Law Logic Summerschool 2012 Session 1.1.2: Argumentation and Defeasibility

1. Law Logic Summerschool 2012Session 1.1.2:Argumentation and Defeasibility Henry Prakken & Giovanni Sartor July 16, 2012

2. What is argumentation? • Giving reasons to support or criticise claims that are open to doubt • logic + dialectic • Often to persuade someone else • rhetoric Proponent: Regarding downloading Mp3s as copying for private use is wrong Respondent: Why? Proponent: Because it makes normal commercial exploitation of music impossible Respondent: Why? Proponent: Because it’s so easy to copy, upload and download MP3s

3. What is argumentation? • Giving reasons to support or criticise claims that are open to doubt • logic + dialectic • Often to persuade someone else • rhetoric Proponent: Regarding downloading Mp3s as copying for private use is wrong Respondent: Why? Proponent: Because it makes normal commercial exploitation of music impossible Respondent: Why? Proponent: Because it’s so easy to copy, upload and download MP3s Respondent: But there are quite profitable ways to sell Mp3s online Proponent: Really? Respondent: Look at iTunes

4. The structure of arguments

5. The structure of arguments:basic elements • (Basic) arguments have: • Premises (grounds) • A conclusion • A reasoning step from the premises to the conclusion Conclusion therefore ….. Premise 1 Premise n

6. P E is expert on P E says that P The offer was written The offer was made in an email The offer was made in a letter Three types of support Cumulative (all premises needed for conclusion) Alternative (one premise suffices for conclusion) S was at crime scene Aggregate (the more support the better) S’s DNA matches DNA found at crime scene Witness W saw S at crime scene

7. Alternative support is in fact alternative arguments The offer was written The offer was written The offer was made in a letter The offer was made in an email

8. Implicit premises The offer was written The offer was made in a letter If the offer was made in a letter or email then it was written

9. Implicit premises The offer was written The offer was made in an email If the offer was made in a letter or email then it was written

10. Manslaughter Intent Killed Recklessness Collision Victim died Caused by collision Drove 180 where max 80 Police radar Witness: “collision” Police report: “collision” Report coroner Report coroner Computer log file

11. Manslaughter Intent Killed Art. 287 CC Recklessness Collision Victim died Caused by collision Drove 180 where max 80 Police radar Witness: “collision” Police report: “collision” Report coroner Report coroner Computer log file

12. Manslaughter Intent Killed Art. 287 CC Causing a collision in consequence of which someone dies is killing Recklessness Collision Victim died Caused by collision Drove 180 where max 80 Police radar Witness: “collision” Police report: “collision” Report coroner Report coroner Computer log file

13. Manslaughter Driving 180 where maximum speed is 80 is consciously taking the risk of a collision, which is Recklessness Intent Killed Art. 287 CC Recklessness Collision Victim died Caused by collision Drove 180 where max 80 Police radar Witness: “collision” Police report: “collision” Report coroner Report coroner Computer log file

14. Manslaughter Intent Killed Art. 287 CC Recklessness Police radars are a reliable source of information on speed Collision Victim died Caused by collision Drove 180 where max 80 Police radar Witness: “collision” Police report: “collision” Report coroner Report coroner Computer log file

15. Manslaughter Intent Killed Art. 287 CC Recklessness Collision Victim died Caused by collision Drove 180 where max 80 This type of computer log file is a reliable indicator of what the radar has measured Police radar Witness: “collision” Police report: “collision” Report coroner Report coroner Computer log file

16. Two important features of arguments • Arguments can be constructed step by step • These steps often leave rules or generalisations implicit • When testing arguments, they must be made explicit to reveal sources of doubt • They can be unfounded • They can have exceptions

17. Arguments and counterarguments

18. Three types of counterarguments • (Basic) arguments have: • Premises (grounds) • A conclusion • A reasoning step from the premises to the conclusion • So arguments can be attacked on: • Their premises • Their conclusion • The reasoning step from premises to conclusion • Except if deductive

19. Deductive vs defeasible arguments Documents that look like avidavits usually are avidavits This document looks like an avidavit Therefore this is an avidavit All men are mortal Socrates is a man Therfore, Socrates is mortal

20. Deductive vs defeasible arguments Documents that look like avidavits usually are avidavits This document looks like an avidavit Therefore (presumably) this is an avidavit All men are mortal Socrates is a man Therefore, Socrates is mortal

21. Attack on conclusion Smoking increases the chance of cancer Smoking does not increase the chance of cancer E1 is oncologist E1 says so E2 says so E2 is oncologist

22. Attack on premise Smoking increases the chance of cancer E1 only says that there is no evidence that smoking does not increase the chance of cancer E1 is oncologist E1 says so Fragment 1 experts examination report

23. Attack on premise is often attack on intermediate conclusion Smoking increases the chance of cancer E1 only says that there is no evidence that smoking does not increase the chance of cancer E1 is oncologist E1 says so Fragment 2 experts examination report Fragment 1 experts examination report

24. Attack on inference step Smoking increases the chance of cancer Smoking does not increase the chance of cancer E1 is oncologist E1 says so E2 says so E2 is oncologist E2 is biased Experts are often biased towards who pays them E2 is paid by Marlboro

25. Attack on conclusion (Monge) If a person cannot be fired in malice and and s/he was fired fired, then firing him/her is a breach of contract If a person can be fired for any reason or no reason at all and s/he was fired, then firing him/her is not a breach of contract Firing Olga Monge was no breach of contract Firing Olga Monge was breach of contract Olga Monge cannot be fired in malice Olga Monge can be fired for any reason or no reason at all … Olga Monge was fired in malice … Olga Monge was fired Employees cannot be fired in malice Olga Monge was employed If a person is employed at will then he or she can be fired for any reason or no reason at all Olga Monge was employed at will

26. Attack on premise (Monge) Firing Olga Monge was no breach of contract Firing Olga Monge was breach of contract Olga Monge cannot be fired in malice Olga Monge can be fired for any reason or no reason at all … Olga Monge was fired in malice … Olga Monge was fired If a person is employed at will then he or she can be fired for any reason or no reason at all Olga Monge was employed at will Employees cannot be fired in malice Olga Monge was employed

27. Evaluating arguments • Does each step instantiate an acceptable argument form/scheme? • See next sessions • Have all its counterarguments been refuted? • Are its premises acceptable? • If defeasible: what about attacks on inference or conclusion? • (Has the search for counterarguments been thorough enough?) Can be indirect

28. Justifying your argument: refute every attack Argument for claim Proponent: Proponent of argument must strictly defeat opponents arguments, Opponent may weakly defeat proponent’s arguments Argument A weakly defeats argument B if A attacks B and is not weaker than B Argument A strictly defeats argument B if A attacks B and is stronger than B

29. Justifying your argument: refute every attack Argument for claim Proponent: Defeater 1 Defeater 2 Opponent:

30. Justifying your argument: refute every attack Argument for claim Proponent: Defeater 1 Defeater 2 Opponent: Strict defeater 1.1 Strict defeater 2.1 Proponent:

31. Justifying your argument: refute every attack Argument for claim Proponent: Defeater 1 Defeater 2 Opponent: Strict defeater 1.1 Strict defeater 2.1 Proponent: Defeater 2.1.1 Defeater 2.1.2 Opponent:

32. Justifying your argument: refute every attack Argument for claim Proponent: Defeater 1 Defeater 2 Opponent: Strict defeater 1.1 Strict defeater 2.1 Proponent: Defeater 2.1.1 Defeater 2.1.2 Opponent: Strict Defeater 2.1.1.1 Strict defeater 2.1.1.2 Proponent:

33. Murder Killing Intent R1 Proponent: Defeater 1 Defeater 2 Opponent: Strict defeater 1.1 Strict defeater 2.1 Proponent: Defeater 2.1.1 Defeater 2.1.2 Opponent: Strict Defeater 2.1.1.1 Strict defeater 2.1.1.2 Proponent:

34. Exception to R1 Murder Selfdefence R2 Killing Intent R1 Threat to life Killing R3 R3 Knife W1 says “knife” Proponent: Defeater 1 Opponent: Strict defeater 1.1 Strict defeater 2.1 Proponent: Defeater 2.1.1 Defeater 2.1.2 Opponent: Strict Defeater 2.1.1.1 Strict defeater 2.1.1.2 Proponent:

35. Summary and outlook Arguments are constructed by combining applications of inference rules into inference trees Some inference rules are deductive Only premise attack Other inference rules are defeasible/presumptive Also inference and conclusion attack ‘Standard’ logic identifies the deductive inference rules Sound and complete wrt semantics Argumentation theory / epistemology identify defeasible inference rules What justifies them? Dialectical evaluation of arguments can be formalised Argument construction Refutation of counterarguments