ARGUMENTATION: SEMANTICS AND ARGUMENTATION SYSTEMS

• “An Argumentation-Based Approach to Modeling Decision Support Contexts with What-If Capabilities”,
  P. Baroni, F. Cerutti, M. Giacomin, G. Guida 
  AAAI 2009 Fall Symposium, Washington, DC, November 2009.
    
  This paper describes a preliminary proposal of an argumentation-based approach to modeling articulated decision support contexts. The proposed approach encompasses a variety of argument and attack schemes aimed at representing basic knowledge and reasoning patterns for decision support. Some of the defined attack schemes involve attacks directed towards other attacks, which are not allowed in traditional argumentation frameworks but turn out to be useful as a knowledge and reasoning modeling tool: in particular, we demonstrate their use to support what-if reasoning capabilities, which are of primary importance in decision support. Formal backing to this approach is provided by the AFRA formalism, a recently proposed extension of Dung's argumentation framework. A literature example concerning a decision problem about medical treatments is adopted to illustrate the approach.
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• “Computational Properties of Resolution-based Grounded Semantics”, 
  P. Baroni, P.E. Dunne, M. Giacomin 
  Proc. of 21^st International Joint Conference on Artificial Intelligence (IJCAI 2009), 683-689, Pasadena, California, July 2009.
    
  In the context of Dung's theory of abstract argumentation frameworks, the recently introduced resolution-based grounded semantics features the unique property of fully complying with a set of general requirements, only partially satisfied by previous literature proposals. This paper contributes to the investigation of resolution-based grounded semantics by analyzing its computational properties with reference to a standard set of decision problems for abstract argumentation semantics: (a) checking the property of being an extension for a set of arguments; (b) checking agreement with traditional grounded semantics; (c) checking the existence of a non-empty extension; (d) checking credulous acceptance of an argument; (e) checking skeptical acceptance of an argument. It is shown that problems (a)-(c) admit polynomial time decision processes, while (d) is NP-complete and (e) coNP-complete.
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• “Encompassing Attacks to Attacks in Abstract Argumentation Frameworks”,
  P. Baroni, F. Cerutti, M. Giacomin, G. Guida  
  Proc. of 10th European Conference on Symbolic and Quantitative Approaches to Reasoning with Uncertainty (ECSQARU 2009), 83-94, Verona, Italy, July 2009.
    
  In the traditional definition of Dung's abstract argumentation framework (AF), the notion of attack is understood as a relation between arguments, thus bounding attacks to start from and be directed to arguments. This paper introduces a generalized definition of abstract argumentation framework called AFRA (Argumentation Framework with Recursive Attacks), where an attack is allowed to be directed towards another attack. From a conceptual point of view, we claim that this generalization supports a straightforward representation of reasoning situations which are not easily accommodated within the traditional framework. From the technical side, we first investigate the extension to the generalized framework of the basic notions of conflict-free set, acceptable argument, admissible set and of Dung's fundamental lemma. Then we propose a correspondence from the AFRA to the AF formalism, showing that it satisfies some basic desirable properties. Finally we analyze the relationships between AFRA and a similar extension of Dung's abstract argumentation framework, called EAF⁺ and derived from the recently proposed formalism EAF.
  © Springer - LNAI 5590  
    
    

• “Skepticism relations for comparing argumentation semantics",
  P. Baroni, M. Giacomin
  International Journal of Approximate Reasoning, vol. 50(6), 854-866, Elsevier Science, 2009.
    
  The issue of formalizing skepticism relations between argumentation semantics has been considered only recently in the literature. In this paper we provide a twofold contribution to this kind of analysis. First, starting from the traditional concepts of skeptical and credulous acceptance, we introduce a comprehensive set of seven skepticism relations, which provide a formal counterpart to several alternative notions of skepticism at an intuitive level. Then we carry out a systematic comparison of a significant set of literature semantics (namely grounded, complete, preferred, stable, semi-stable, ideal, prudent, and CF2 semantics) on the basis of the proposed skepticism relations.
  © Elsevier Science
    
    

• “A systematic classification of argumentation frameworks where semantics agree”,
  P. Baroni, M. Giacomin
  Proc. of 2^nd International Conference on Computational Models of Argument (COMMA 2008), 37-48, Toulouse, France, May 2008.
    
  The issue of characterizing classes of argumentation frameworks where different semantics agree has been considered in the literature with main focus on the relationships between agreement and topological properties. This paper contributes to this kind of investigation from a complementary perspective, by introducing a systematic classification of agreement classes concerning a comprehensive set of argumentation semantics on the basis of properties of their sets of extensions only. In particular, it is shown that 14 distinct classes out of 120 nominal ones exist, and a complete analysis of their set-theoretical relationships is carried out.
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• “Resolution-based argumentation semantics”,
  P. Baroni, M. Giacomin
  Proc. of 2^nd International Conference on Computational Models of Argument (COMMA 2008), 25-36, Toulouse, France, May 2008.
    
  In a recent work we have proposed a comprehensive set of evaluation criteria for argumentation semantics and we have shown that none of a set of semantics including both traditional and recent proposals is able to meet all criteria. This naturally raises the question whether such criteria are actually satisfiable altogether: this paper provides a positive answer to this question by introducing a new family of argumentation semantics, called resolution-based and showing that all the desirable criteria are met by the resolution-based version of grounded semantics.
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• “Comparing Argumentation Semantics with respect to Skepticism”,
  P. Baroni, M. Giacomin
  Proc. of 9^th European Conference on Symbolic and Quantitative Approaches to Reasoning with Uncertainty (ECSQARU 2007), 210-221, Hammamet, Tunisia, October/November 2007.
    
  The issue of formalizing skepticism relations between argumentation semantics has been considered only recently in the literature. In this paper, we contribute to this kind of analysis by providing a systematic comparison of a significant set of literature semantics (namely grounded, complete, preferred, stable, semi-stable, ideal, prudent, and CF2 semantics) using both a weak and a strong skepticism relation.
  © Springer-Verlag - LNAI 4724
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• “On principle-based evaluation of extension-based argumentation semantics”, 
  P. Baroni, M. Giacomin 
  Artificial Intelligence, vol. 171(10-15), 675-700, Elsevier Science, 2007.
     
  The increasing variety of semantics proposed in the context of Dung's theory of argumentation makes more and more inadequate the example-based approach commonly adopted for evaluating and comparing different semantics. To fill this gap, this paper provides two main contributions. First, a set of general criteria for semantics evaluation is introduced by proposing a formal counterpart to several intuitive notions related to the concepts of maximality, defense, directionality, and skepticism. Then, the proposed criteria are applied in a systematic way to a representative set of argumentation semantics available in the literature, namely grounded, complete, preferred, stable, semi-stable, ideal, prudent, and CF2 semantics.
  © Elsevier Science
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• “Characterizing defeat graphs where argumentation semantics agree”,
  P. Baroni, M. Giacomin
  Proc. of Argumentation and Non-Monotonic Reasoning LPNMR Workshop (ArgNMR), 33-48, Tempe, Arizona, May 2007.
    
  In the context of Dung's theory of argumentation frameworks, comparisons between argumentation semantics are often focused on the different behavior they show in some (more or less peculiar) cases. It is also interesting however to characterize situations where (under some reasonably general assumptions) different semantics behave exactly in the same way. Focusing on the general family of SCC-recursive argumentation semantics, the paper provides some novel results in this line. In particular, we study the characterization of defeat graphs where any SCC-recursive semantics admits exactly one extension coinciding with the grounded extension. Then, we consider the problem of agreement with stable semantics and identify the family of SCC-symmetric argumentation frameworks, where agreement is ensured for a class of multiple-status argumentation semantics including stable, preferred and CF2 semantics.  
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• “Evaluation and comparison criteria for extension-based argumentation semantics”,
  P. Baroni, M. Giacomin
  Proc. of 1^th International Conference on Computational Models of Argument (COMMA 2006), 157-168, Liverpool, England, September 2006.
    
  In the context of Dung's theory of abstract argumentation frameworks, the comparison between different semantics is often carried out by resorting to some specific examples considered particularly meaningful. This kind of comparison needs to be complemented by more general evaluation criteria based on "example-independent" basic principles. We review several principles for argumentation semantics, identify their formal counterpart in terms of extensions, and analyze their relationships with the notion of argument justification state. Then, we evaluate and compare several semantics on the basis of the introduced principles.
  © IOS Press
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• “Refining SCC decomposition in argumentation semantics: a first investigation”,
  P. Baroni, M. Giacomin
  Proc. of 11^th International Workshop on Non-Monotonic Reasoning (NMR 2006), 442-450, Lake District, England, May-June 2006.
    
  In the recently proposed SCC-recursive approach to argumentation semantics, the strongly connected components of an argumentation framework are used as the basic elements for the incremental construction of extensions. In this paper we argue that a finer decomposition, considering some suitably defined internal substructures of strongly connected components, called autonomous fragments, may be appropriate and support, in some cases, more intuitive results than the original approach. We cast this proposal within the SCC-recursive framework, show that it satisfies some fundamental requirements and provide some examples of its potential advantages.    
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• “SCC-recursiveness: a general schema for argumentation semantics”,
  P. Baroni, M. Giacomin, G. Guida
  Artificial Intelligence, vol. 168 (1-2), 162-210, Elsevier Science, 2005.
   
  In argumentation theory, Dung's abstract framework provides a unifying view of several alternative semantics based on the notion of extension. In this context, we propose a general recursive schema for argumentation semantics, based on decomposition along the strongly connected components of the argumentation framework. We introduce the fundamental notion of SCC-recursiveness and we show that all Dung's admissibility-based semantics are SCC-recursive, and therefore a special case of our schema. On these grounds, we argue that the concept of SCC-recursiveness plays a fundamental role in the study and definition of argumentation semantics. In particular, the space of SCC-recursive semantics provides an ideal basis for the investigation of new proposals: starting from the analysis of several examples where Dung's preferred semantics gives rise to questionable results, we introduce four novel SCC-recursive semantics, able to overcome the limitations of preferred semantics, while differing in other respects.
  © Elsevier Science
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• “Evaluating Argumentation Semantics with respect to Skepticism Adequacy”,
  P. Baroni, M. Giacomin
  Proc. of 8^th European Conference on Symbolic and Quantitative Approaches to Reasoning with Uncertainty (ECSQARU 2005), 329-340, Barcelona, Spain, July 2005.
    
  Analyzing argumentation semantics with respect to the notion of skepticism is an important issue for developing general and well-founded comparisons among existing approaches. In this paper, we show that the notion of skepticism plays also a significant role in order to better understand the behavior of a specific semantics in different situations. Building on an articulated classification of argument justification states into seven distinct classes and on the definition of a weak and a strong version of skepticism relation, we define the property of skepticism adequacy of an argumentation semantics, which basically consists in requiring a lesser commitment when transforming a unidirectional attack into a mutual one. We then verify the skepticism adequacy of some literature proposals and obtain the rather surprising result that some semantics fail to satisfy this basic property.
  © Springer-Verlag - LNAI 3571 
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• “Towards a Formalization of Skepticism in Extension-based Argumentation Semantics”,
  P. Baroni, M. Giacomin, G. Guida
  Proc. of 4^th Workshop on Computational Models of Natural Argument (CMNA 2004), 47-52, Valencia, Spain, August 2004.
    
  This paper provides a preliminary investigation towards the definition of a general framework for the comparison of extension-based argumentation semantics with respect to the notion of skepticism. We identify seven justification states for arguments and define two alternative skepticism relations between semantics, which induce a partial order on the justification states, reflecting the relevant levels of commitment.
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• “A General Recursive Schema for Argumentation Semantics”,
  P. Baroni, M. Giacomin
  Proc. of 16^th European Conference on Artificial Intelligence (ECAI 2004), 783-787, Valencia, Spain, August 2004.
  
  In argumentation theory, Dung's abstract framework provides a unifying view of several alternative semantics based on the notion of extension. Recently, a new semantics has been introduced to solve the problems related to counterintuitive results produced by literature proposals. In this semantics, an important role is played by a recursive schema in the definition of extensions. This paper proves that all the semantics encompassed by Dung's framework adhere to this property, not previously considered in the literature, which we call SCC-recursiveness. We argue that this notion plays a general role in the definition and computation of argumentation semantics.
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• “A recursive approach to argumentation: motivation and perspectives”,
  P. Baroni, M. Giacomin
  Proc. of 10^th International Workshop on Non-Monotonic Reasoning (NMR 2004), 50-58, Whistler BC, Canada, June 2004.
  
  In the context of Dung's argumentation framework, we propose a general recursive schema for argumentation semantics, based on decomposition along the strongly connected components of the argumentation framework. We introduce the fundamental notion of SCC-recursiveness and show that all Dung's admissibility-based semantics are SCC-recursive, and therefore a special case of our schema. The space of SCC-recursive semantics provides a basis for the investigation of novel proposals: starting from the analysis of several examples where Dung's preferred semantics gives rise to counterintuitive results, we introduce four novel SCC-recursive semantics, able to overcome the limitations of preferred semantics, while differing in other respects. Among them, the CF2 semantics, which more radically departs from the traditional notion of admissibility, turns out to be the most satisfactory in all cases and, in particular, is able to correctly deal with the thorny problem of self-defeating arguments.
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• “On the role of strongly connected components in argumentation”,
  P. Baroni, M. Giacomin
  Proc. of 10^th Information Processing and Management of Uncertainty in Knowledge-Based System Conference (IPMU 2004), 1887-1894, Perugia, Italy, July 2004.
  
  In argumentation theory, Dung's abstract framework provides a unifying view of several alternative semantics based on the notion of extension. Recently, a new semantics has been introduced to solve the problems related to counterintuitive results produced by literature proposals. In this semantics, extensions can be decomposed and constructed along the strongly connected components of the defeat graph. This paper proves that this property holds also in the context of all semantics encompassed by Dung's framework, showing that strongly connected components may play a general role in the definition and computation of argumentation semantics.
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• “Solving Semantic Problems with Odd-Length Cycles in Argumentation”,
  P. Baroni, M. Giacomin
  Proc. of 7^th European Conference on Symbolic and Quantitative Approaches to Reasoning with Uncertainty (ECSQARU 2003), LNAI 2711 Springer-Verlag, 440-451,  Aalborg, Denmark, July 2003.
  
  In the context of Dung's abstract framework for argumentation, two main semantics have been considered to assign a defeat status to arguments: the grounded semantics and the preferred semantics. While the two semantics agree in most situations, there are cases where the preferred semantics appears to be more powerful. However, we notice that the preferred semantics gives rise to counterintuitive results in some other cases, related to the presence of odd-length cycles in the attack relation between arguments. To solve these problems, we propose a new semantics which preserves the desirable properties of the preferred semantics, while correctly dealing with odd-length cycles. We check the behavior of the proposed semantics in a number of examples and discuss its relationships with both grounded and preferred semantics.
  © Springer-Verlag - LNAI 2711[PDF]
   
   

• “On the notion of strength in argumentation: overcoming the epistemic/practical dichotomy”
  P. Baroni, M. Giacomin, G. Guida
  Simon Parsons and Tony Hunter (Eds.), Proc. of ECSQARU Workshop Adventures in Argumentation, 1-8, Toulouse, France, September 2001.
  
  In this paper we provide an analysis of various argumentation approaches (mainly due to Pollock) which explicitely deal with the notion of strength. While the attribution of a strength value to arguments seems to be necessary in actual practice, the basic meaning of the notion of strength is not clear, nor its role in resolving conflicts among arguments and determining their justification. The main contribution of the paper is to show that justification is not a purely epistemic notion, and a more articulated model of reasoning activity is a possible way to settle many problems emerging in existing approaches.
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• “Extending abstract argumentation systems theory”
  P. Baroni, M. Giacomin, G. Guida
  Artificial Intelligence, vol. 120 (2), 251-270, Elsevier Science, 2000.
  
  In this paper, we extend the theory of abstract argumentation systems proposed by Vreeswijk (1997). This framework stands at a high abstraction level and provides a general model for argumentation activity. However, the theory reveals an inherent limitation in that the premises of the argumentation process are assumed to be indefeasible, and this introduces the need of an implicit constraint on the strength of the arguments, in order to preserve correctness. In many application contexts the information available to start reasoning is not guaranteed to be completely reliable, therefore it is natural to assume that premises can be discarded during the argumentation process. We extend the theory by admitting that premises can be defeated and relaxing the implicit assumption about their strength.
  Besides fixing the technical problems related to this hidden assumption (e.g. ensuring that warranted arguments are compatible), our proposal provides an integrated model for belief revision and defeasible reasoning, confirming the suitability of argumentation as a general model for the activity of intelligent reasoning in presence of various kinds of uncertainty.
  © Elsevier Science

DISTRIBUTED ARGUMENTATION, ARGUMENTATION IN MAS

• “Self-stabilizing defeat status computation: dealing with conflict-management in multi-agent systems”
  P. Baroni, M. Giacomin, G. Guida
  Artificial Intelligence, vol. 165 (2), 187-259, Elsevier Science, 2005.
  
  The role of argumentation in supporting various forms of interaction among possibly conflicting autonomous agents has been explicitly recognized in the literature. In argumentation, conflict management is carried out by the formal process of defeat status computation. In this paper we consider the generalization of this process to a distributed setting. We show that significant stabilization problems may arise even in relatively simple cases. A fundamental negative result is then proved: no general self-stabilizing algorithm exists for distributed defeat status computation, indicating that self-stabilizing algorithms for this problem can be defined only under specific conditions. Accordingly, we focus on two cases: an algorithm tailored to a specific family of inference graphs, that include only rebutting defeaters, and an algorithm that applies to any inference graph, also including undercutting defeaters, but may provide (cautiously) incorrect results for some nodes. For both algorithms the worst-case round complexity is analyzed and it is proved that no algorithms with lower complexity exist for the same tasks.
  © Elsevier Science
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• “Argumentation in Multi-Agent Systems: Self-stabilizing Defeat Status Computation”
  P. Baroni, M. Giacomin, G. Guida
  Atti dell’Ottavo Convegno dell’Associazione Italiana per l’Intelligenza Artificiale, 40, Siena, Italy, Settembre 2002.
  
  In this paper, we consider a multi-agent system where agents perform argumentation activity on the basis of knowledge both stated in their knowledge-bases and acquired from other agents. Starting from a previous proposal, we introduce a distributed approach to argumentation, removing some limiting assumptions, and we show by means of an example that the system can not be guaranteed to reach a state which satisfy a well-founded semantics, and not even to terminate. In order to tackle this problem, we devise two self-stabilizing algorithms for the computation of the defeat status, one specifically tailored to rebutting defeat, and the other able to deal with any form of defeat.
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• “Self-stabilizing Distributed Algorithms for Defeat Status Computation in Argumentation”
  M. Giacomin 
  V. Marik, O. Stepankova, H. Krautwurmova and M. Luck. (Eds.), Multi-Agent Systems and Applications II - Lecture Notes in Artificial Intelligence 2322, 137-147, Springer-Verlag, 2002.
  
  Argumentation is receiving an increasing attention as a technique for practical and uncertain reasoning underlying the realization of intelligent autonomous agents. In line with the evolution of agent architectures towards distribution, we propose a distributed approach to argumentation, where independent processes construct arguments and revise their defeat status by exploiting local information only. We present and compare two distributed self-stabilizing algorithms for defeat status computation, one specifically tailored to rebutting defeat, and the other able to handle any form of defeat. The property of self-stabilization is enforced to ensure a globally correct behavior of the system.
  © Springer-Verlag - LNAI 2322 [PDF]
   
   

• “Self-stabilizing Distributed Algorithms for Argumentation”
  M. Giacomin 
  V. Marik, O. Stepankova, H. Krautwurmova, J-P. Briot (Eds.), Proc. of Multi-Agent Systems and Applications – ACAI 2001 & EASSS 2001 Student Sessions, 40-47, Prague, Czech Republic, July 2001.
  
  Argumentation is receiving an increasing attention as a technique for practical and uncertain reasoning underlying the realization of intelligent autonomous agents. In line with the evolution of agent architectures towards distribution, we propose a distributed approach to argumentation, where independent processes construct arguments and revise their defeat status by exploiting local information only. We present and compare two distributed self-stabilizing algorithms for defeat status computation. The property of self-stabilization is enforced to ensure a globally correct behavior of the system.
   
   

• “Argumentation through a distributed self-stabilizing approach”
  P. Baroni, M. Giacomin
  Journal of Experimental and Theoretical Artificial Intelligence, vol. 14 (4), 273-301, Taylor & Francis, 2002.
  
  Argumentation is receiving an increasing attention as a technique for practical and uncertain reasoning underlying the realization of intelligent autonomous agents. Since a decentralized organization has been proposed by several authors as an appropriate paradigm for the design of agent architectures, we propose in this paper a distributed approach to argumentation, in which several independent asynchronous processes carry out argumentation activity by exploiting local information only. The final result of this process is the computation of the defeat status of the arguments: we devise a general distributed algorithm, which does not rely on any specific notion of defeat between arguments. The issue of coordination has been explicitly tackled by ensuring the property of self-stabilization for the algorithm. A proof of its correctness, as well as an analysis of its complexity, is provided.
  © Taylor & Francis
   
   

• “A Distributed Self-stabilizing Approach to Defeat Status Computation in Argumentation”
  P. Baroni, M. Giacomin
  Proc. of 9^th Information Processing and Management of Uncertainty in Knowledge-Based System Conference (IPMU 2002), 1687-1694, Annecy, France, July 2002.
  
  Argumentation is an approach to practical uncertain reasoning which is receiving an increasing attention in the field of autonomous agents. Since decentralized agent architectures have been advocated by several authors, we propose a distributed approach to argumentation, in which independent processes carry out argumentation activity exploiting local information only. The issue of coordination has been explicitely tackled by devising a general self-stabilizing algorithm for the defeat status computation of arguments.
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• “A distributed self-stabilizing algorithm for argumentation”
  P. Baroni, M. Giacomin
  Proc. of 15^th 2001 International Parallel and Distributed Processing Symposium (IPDPS2001), IEEE Press, San Francisco, CA, April 2001.
  
  We propose a distributed approach to argumentation in which arguments are constructed by asynchronous processes which compute their own defeat status on the basis of local information. We present a self-stabilizing algorithm for the computation of defeat status, along with a proof of its correctness. The proposed approach introduces a remarkable flexibility in the management of argumentation activity with respect to a centralized approach. Moreover, it is in line with the evolution of autonomous agents architectures, one of the main application fields of argumentation systems.
  © IEEE (see copyright notice) [PDF]

AGENT ARCHITECTURES
 

• “Distributed Cognition through Active Mental Entities: an Argumentation-Based Approach”
  P. Baroni, D. Fogli, M. Giacomin, G. Guida
  Cognitive Science Quarterly, vol. 2 (3-4), 385-404, Lavoisier, 2002.
  
  This paper introduces the active mental entities approach to the design of intelligent autonomous agents. In this approach, mental attitudes are conceived as active computational entities and the overall mental activity of an agent results from their independent operation and interaction. In order to provide a formal support to this approach, we propose argumentation theory as a basis for the definition of interaction mechanisms among active mental entities. To this purpose, a novel distributed argumentation algorithm is devised. Finally, we give some operation examples in the context of a robotic application.
  © Lavoisier