, one ought to kill gently.

A computational theory of normative positions

The Kanger-Lindahl theory of normative positions attempts to use a combination of deontic logic (the logic of obligation and permission) and a logic of action/agency to give a formal account of obligations, duties, rights, and other complex normative concepts. This paper presents a generalization and further development of this theory, together with methods for its automation and application to practical examples. The resulting theory is intended to be applied in the representation and analysis of laws, regulations, and contracts, in the specification of aspects of computer systems, in multiagent systems, and as a contribution to the formal theory of organizations. Particular attention is paid to representations at varying levels of detail and the relationships that hold between them. The last part presents Norman-G, an automated support system intended to facilitate application of the theory to the analysis of practical problems, with a small example to illustrate its use.

Status-Based Access Control

Despite their widespread adoption, Role-based Access Control (RBAC) models exhibit certain shortcomings that make them less than ideal for deployment in, for example, distributed access control. In the distributed case, standard RBAC assumptions (e.g., of relatively static access policies, managed by human users, with complete information available about users and job functions) do not necessarily apply. Moreover, RBAC is restricted in the sense that it is based on one type of ascribed status, an assignment of a user to a role. In this article, we introduce the status-based access control (SBAC) model for distributed access control. The SBAC model (or family of models) is based on the notion of users having an action status as well as an ascribed status. A users action status is established, in part, from a history of events that relate to the user; this history enables changing access policy requirements to be naturally accommodated. The approach can be implemented as an autonomous agent that reasons about the events, actions, and a history (of events and actions), which relates to a requester for access to resources, in order to decide whether the requester is permitted the access sought. We define a number of algebras for composing SBAC policies, algebras that exploit the language that we introduce for SBAC policy representation: identification-based logic programs. The SBAC model is richer than RBAC models and the policies that can be represented in our approach are more expressive than the policies admitted by a number of monotonic languages that have been hitherto described for representing distributed access control requirements. Our algebras generalize existing algebras that have been defined for access policy composition. We also describe an approach for the efficient implementation of SBAC policies.

Using the event calculus for tracking the normative state of contracts

In this work, we have been principally concerned with the representation of contracts so that their normative state may be tracked in an automated fashion over their deployment lifetime. The normative state of a contract, at a particular time, is the aggregation of instances of normative relations that hold between contract parties at that time, plus the current values of contract variables. The effects of contract events on the normative state of a contract are specified using an XML formalization of the Event Calculus, called ecXML. We use an example mail service agreement from the domain of web services to ground the discussion of our work. We give a characterization of the agreement according to the normative concepts of obligation, power and permission, and show how the ecXML representation may be used to track the state of the agreement, according to a narrative of contract events. We also give a description of a state tracking architecture, and a contract deployment tool, both of which have been im...