Valid attacks in argumentation frameworks with recursive attacks

The purpose of this work is to study a generalisation of Dung’s abstract argumentation frameworks that allows representing recursive attacks, that is, a class of attacks whose targets are other attacks. We do this by developing a theory of argumentation where the classic role of attacks in defeating arguments is replaced by a subset of them, which is extension dependent and which, intuitively, represents a set of “valid attacks” with respect to the extension. The studied theory displays a conservative generalisation of Dung’s semantics (complete, preferred and stable) and also of its principles (conflictfreeness, acceptability and admissibility). Furthermore, despite its conceptual differences, we are also able to show that our theory agrees with the AFRA interpretation of recursive attacks for the complete, preferred and stable semantics

A Proposal to Embed the In Dubio Pro Reo Principle into Abstract Argumentation Semantics based on Topological Ordering and Undecidedness Propagation

Abstract. In this paper we discuss how the in dubio pro reo principle and the corresponding standard of proof beyond reasonable doubt can be modelled in abstract argumentation. The in dubio pro reo principle protects arguments against attacks from doubtful arguments. We identify doubtful arguments with a subset of undecided arguments, called active undecided arguments, consisting of cyclic arguments responsible for generating the undecided situation. We obtain the standard of proof beyond reasonable doubt by imposing that attacks from doubtful undecided arguments are not enough to change the acceptability status of an attacked argument (the reo). The resulting semantics, called SCCvoid semantics, are defined using a SCC-recursive schema. The semantics are conflict-free, non-admissible (in Dung’s sense), but employing a more relaxed defence-based notion of admissibility; they allow reinstatement and they accept credulously what the corresponding complete semantics accepts at least credulously

Towards a Computational Analysis of Probabilistic Argumentation Frameworks

In this paper we analyze probabilistic argumentation frameworks (PAFs), defined as an extension of Dung abstract argumentation frameworks in which each argument n is asserted with a probability p(n). The debate around PAFs has so far centered on their theoretical definition and basic properties. This work contributes to their computational analysis by proposing a first recursive algorithm to compute the probability of acceptance of each argument under grounded and preferred semantics, and by studying the behavior of PAFs with respect to reinstatement, cycles and changes in argument structure. The computational tools proposed may provide strategic information for agents selecting the next step in an open argumentation process and they represent a contribution in the debate about gradualism in abstract argumentation

Beyond reasonable doubt: a proposal for undecidedness blocking in abstract argumentation

In Dung’s abstract semantics, the label undecided is always propagated from the attacker to the attacked argument, unless the latter is also attacked by an accepted argument. In this work we propose undecidedness blocking abstract argumentation semantics where the undecided label is confined to the strong connected component where it was generated and it is not propagated to the other parts of the argumentation graph. We show how undecidedness blocking is a fundamental reasoning pattern absent in abstract argumentation but present in similar fashion in the ambiguity blocking semantics of Defeasible logic, in the beyond reasonable doubt legal principle or when someone gives someone else the benefit of the doubt. The resulting semantics, called SCC-void semantics, are defined using an SCC-recursive schema. The semantics are conflict-free and non-admissible, but they incorporate a more relaxed defence-based notion of admissibility. They allow reinstatement and they credulously accept what the corresponding Dung’s complete semantics accepts at least credulously

Joint attacks and accrual in argumentation frameworks

While modelling arguments, it is often useful to represent joint attacks, i.e., cases where multiple arguments jointly attack another (note that this is different from the case where multiple arguments attack another in isolation). Based on this remark, the notion of joint attacks has been proposed as a useful extension of classical Abstract Argumentation Frameworks, and has been shown to constitute a genuine extension in terms of expressive power. In this chapter, we review various works considering the notion of joint attacks from various perspectives, including abstract and structured frameworks. Moreover, we present results detailing the relation among frameworks with joint attacks and classical argumentation frameworks, computational aspects, and applications of joint attacks. Last but not least, we propose a roadmap for future research on the subject, identifying gaps in current research and important research directions.Fil: Bikakis, Antonis. University College London; Estados UnidosFil: Cohen, Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Ciencias e Ingeniería de la Computación. Universidad Nacional del Sur. Departamento de Ciencias e Ingeniería de la Computación. Instituto de Ciencias e Ingeniería de la Computación; ArgentinaFil: Dvoák, Wolfgang. Technische Universitat Wien; AustriaFil: Flouris, Giorgos. Foundation for Research and Technology; GreciaFil: Parsons, Simon. University of Lincoln; Reino Unid

Extension-based Semantics of Abstract Dialectical Frameworks

One of the most prominent tools for abstract argumentation is the Dung's framework, AF for short. It is accompanied by a variety of semantics including grounded, complete, preferred and stable. Although powerful, AFs have their shortcomings, which led to development of numerous enrichments. Among the most general ones are the abstract dialectical frameworks, also known as the ADFs. They make use of the so-called acceptance conditions to represent arbitrary relations. This level of abstraction brings not only new challenges, but also requires addressing existing problems in the field. One of the most controversial issues, recognized not only in argumentation, concerns the support cycles. In this paper we introduce a new method to ensure acyclicity of the chosen arguments and present a family of extension-based semantics built on it. We also continue our research on the semantics that permit cycles and fill in the gaps from the previous works. Moreover, we provide ADF versions of the properties known from the Dung setting. Finally, we also introduce a classification of the developed sub-semantics and relate them to the existing labeling-based approaches.Comment: To appear in the Proceedings of the 15th International Workshop on Non-Monotonic Reasoning (NMR 2014

Algorithms for enriched abstract argumentation frameworks for large-scale cases

La théorie de l'argumentation abstraite propose des méthodes pour représenter et traiter les informations potentiellement incohérentes, et pour en tirer des conclusions ou prendre des décisions. Une telle approche est dite abstraite car elle se concentre uniquement sur la manière dont les arguments s'influencent mutuellement et pas sur la constitution des arguments. Les arguments sont donc considérés comme des entités génériques qui interagissent positivement (relation de support) ou négativement (relation d'attaque) les unes avec les autres. Ce niveau d'abstraction permet de proposer des processus de raisonnement génériques qui peuvent être appliqués à toute définition ou formalisme concret des arguments. Le modèle de raisonnement basé sur l'argumentation est appliqué dans les systèmes multi-agents depuis des années. Le développement des techniques d'argumentation et de leur calcul est un point clé de ces applications. C'est la motivation même de mon travail : améliorer l'utilisation de l'argumentation abstraite en développant de meilleurs outils pour sa mise en oeuvre. De nombreux cadres d'argumentation et sémantiques associées ont été proposés dans la littérature pour améliorer l'expressivité de l'argumentation abstraite. Alors qu'un cadre donné spécifie la manière de représenter et d'exprimer un problème d'argumentation (types de relations entre les arguments, poids des attaques ou des arguments, relation d'ordre supérieur, etc.), une sémantique, pour un cadre d'argumentation spécifique, capture ce qui est une solution d'un problème d'argumentation, dans le sens de ce qui est acceptable. Dans mon travail, je me suis d'abord concentré sur la résolution efficace de certains problèmes d'argumentation qui sont exprimés dans le cadre d'argumentation classique et les sémantiques définis par Dung. Les sémantiques de Dung produisent des ensembles d'arguments conjointement acceptables, appelés extensions. Mon travail a conduit à la proposition d'un nouvel algorithme distribué et basé sur une technique de clustering pour calculer les extensions sous les sémantiques de Dung. Il a été conçu pour certains types de cadres d'argumentation de "grande échelle", produisant un grand nombre d'extensions. Il a été implémenté et testé. Les résultats des tests montrent toute son efficacité pour les cadres d'argumentation à grande échelle ciblés. Je me suis ensuite intéressé aux cadres d'argumentation d'ordre supérieur, et en particulier au cadre d'argumentation récursif (RAF). Dans ce contexte, une attaque peut avoir comme cible une autre attaque : un argument peut ainsi être acceptable alors même qu'il est attaqué parce que cette attaque (recevant elle-même une attaque) peut être invalide, et donc non pertinente contre sa cible. Là où le cadre de Dung produit des extensions, les sémantiques des RAF produisent des "structures", des paires dont le premier élément est un ensemble d'arguments et le second un ensemble d'attaques. Si des algorithmes existaient déjà pour le cadre de Dung, il n'en était pas de même pour les RAF. J'ai donc commencé par étudier la complexité des sémantiques des RAF. J'ai ensuite étendu la notion de labelling aux RAF, une autre caractérisation de l'acceptabilité déjà existante dans le cadre de Dung. La notion de "composante fortement connexe" a été élargie aux RAF, et les propriétés de décomposabilité des sémantiques des RAF ont été étudiées. Toutes ces contributions ouvrent la voie à de futurs algorithmes pour calculer l'acceptabilité sous plusieurs sémantiques des RAF.Abstract argumentation theory proposes methods to represent and deal with contentious information, and to draw conclusions or take decision from it. Such an abstract approach focuses on how arguments affect each other. Arguments are seen as generic entities which interact positively (support relation) or negatively (attack relation) with each other. This abstraction level allows to propose generic reasoning processes that can be applied to any concrete definition or formalism for arguments. Argumentation-based reasoning model has been of application in multi-agent systems for years now. The development of argumentation techniques and of their computation drives such applications. This is the very motivation of this thesis: enhancing the use of abstract argumentation by developing better tools for its application. A lot of frameworks and semantics have been proposed to enhance expressivity in abstract argumentation. While a given framework specifies the way of representing and expressing an argumentation problem (types of relations between arguments, weight on attacks or arguments, higher-order relation, etc.), a semantics, defined for a specific argumentation framework, captures what is a solution of an argumentation problem, in the sense of what is acceptable. In this thesis, I first focus on solving more efficiently argumentation problems which are expressed in the basic, seminal argumentation framework and semantics defined by Dung. Dung's semantics produce sets of jointly acceptable arguments, called extensions. A new distributed and clustering based algorithm to compute Dung's semantics is my first contribution. This algorithm has been designed for certain types of large-scale argumentation frameworks, that produce a large number of extensions. It has been implemented and tested. The results of these tests show its efficiency in the context of the large scale argumentation frameworks which are targeted. Second, I focus on argumentation frameworks with higher order attacks, and especially Recursive Argumentation Frameworks (RAF). In this context, an attack may have as target an attack: an argument may thus be acceptable while one of its attack (receiving itself an attack) may be invalid, and so non pertinent against its target. Similarly to Dung's semantics which produce extensions, the RAF semantics produce "structures", pairs whose first element is a set of arguments and the second a set of attacks. If algorithms already existed for Dung's framework, it was not the case for RAF. In order to address this issue, I start with studying the complexity of RAF semantics. I then extend the notion of labelling to RAF, another kind of characterization of acceptability which already existed for Dung's framework. The notion of "strongly connected component" is extended to RAF and decomposability properties of RAF semantics are studied. All these contributions pave the way for future algorithms to compute acceptability under RAF semantics