Bernd van Linder

A Logic of Capabilities

In this paper a logic of capabilities is introduced. We start by defining a language in which not only knowledge and actions of agents can be expressed, but also the abilities. The semantics for this language is given by extended Kripke models. Equivalence transformations on actions are defined in such a way that equivalent actions have equivalent results and that the abilities of an agent are closed under these transformations. The planning capacities of the agent are considered by looking at the Can-predicate and the Cannot-predicate. The former indicates that an agent knows that he can achieve a given goal by performing a given action, the latter indicates that the agent knows that that he cannot achieve the given goal by performing the given action. It turns out that the possibility to express the abilities of an agent provides for a flexible and intuitively appealing framework.

Modelling Social Agents: Communication as Action

In this paper we present a formal framework for social agents. The social agents consist of four components: the information component (containing knowledge and belief), the action component, the motivational component (where goals, intentions, etc. play arole) and the social component (containing aspects of speech acts and relations between agents). The main aim of this work was to describe all components in a uniform way, such that it is possible to verify each component separately but also formally describe the interactions between the different components. E.g. the effect of a speech act on the believes of an agent or on the commitment to a goal it pursues.

Communicating Rational Agents

We demonstrate how communication between agents can be incorporated in a dynamic/epistemic multi-agent system, in which the knowledge and ability of agents, and the opportunity for, and the result of their actions are formalized. We deal with two sorts of communication inside this system: one in which a didactic agent passes on its knowledge to every other agent, and one in which agents only send information upon request. The ability and opportunity to communicate depends on the knowledge and lack of knowledge of agents, and the trust and dependence relations that exist between them. The semantics of communication actions is given by means of the machinery of epistemic updates, which is a useful instrument when modelling knowledge-producing actions. Our approach allows the whole process of communication to be defined inside a Kripke model. We compare our communication system to other multi-agent epistemic notions, viz. belief dependence, and distributed and common knowledge.

Preferential Models of Query by Navigation

The Information Retrieval (IR) problem can be described as the quest to find the set of relevant information objects corresponding to a given information need, which is represented by a request. IR begins with a user who wishes to satisfy an information need. The information need is typically formulated in the form of a request, denoted by q. The intention is that the request q be as good as possible description of the information need N. The information being queried is modelled as a set of information carriers, or documents. An often used premise in IR is the following: if a given document d is about the request q, then there is a high likelihood that d is relevant with respect to the associated information need. Thus, the IR problem is reduced to deciding the aboutness relation between documents and requests. Many IR models have been developed, and there is a wide variation in how they determine aboutness. A common approach is to equate aboutness with overlap, meaning if there is sufficient overlap between the document and the query, the document is deemed to be about the query. The degree of overlap determined by the retrieval model is usually expressed as a real number between zero (no overlap) and unity (complete overlap). In most models, “sufficient” overlap is not prescribed by the model but by experimentation within a given IR setting. Such experimentation may lead to the conclusion, say, that sufficient overlap is apparent when the overlap measure is greater than α, 0 < α ≤ 1. The value α varies from setting to setting.

Group knowledge is not always distributed (neither is it always implicit)

Abstract In this paper we study the notion of group knowledge in a modal epistemic context. Starting with the standard definition of this kind of knowledge on Kripke models, we show that this definition gives rise to some quite counter-intuitive behaviour. Firstly, using a strong notion of derivability, we show that group knowledge in a state can always, but trivially, be derived from each of the agents’ individual knowledge. In that sense, group knowledge is not really implicit, but rather explicit knowledge of the group. Thus, a weaker notion of derivability seems to be more adequate. However, adopting this more ‘local view’, we argue that group knowledge need not be distributed over (the members of) the group: we give an example in which (the traditional concept of) group knowledge is stronger than what can be derived from the individual agents’ knowledge. We then propose two additional properties on Kripke models: we show that together they are sufficient to guarantee ‘distributivity’, while, when leaving one out, one may construct models that do not fulfil this principle.

Seeing is Believing

In this paper a formal framework is proposed in which variousinformative actions are combined, corresponding to the different ways in whichrational agents can acquire information. In order to solve the variousconflicts that could possibly occur when acquiring information fromdifferent sources, we propose a classification of the informationthat an agent possesses according to credibility. Based on this classification, we formalize what itmeans for agents to have seen or heard something, or to believesomething by default. We present a formalization of observations,communication actions, and the attempted jumps to conclusions thatconstitutes default reasoning. To implement these informative actionswe use a general belief revision action which satisfies theAGM postulates; dependent on the credibility of the incominginformation this revision action acts on one or more parts ofthe classified belief sets of the agents. The abilities of agents formalizeboth the limited capacities of agents to acquire information, and the preference of one kind of information acquisition to another. A very important feature of our approach is that it shows how to integratevarious aspects of agency, in particular the (informational) attitudesof dealing with information from observation, communication and defaultreasoning into one coherent framework, both model-theoretically andsyntactically.

On agents that have the ability to choose

We demonstrate ways to incorporate nondeterminism in a system designed to formalize the reasoning of agents concerning their abilities and the results of the actions that they may perform. We distinguish between two kinds of nondeterministic choice operators: one that expresses an internal choice, in which the agent decides what action to take, and one that expresses an external choice, which cannot be influenced by the agent. The presence of abilities in our system is the reason why the usual approaches towards nondeterminism cannot be used here. The semantics that we define for nondeterministic actions is based on the idea that composite actions are unravelled in the strings of atomic actions and tests that constitute them. The main notions used in defining this semantics are finite computation sequences and finite computation runs of actions. The results that we obtain meet our intuitions regarding events and abilities in the presence of nondeterminism.