| 1 › Floris Bex / Centre for Law and ICT › Henry Prakken / Centre for Law and ICT Dept. of ICS, Utrecht University Investigating stories in a formal dialogue game

| 2 Introduction ›Logical framework for analysing and reasoning with evidence and stories in criminal cases. ›Limitation: provides a static viewpoint. ›Formal dialogue game to model the dynamics of developing and refining an analysis of a case. Analysts build and refine stories (explanations) and support them with evidence. Determine the best story in an adversarial setting.

| 3 Contents of this talk ›Summary of the framework for evidential reasoning with stories and arguments. ›A dialogue game for the analysis of evidential stories and arguments. ›Example dialogue. ›Conclusion

| 4 Two approaches to evidential reasoning ›Story-based approach Construct and compare stories about what happened in a case. Modelled as abductive inference to the best explanation (IBE). ›(Wigmorean) argument graph approach. Construct argument graphs from sources of evidence to conclusion. Modelled with formal argumentation theory.

| 5 Abductive inference to the best explanation ›Stories are modelled as causal networks. ›Given: a causal theory T consisting of causal rules of the form event1  C event2. explananda F (facts to be explained). ›Hypothesize a set of causes H such that H  T logically implies F (“explains F”). ›Compare different hypothesis according to some criteria.

| 6 Different explanations Event1 Event3Event2 Explanandum

| 7 Different explanations Event1 Event3Event2 Explanandum

| 8 Evidential arguments ›Arguments are constructed using input (evidence) and evidential generalizations of the form P  E Q. ›Arguments have a tree structure. ›An argument can be rebut or undercut by another argument. ›An argument can be justified, overruled or defensible.

| 9 Arguments ›Attacking arguments John says ”Rijkbloem shot my husband!” John shot peter Witness 1 says ”John shot Peter” Witness 1 says ”John shot Peter” Witness says “P”  E P Witness 2 says ”John did not shoot Peter” John did not shoot Peter Witness 1 says ”John shot Peter”

| 10 Arguments John says ”Rijkbloem shot my husband!” John shot peter Witness 1 says ”John shot Peter” Witness 1 says ”John shot Peter” Witness 1 is not trustworthy John shot peter ›Attacking arguments

| 11 Combining the theories ›The stories are modelled as causal networks Different stories = different explanations ›Sources of evidence are connected to the stories using evidential arguments. ›Explanations are compared by how many sources of evidence are covered: An explanation S covers a piece of evidence P if there is a (non-overruled) argument from P to an event in S

| 12 The dialogue game ›Game between two players. ›Players build a model that contains explanations supported by arguments. ›Elements of the dialogue game: Communication language (speech acts) Commitment rules Protocol Turntaking and winning

| 13 Communication language ›Argue AR The speaker states an argument AR ›Explain (E, S) The speaker provides an explanation S for E ›Concede / retract p The speaker concedes or retracts p

| 14 Protocol ›Legality of moves: a move must be a sensible operation on the evidential framework, e.g.: ›Arguments may be stated: to attack other arguments to attack explanations to increase evidential support ›Explanations may be given for propositions.

| 15 Turntaking and winning ›Adversarial setting: each player must try to advance and support his own explanation. ›Current winner: player who is committed to the explanation with the best evidential coverage. ›Players must try to become the current winner by giving explanations, supporting and attacking explanations.

| 16 Example: the Haaknat case ›A supermarket is robbed and the masked robbers flee. ›Police conduct a search operation in a park near the supermarket, hoping to find the robber. ›Haaknat was found hiding in a moat in the park and the police, believing that Haaknat was the robber, apprehended him. ›Haaknat, however, argued that he was hiding in the moat because earlier that day, he had an argument with a man called Benny over some money. ›According to Haaknat, Benny drew a knife so Haaknat fled and hid himself in the moat where the police found him.

| 17 Example: giving explanations ›p 1 : Explain ({H is found}, {H robs supermarket}  T 1 ) H is found H hides in a moat H flees H robs supermarket

| 18 Example: giving explanations ›p 1 : Explain ({H is found}, {H robs supermarket}  T 1 ) ›p 2 : Explain ({H is found}, {argument between H and B}  T 2 ) H is found H hides in a moat H flees H robs supermarket Argument between H and B

| 19 Example: supporting explanations with evidence ›p 2 : argue H testified that he had an argument with B H is found H hides in a moat H flees H robs supermarket Argument between H and B

| 20 Example: supporting explanations with evidence ›p 2 : argue AR 1 H is found H hides in a moat H flees H robs supermarket Argument between H and B Haaknat says: “I had an argument with B” ge 1 : Witness says “P”  E P

| 21 Example: attacking supporting arguments ›p 1 : Haaknat is a suspect and suspects do not make reliable witnesses so  ge 1 H is found H hides in a moat H flees H robs supermarket Argument between H and B AR 1 H is a suspect

| 22 Example: expanding and supporting explanations ›p 1 : Explain ({H robs supermarket}, {H is from S }  T 3 ) H is found H hides in a moat H flees H robs supermarket Argument between H and B H is from Suriname

| 23 Example: expanding and supporting explanations ›p 1 : Explain ({H robs supermarket}, {H is from S }  T 3 ) ›p 1 : Argue I have evidence that R is from Suriname H is found H hides in a moat H flees H robs supermarket Argument between H and B H is from Suriname Evidence

| 24 Example: attacking explanations ›p 2 : Argue your causal generalization is based on prejudice H is found H hides in a moat H flees H robs supermarket Argument between H and B H is from Suriname Evidence Prejudiced

| 25 Conclusions ›Dynamic dialogue game for analysing stories and evidence. ›Find the best explanation in an adversarial setting. ›Players jointly build a model. ›Combination of enquiry and persuasion dialogue. ›Game can provide guidelines for discussions. ›Theory can serve as basis for system AVERs.

| 26 Thank you for your attention