An Argumentation Framework Supporting Evidential Reasoning with Applications to Contract ...

131 Pages·2007·0.9 MB·English

An Argumentation Framework Supporting
Evidential Reasoning with Applications to
Contract Monitoring
Nir Oren
Athesispresentedforthedegreeof
DoctorofPhilosophy
atthe
UniversityofAberdeen.
DepartmentofComputingScience
2007
Declaration
Noportionoftheworkcontainedinthisdocumenthasbeensubmittedinsupportofanapplication
for a degree or qualification of this or any other university or other institution of learning. All
verbatimextractshavebeendistinguishedbyquotationmarks,andallsourcesofinformationhave
beenspecificallyacknowledged.
Signed:
Date: 2007
Abstract
Inthisthesiswepresentaframeworkforargumentwhichaimstosupportconceptsusefulforevi-
dentialreasoning. Theframeworkispresentedattwolevels,anabstractlevel,inwhicharguments
are not treated as concrete entities, and a concrete level, in which arguments are underpinned by
SubjectiveLogic. Attheabstractlevel, theframeworkprovidesthetheoreticalunderpinningsfor
the concept ofsupport in a manner distinctfrom other frameworks. Support for an argumentcan
meanthattheargumentmaybeinferredfromexistingarguments,orthattheargumentisaprima
facieargument. Attheconcretelevel,supportforburdenofproof,argumentschemesandsensing
actions is introduced. A simple dialogue game is also presented, which shows how agents can
usetheframeworktoreasonabouttheirenvironment. Finallyweintroduceaheuristicthatallows
agentsparticipatinginadialoguetodecide,givenmultiplepossibleutterances,whichutteranceto
advance.
Thesecondpartofthethesisconcernsitselfwiththeapplicationoftheargumentframework
tothecontractmonitoringdomain. Weshowhowtheconcreteformoftheframeworkallowsaset
of agents, possibly with conflicting goals, to agree on the most likely state of a contract, and to
reachadecisionregardingwhatpenaltiesshouldbeimposedifthecontracthasbeenviolated.
The main contributions of this thesis include an abstract argumentation framework that in-
troduces enhancements over traditional frameworks, and is geared towards evidential reasoning;
theframeworkisinstantiatedinanovelwayusingSubjectiveLogic. Thisinstantiationallowsus
to represent important concepts such as burden of proof and argument schemes. Another contri-
bution involves the ability to introduce evidence via the sensors within the framework. Sensors
arecriticalforevidentialreasoning,buthavenotreceivedmuchattentionfromtheargumentation
community.
We then introduce two contributions that demonstrate the validity of the model in a practi-
cal application, namely in the area of automated contracting. We describe a simple contracting
language, and show how agents may use the argument framework to reason about the state of a
contract.
Salient Points of the Thesis
• We introduce an evidence based abstract argument framework containing the notions of
evidentialsupportandattackbetweenarguments.
• We describe a concrete argument framework based on Subjective Logic opinions. This
framework provides support for a number of important concepts including argument
schemes,burdenofproof,andsomeinstancesofaccrualofargument.
• Adialoguegameallowingagenttointroduceevidenceintoanargumentisintroduced.
• We show how an agent may decide which argument to advance, and which evidence to
introduceintoanongoingdialogue.
• Wedescribeanewcontractinglanguage.
• We show how the argument framework may be used in conjunction with the contracting
language to allow a set of agents to reason about the state of a contract within a partially
observabledomain.
Acknowledgements
First and foremost, I’d like to thank Ruth Woods for, well, pretty much everything. This thesis
wouldn’tbeheretodaywithouthersupportandencouragement. I’malsogratefultomysupervi-
sors,TimNormanandAlunPreece,foralltheiradvice,ideasandhelp.
Of course, these are not the only people who have had a direct, or indirect hand in the com-
pletion of this thesis. So, I’d like to thank, in no particular order, the following people, without
whomthisthesiswouldhavetakenalotmore(or,insomeinstances,alotless)time:
Gideon, HanaandIrisOren, MadalinaCroitoru, FrankGuerin, JoeyLam, GunnarGrimnes,
MartinKollingbaum,TinaGotschi,andtheentireWoodsclan.
This research was partly funded by the DTI/EPSRC E-Science Core Program and British
Telecom, via a grant for the CONOISE-G project. It is continuing through participation in the
InternationalTechnologyAlliancesponsoredbytheU.S.ArmyResearchLaboratoryandtheU.K.
MinistryofDefence.
Contents
1 Introduction 1
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 ResearchHypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4 ContributionsofthisResearch . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 ThesisStructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.6 RelatedPublications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Background 11
2.1 Argumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 ArgumentationFrameworks . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.2 AccrualsofArgument . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.3 ArgumentSchemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.1.4 Toulmin’sRepresentationofArgument . . . . . . . . . . . . . . . . . . 23
2.1.5 BurdenofProof. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.1.6 DialogueGames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.1.7 HeuristicsforArgument . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Contracting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 SubjectiveLogic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3 EvidentialArgumentationFrameworks 36
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 TheNeedforSupport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3 ReasoningwithSupport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.1 Arguments,AttackandSupport . . . . . . . . . . . . . . . . . . . . . . 40
3.3.2 E-PreferredExtensions . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.3.3 E-StableExtensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.3.4 E-GroundedExtensions . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4 AConcreteArgumentationFramework 49
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2 TheLogicLayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Contents v
4.3 TheDialecticLayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4 TheProceduralLayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.5 TheHeuristicLayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.5.1 LearningfromEvidence . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.6.1 TheLogicalandDialecticLayers . . . . . . . . . . . . . . . . . . . . . 70
4.6.2 TheProceduralandHeuristicLayers . . . . . . . . . . . . . . . . . . . . 74
4.6.3 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5 Contracting 78
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.2 Contracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.3 ContractingLanguageFeatures . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.4 ASimpleContractingLanguage . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.5 UsingContractsinaDialogue . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.5.1 TheFamilyofFundamentalArgumentSchemesforContracting . . . . . 88
5.5.2 HandlingVariablesinContracts . . . . . . . . . . . . . . . . . . . . . . 89
5.5.3 DetectingClauseApplicability . . . . . . . . . . . . . . . . . . . . . . . 93
5.5.4 ViolationsandNorms . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.5.5 ClausesandNorms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.5.6 DealingwithViolations . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.6 ArguingaboutContracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6 Conclusions 106
6.1 FutureWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.1.1 TheAbstractArgumentFramework . . . . . . . . . . . . . . . . . . . . 106
6.1.2 TheConcreteArgumentationFramework . . . . . . . . . . . . . . . . . 107
6.1.3 Contracting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6.2 ValidatingtheResearchHypothesis . . . . . . . . . . . . . . . . . . . . . . . . 110
6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
List of Tables
5.1 Thesimilaritiesbetweenanargumentschemeandacontractclause. . . . . . . . 81
5.2 ThecontractinglanguagesyntaxinBNFform.. . . . . . . . . . . . . . . . . . . 87
5.3 Representingthecontractusingthenewcontractinglanguage. . . . . . . . . . . 88
List of Figures
2.1 ASummaryofthetopicscoveredinthebackgroundchapter. . . . . . . . . . . . 12
2.2 Asimpleargumentationframework. . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 ThecomponentsofanargumentaccordingtoToulmin. . . . . . . . . . . . . . . 23
2.4 Aframeofdiscernment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.1 Possiblerepresentationsinanargumentframeworkofthesentencep,p → q,q. . 37
3.2 An implicit representation of support in an argument framework containing only
theattackrelation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3 A set of arguments and their interactions in a framework containing support and
attack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4 Anillustrationofsupportwithintheevidentialframework. . . . . . . . . . . . . 41
3.5 Anillustrationofasupportedattack. . . . . . . . . . . . . . . . . . . . . . . . . 42
3.6 Anargumentsystemshowingsupportedarguments,supportattacks,acceptability
andadmissibility. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.7 Representingtheflyingexampleusingtheevidentialframework. . . . . . . . . . 46
4.1 Anexampleillustratingtheshortcomingsoftheabstractframework. . . . . . . . 49
4.2 Anargumentgraphwithevidenceandwarrantnodes. . . . . . . . . . . . . . . . 51
4.3 An illustration of how evidential nodes and argument schemes may interact with
eachother. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4 An algorithm to compute conclusions given a set of argument schemes, instanti-
atedarguments,and,optionally,someopinions. . . . . . . . . . . . . . . . . . . 61
4.5 A “translated” argument graph containing the mutually exclusive pieces of evi-
denceaandb. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.6 Screenshotoftheuserinterfacetotheimplementedconcreteframework. . . . . . 76
5.1 AnEnglish-languagecontract. . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2 Apossible,butfaulty,formalisationofanargumentschemerepresentingcontract
clauses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.3 Thecorrectedargumentschemerepresentationofacontractclause.. . . . . . . . 83
5.4 Thealgorithmusedtodeterminewhichvariablesmaybeboundwithinaclause. . 91
5.5 Agraphicalrepresentationofanutterance. . . . . . . . . . . . . . . . . . . . . . 100
5.6 Agraphicalrepresentationofhowonetypeofaccrualmaybedealtwith. . . . . . 101
ListofFigures viii
6.1 A sample set of arguments showing why valuations and admissibility computa-
tionsareinterdependent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107