Reasoning with testimony

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

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

3. 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

4. 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

5. 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

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

7. 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”

8. 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”

9. 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”

10. 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

11. 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

12. 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

13. 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!

14. 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”

15. 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

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

17. 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

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

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

20. 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!

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