Investigating stories in a formal dialogue game

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

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 Tconsisting of causal rules of the form event1 Cevent2. • 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 Explanandum Event2 Event3

7. Different explanations Event1 Explanandum Event2 Event3

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 did not shoot Peter John shot peter Witness says “P” EP Witness 2 says ”John did not shoot Peter” John says ”Rijkbloem shot my husband!” Witness 1 says ”John shot Peter” Witness 1 says ”John shot Peter” Witness 1 says ”John shot Peter”

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

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 • p1: Explain ({H is found}, {H robs supermarket}  T1) H robs supermarket H flees H hides in a moat H is found

18. Example: giving explanations • p1: Explain ({H is found}, {H robs supermarket}  T1) • p2: Explain ({H is found}, {argument between H and B}  T2) H robs supermarket H flees H hides in a moat H is found Argument between H and B

19. Example: supporting explanations with evidence • p2: argue H testified that he had an argument with B H robs supermarket H flees H hides in a moat H is found Argument between H and B

20. Example: supporting explanations with evidence • p2: argue AR1 H robs supermarket H flees H hides in a moat H is found Argument between H and B ge1: Witness says “P” EP Haaknat says: “I had an argument with B”

21. Example: attacking supporting arguments • p1: Haaknat is a suspect and suspects do not make reliable witnesses so ge1 H robs supermarket H flees H hides in a moat H is found Argument between H and B H is a suspect AR1

22. Example: expanding and supporting explanations • p1: Explain ({H robs supermarket}, {H is from S }  T3) H robs supermarket H flees H hides in a moat H is found H is from Suriname Argument between H and B

23. Example: expanding and supporting explanations • p1: Explain ({H robs supermarket}, {H is from S }  T3) • p1: Argue I have evidence that R is from Suriname H robs supermarket H flees H hides in a moat H is found H is from Suriname Argument between H and B Evidence

24. Example: attacking explanations • p2: Argue your causal generalization is based on prejudice Prejudiced H robs supermarket H flees H hides in a moat H is found H is from Suriname Argument between H and B Evidence

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