Faculty of Law

Argumentative Story-based Analysis of Evidence

Floris Bex (Law and ICT, U. Groningen)

Henry Prakken (Law and ICT, U. Groningen / Information and Computing Sciences, U.

Utrecht)

Bart Verheij (Artificial Intelligence, U. Groningen)

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Contents

Evidential reasoning in legal theory/psychology Stories (Pennington & Hastie, Crombag et al.) Argument-graphs (Wigmore, Twining, Schum)

Evidential reasoning in AI (& Law) Inference to the Best Explanation (Thagard) Argumentation theory (Prakken, Walton, Gordon)

Combined theory Example Conclusions and future research

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Stories

Legal decisions are based on stories: “sequences of events which form a meaningful whole”

Stories are compared and the “best” story is chosen

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Stories

Legal decisions are based on stories: “sequences of events which form a meaningful whole”

Stories are compared and the “best” story is chosen

Problem: Relations between evidence, story and generalisations are unclear Causal relations between events Sources of evidence

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Argument structures

Structured argument-graphs from sources of evidence to conclusion (usually an event)

Generalisations are the “glue”

1 2 3 4

5 6 7 8

9

10

Sources of evidence

Event to be proven

Generalisation (inference warrant)

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Argument structures

Structured argument-graphs from sources of evidence to conclusion (usually an event)

Generalisations are the “glue”

Problem: sequence of events unclear Passage of time Causal relations between events

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Aims

Connect evidence to story using arguments

Formalise the combined theory in order to clarify the different relations

event eventeventevent

story

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Reasoning with evidence in AI (& Law)

Two approaches: Inference to the Best Explanation (IBE)

Form causal scenarios about “what happened” and compare these scenarios

Argumentation theory Form arguments from premisses to

conclusion

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the Rijkbloem case (1)

Nicole Lammers, a baker’s daughter had a relationship with Rijkbloem, a small-time criminal

After breaking up, Nicole and her parents go to Rijkbloem’s house to pick up some of her belongings

A fight develops, which ends in Mr. Lammer’s death

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the Rijkbloem case (2)

Fact: Mr. Lammers was shot through the head in Rijkbloem’s house

Prosecution’s story: The fight between father and Rijkbloem

started Rijkbloem pulled out a gun Rijkbloem shot father through the head Father died

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the Rijkbloem case (3)

Fact: Mr. Lammers was shot through the head in Rijkbloem’s house

Defence’s story: The fight between father and Rijkbloem started Mrs. Lammers pulled a small gun out of her

handbag and aimed the gun at Rijkbloem Rijkbloem tried to push the gun away The gun accidentally went off Father was hit in the head and died

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IBE – causal reasoning

Stories involve causal reasoning Stories are (at least) a sequence of events

on a timeline Events are supposedly caused by earlier

events Physical causation Mental causation

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IBE - explanations

Given: Causal rules T Facts that need to be explained F

Hypothesise a set of causes H such that the H T logically implies F (“explains F”)

Rijkbloem shoots father

Father is hit

Father dies

Rijkbloem pushes

away gun

gun goes off

Mother pullsout gun

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IBE – choice

Choose between the different explanations:

Father dies

Prosecution’s explanation

Defence’s explanation

choice

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Arguments - evidential reasoning

Reasoning with sources of evidence is evidential

Witness W saying “P” is evidence for P Gunpowder on Rijkbloem’s hands is

evidence for Rijkbloem having fired a gun

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Arguments

(formal) argumentation theory

Mrs. Lammers says ”Rijkbloem shot my

husband!”

Rijkbloem shot mr. Lammers

If a witness says “P” then usually P

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Arguments - attacking

Attacking arguments

Rijkbloem says “I did not shoot mr. Lammers!”

Rijkbloem did not shoot

mr. Lammers

Mrs. Lammers says ”Rijkbloem shot my

husband!”

Rijkbloem shot mr. Lammers

If a witness says “P” then usually P

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Arguments - attacking

Attacking arguments

Mrs. Lammers says ”Rijkbloem shot my

husband!”

Rijkbloem shot mr. Lammers

If a witness says “P” then usually P

Mrs. Lammers is not trustworthy

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Combining the theories

The stories are modelled as explanations Sources of evidence are connected to the

stories using evidential arguments Explanations are compared

How much additional evidence is explained? How much additional evidence is

contradicted? Possible to reason about causal

generalisations in the stories

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Combining the theoriesExample

FightRijkbloem

shoots father

Father is hit

Father dies

Police officer’s testimony

Forensic report

Mrs. Lammers’ testimony

Mrs. Lammers is not trustworthy

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Combining the theoriesExample

Prosecutions story

Defence’s story

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Conclusions

Stories and evidence have a seperate place in the theory

Stories and their supporting evidence can be easily combined

Better criteria for comparing stories

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Future work

Other criteria for comparing stories Coherence Plausibility