Reasoning with testimony

Argumentation vs. Explanatory Coherence

Floris Bex - University of GroningenHenry 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 CC consistent

with my beliefs?A credible?

Accept C

Construct explanatory

network

Does Cmaximize

coherence? Reject C

yes

yesno

no

Default pathway

Reflective pathway

Representing causal knowledge

Explanation with evidential rules:

‘Deduction’:

Explanation with causal rules:

Abduction:

Effect CauseEffect Cause

Cause EffectEffect 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”

smokemachine

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.

smokemachine

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

smokemachine

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

smokemachine

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

smokemachine

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!

smokemachine

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

smokemachine

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

smokemachine

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