Reasoning with testimony Argumentation vs. Explanatory Coherence Floris Bex - University of Groningen Henry Prakken - University of Groningen - Utrecht University

Introduction Thagard’s dual pathway model of testimony Modelling it in our approach (2x) Modelling it in Thagard’s ECHO Comparison

Thagard on testimonies A claims C C consistent with my beliefs? A credible? Accept C Construct explanatory network Does C maximize coherence? Reject C yes no Default pathway Reflective pathway

Representing causal knowledge Explanation with evidential rules: ‘Deduction’: Explanation with causal rules: Abduction: Effect  Cause Effect Cause Cause  Effect Effect Cause Fire causes Smoke Smoke Fire Smoke means Fire Smoke Fire

Modelling Thagard’s ideas in our approach (1): both causal and evidential rules Default pathway: whenever a witness says that P, believe P (unless …) Can be formalised as argumentation with evidential rules Causal pathway: represent all possible causes of the testimony that P: P is true The witness has reason to lie that P His senses deceived him that P His memory deceived him that P … Then determine the most likely cause Can be modelled as abduction with causal rules

Default pathway R1: Witness W says that P => e P R2: W has reason to lie that P => e exception to R1 … (more exceptions)

Default pathway - example Say that “smoke” is observed (a fact) If we only know that Witness 2 says “smoke machine”, we can conclude that “smoke machine” smoke machine f1: smoke R1 Witness 2 says “smoke machine” fire

Default pathway - example If we also know, that witness 2 has reason to lie about machine, this conclusion is blocked. smoke machine f1: smoke R1 Witness 2 says “smoke machine” Witness 2 has reason to lie

Default pathway - example What if we have evidence that W may have reason to lie that machine? => this is where we shift to reflective pathway smoke machine f1: smoke R1 Witness 2 says “smoke machine” Witness 2 has reason to lie

Reflective pathway Two explanations for the observations “smoke machine” “fire” and “witness has reason to lie” f1: smoke fire smoke machine f2: witness says “smoke machine” witness has reason to lie

Reflective pathway If we also have evidence that W may have reason to lie, this might create a preference for the “fire-explanation”. f1: smoke fire smoke machine f2: witness says “smoke machine” witness has reason to lie f3

Reflective pathway But if we have no additional evidence, we have no reason to prefer the “fire- explanation”! fire & reason to lie smoke machine smoke

Intermediate conclusion Our first proposal to model Thagard’s ideas in our approach requires that a shift from the default to the reflective pathway is modelled as a shift in problem representation Abduction alone cannot justify believing the witness by default And the truth of P is the usual cause of a witness statement that P!

Both pathways in argumentation If we only know that Witness says that P, we can conclude that P But first we must spend some effort in searching for the exceptions! smoke machine f1: smoke R1 Witness 2 says “smoke machine” fire ? ?

Principles of coherence Two propositions A and B cohere iff: A explains B or vice versa (symmetrical) A and B together explain C Two propositions A and B are in competition iff: A explains C and B explains C They are contradictory

A coherence network f1: smoke fire smoke machine f2: witness says “smoke machine” witness has reason to lie

Activation in the network Activation is between 1 and -1 Evidence nodes (f1…fn) have an activation of 1 Coherence relation is an excitatory link Competition relation is an inhibitory link

Activation in the network f1: smoke fire smoke machine f2: witness says “smoke machine” witness has reason to lie

Some comments  Good principles of coherence  The “right” result  Not transparent (black box)  More complex examples?  No modelling of the default pathway!

A claims C C consistent with my beliefs? A credible? A coherence network needs to be built to answer this question! Not the only critical question!

Conclusion In our approach Thagard’s dual pathway model can be modelled as argumentation if embedded in investigation Thagard only models the reflective pathway