Reasoning with testimony
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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)

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Reasoning with testimony

Reasoning with testimony

Argumentation vs. Explanatory Coherence

Floris Bex - University of Groningen

Henry Prakken - University of Groningen

- Utrecht University


Introduction

Introduction

  • Thagard’s dual pathway model of testimony

  • Modelling it in our approach (2x)

  • Modelling it in Thagard’s ECHO

  • Comparison


Thagard on testimonies

Thagard on testimonies

C consistent

with my beliefs?

A credible?

yes

A claims C

Accept C

Default pathway

yes

no

Construct

explanatory

network

Does C

maximize

coherence?

Reject C

Reflective pathway

no


Representing causal knowledge

Representing causal knowledge

  • Explanation with evidential rules:

    ‘Deduction’:

  • Explanation with causal rules:

    Abduction:

Smoke means Fire

Smoke

Fire

Effect  Cause

Effect

Cause

Cause  Effect

Effect

Cause

Fire causes Smoke

Smoke

Fire


Modelling thagard s ideas in our approach 1 both causal and evidential rules

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

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

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”

fire

smoke

machine

f1: smoke

R1

Witness 2 says

“smoke machine”


Default pathway example1

Default pathway - example

  • If we also know, that witness 2 has reason to lie about machine, this conclusion is blocked.

smoke

machine

f1: smoke

Witness 2 has

reason to lie

R1

Witness 2 says

“smoke machine”


Default pathway example2

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

?

Witness 2 has

reason to lie

R1

Witness 2 says

“smoke machine”


Reflective pathway

Reflective pathway

  • Two explanations for the observations

    • “smoke machine”

    • “fire” and “witness has reason to lie”

fire

f1: smoke

smoke

machine

f2: witness says

“smoke machine”

witness has

reason to lie


Reflective pathway1

Reflective pathway

  • If we also have evidence that W may have reason to lie, this might create a preference for the “fire-explanation”.

fire

f1: smoke

smoke

machine

f2: witness says

“smoke machine”

witness has

reason to lie

f3


Reflective pathway2

Reflective pathway

  • But if we have no additional evidence, we have no reason to prefer the “fire- explanation”!

fire & reason to lie

?

smoke

smoke machine


Intermediate conclusion

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

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!

fire

smoke

machine

f1: smoke

?

R1

?

Witness 2 says

“smoke machine”


Principles of coherence

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

A coherence network

fire

f1: smoke

smoke

machine

f2: witness says

“smoke machine”

witness has

reason to lie


Activation in the network

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 network1

Activation in the network

fire

f1: smoke

smoke

machine

f2: witness says

“smoke machine”

witness has

reason to lie


Some comments

Some comments

  • Good principles of coherence

  • The “right” result

  • Not transparent (black box)

  • More complex examples?

  • No modelling of the default pathway!


Reasoning with testimony

C consistent

with my beliefs?

A credible?

A coherence network needs to be built to answer this question!

A claims C

Not the only critical question!


Conclusion

Conclusion

  • In our approach Thagard’s dual pathway model can be modelled as argumentation

    • if embedded in investigation

  • Thagard only models the reflective pathway


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