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Projekt 1: Rekonstruktion der Sprechakttheorie
Projekt 2: Pragmatische Implikationen
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Projekt 5: Computationale Dialektik
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Folgende Texte sind im Rahmen der Forschergruppe bisher entstanden:
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Booth 2000
The lexicographic closure as a revision process
In: Proceedings of Belief Revision Track of NMR2000, International Workshop on Nonmonotonic Reasoning, Breckenridge
Erweiterte Fassung erscheint in: Journal of Applied Non-Classical Logics
The connections between nonmonotonic reasoning and belief revision are well-known. A central problem in the area of nonmonotonic reasoning is the problem of default entailment, i.e., when should an item of default information representing "if A is true then, normally, B is true" be said to follow from a given set of items of such information. Many answers to this question have been proposed but, surprisingly, virtually none have attempted any explicit connection to belief revision. The aim of this paper is to give an example of how such a connection can be made by showing how the lexicographic closure of a set of defaults may be conceptualised as a process of iterated revision by sets of sentences. Specifically we use the revision method of Nayak.
Booth & Paris 1998
A note on the rational closure of knowledge bases with both positive and negative knowledge
In: Journal of Logic, Language and Information 7
The notion of the rational closure of a positive knowledge base K of conditional assertions A =B (standing for "if A then normally B") was first introduced by Lehmann (1989) and developed by Lehmann and Magidor (1992). Following those authors we would also argue that the rational closure is, in a strong sense, the minimal information, or simplest, rational consequence relation satisfying K. In practice, however, one might expect a knowledge base to consist not just of positive conditional assertions, A =B, but also negative conditional assertions not{A =B}. Restricting ourselves to a finite language we show that the rational closure still exists for satisfiable knowledge bases containing both positive and negative conditional assertions and has similar properties to those exhibited in Lehmann and Magidor (1992). In particular an algorithm in Lehmann and Magidor (1992) which constructs the rational closure can be adapted to this case and yields, in turn, completeness theorems for the conditional assertions entailed by such a mixed knowledge base.
Brewka 1999
Logic Programs with Dynamic Preference Information: Abridged Report
In: D. Dubois & H. Prade (Hrsg.), Fuzzy Sets, Logics and Reasoning About Knowledge, Dordrecht
The paper describes an extension of well-founded semantics for logic programs with two types of negation. In this extension information about preferences between rules can be expressed in the logical language and derived dynamically. This is achieved by using a reserved predicate symbol and a naming technique. Conflicts among rules are resolved whenever possible on the basis of derived preference information. A legal reasoning example illustrates the usefulness of the approach.
Brewka 2000a
Declarative Representation of Revision Strategies
In: W. Horn (Hrsg.), Proceedings of the European Conference on Artificial Intelligence, Berlin
Erweiterte Fassung erscheint in: Journal of Applied Non-Classical Logics
We introduce a nonmonotonic framework for belief revision in which reasoning about the reliability of different pieces of information based on meta-knowledge about the information is possible. The approach is based on a Poole-style system for default reasoning in which entrenchment information is represented in the logical language. A notion of inference based on the least fixed point of a monotone operator is used to make sure that all theories possess a consistent set of conclusions.
Brewka 2000b
Dynamic Argument Systems: A Formal Model of Argumentation Processes Based on Situation Calculus
Erscheint in: Journal of Logic and Computation
We present a formal model of argumentation based on situation calculus which captures both the logical and the procedural aspects of argumentation processes. The logic is used to determine what is accepted by each agent participating in the discussion and by the group as a whole, on the basis of the speech acts performed during argumentation. Argumentation protocols, also called rules of order, describe declaratively which speech acts are legal in a particular state of the argumentation. We first discuss argumentation with fixed rules of order. Our model tolerates protocol violations but makes it possible to object to illegal actions. In realistic settings the rules of order themselves can at any time become the topic of the debate. We show how meta level argumentation of this kind can be modelled in what we call dynamic argument systems. To illustrate the notions introduced in the paper we present a reconstruction of Rescher's theory of formal disputation and a dynamic argument system with three levels which we use to discuss a murder case.
Brewka 2000c
Representing Meta-Knowledge in Poole-Systems
Erscheint in: Studia Logica
We show how Poole-systems can be extended to take meta-information into account adequately. The meta-information is used to guide the choice of formulas accepted by the reasoner as premises. Existence of a consistent set of conclusions is guaranteed by a least fixpoint construction. The proposed formalism has useful applications in defeasible reasoning, knowledge base fusion and belief revision.
Brewka & Dix 1998
Knowledge Representation with Logic Programs
In: J. Dix, L.M. Pereira & T. Przymusinski (Hrsg.), Logic Programming and Knowledge Representation, Heidelberg
In this overview we show how Knowledge Representation (KR) can be done with the help of generalized logic programs. We start by introducing the core of PROLOG, which is based on definite logic programs. Although this class is very restricted (and will be enriched by various additional features in the rest of the paper), it has a very nice property for KR-tasks: there exist efficient Query-answering procedures - a Top-Down approach and a Bottom-Up evaluation. In addition we can not only handle ground queries but also queries with variables and compute answer-substitutions.
It turns out that more advanced KR-tasks can not be properly handled with definite programs. Therefore we extend this basic class of programs by additional features like Negation-as-Failure, Default-Negation, Explicit Negation, Preferences, and Disjunction. The need for theses extensions is motivated by suitable examples and the corresponding semantics are discussed in detail.
Clearly, the more expressive the respective class of programs under a certain semantics is, the less efficient are potential Query-answering methods. This point will be illustrated and discussed for every extension. By well-known recursion-theoretic results, it is obvious that there do not exist complete Query-answering procedures for the general case where variable and function symbols are allowed. Nevertheless we consider it an important topic of further research to extract feasible classes of programs where answer-substitutions can be computed.
Brewka & Eiter 1999
Preferred Answer Sets for Extended Logic Programs
In: Artificial Intelligence 109
Eine kürzere Version dieses Textes ist In: A.G. Cohn, L. Schubert & S.C. Shapiro (Hrsg.), Proceedings of the 6th International Conference on Principles of Knowledge Representation and Reasoning, 1998
In this paper, we address the issue of how Gelfond and Lifschitz's answer set semantics for extended logic programs can be suitably modified to handle prioritized programs. In such programs an ordering on the program rules is used to express preferences. We show how this ordering can be used to define preferred answer sets and thus to increase the set of consequences of a program. We define a strong and a weak notion of preferred answer sets. The first takes preferences more seriously, while the second guarantees the existence of a preferred answer set for programs possessing at least one answer set.
Adding priorities to rules is not new, and has been explored in different contexts. However, we show that many approaches to priority handling, most of which are inherited from closely related formalisms like default logic, are not suitable and fail on intuitive examples. Our approach, which obeys abstract, general principles that any approach to prioritized knowledge representation should satisfy, handles them in the expected way. Moreover, we investigate the complexity of our approach. It appears that strong preference on answer sets does not add on the complexity of the principal reasoning tasks, and weak preference leads only to a mild increase in complexity.
Brewka & Eiter 2000
Prioritizing Default Logic
In: S. Hoelldobler (Hrsg.), Festschrift 60th Anniversary of W. Bibel, Dordrecht
A number of prioritized variants of Reiter's default logic have been described in the literature. In this paper, we introduce two natural principles for preference handling and show that all existing approaches fail to satisfy them. We develop a new approach which does not suffer from these shortcomings. We start with the simplest case, supernormal default theories, where preferences are handled in a straightforward manner. The generalization to prerequisite-free default theories is based on an additional fixed point condition for extensions. The full generalization to arbitrary default theories uses a reduction of default theories to prerequisite-free theories. The reduction can be viewed as dual to the Gelfond/Lifschitz reduction used in logic programming for the definition of answer sets. We finally show how preference information can be represented in the logical language.
Wagner 1998
Foundations of Knowledge Systems with Applications to Databases and Agents
Boston, Dordrecht & London
Nach einer allgemeinen Einführung in einige fundamentale Fragen der
Wissenrepräsentation wird in den folgenden Kapiteln eine Reihe grundlegender positiver Wissenssysteme, wie Relationale, Temporale, Disjunktive und Fuzzy-Datenbanken, in einem einheitlichen Rahmen vorgestellt. Es wird gezeigt, wie diese Systeme zu erweitern sind, so dass sie die Repräsentation negativer Information erlauben. Alle zuvor behandelten extensionalen Wissensbasen können durch Hinzufügung von Deduktionsregeln zu deduktiven Wissensbasen erweitert werden. In deduktiven Wissensbasen können Prädikate sowohl extensional, d.h. durch die Angabe ihrer Extension in der Form einer Tabelle, als auch intensional, d.h. durch die Angabe von Deduktionsregeln, repräsentiert werden. Die Semantik deduktiver Wissensbasen, d.h. die Definition der jeweiligen Inferenzrelation, folgt aus der Definition eines intendierten Abschlusses. Es wird gezeigt, wie der Begriff eines stabil erzeugten Abschlusses (ohne Bezug zur Modelltheorie) zu definieren ist. Zur Spezifikation der Kommunikation zwischen Wissensbasen werden vierteilige Reaktionsregeln vorgeschlagen. Sie werden von einem Kommunikationsereignis getriggert und führen in Abhängigkeit von einer epistemischen Bedingung zu einer kommunikativen Reaktion, die mit einem epistemischen Effekt einhergeht.
Wagner 1999
Knowledge Systems, Artificial Agents, and Computational Dialectics
Als Preprint in: Beiträge der Forschergruppe Kommunikatives Verstehen
In this preliminary report, we discuss the basic concepts of argumentation both in the context of the internal reasoning of an agent and in the context of agent-to-agent communication. We propose a model-theoretic semantics for a fairly general concept of an argumentation system where arguments are stable grounded chains of facts and rules. We introduce the notion of a conservative model of an argumentation base. Not every conservative model of an argumentation base qualifies as a coherent view implicit in it. We propose to take the stable generated conservative models as representing the coherent views that are supported by an argumentation base.
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