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Legal Argumentation 2 Henry Prakken March 28, 2013.

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1 Legal Argumentation 2 Henry Prakken March 28, 2013

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

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

4 Implicit premises The offer was made in a letter The offer was written

5 Implicit premises The offer was made in a letter The offer was written If an offer is made in a letter or then it is written

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

7 Overview of course Week 1: Basic structure of arguments Combinations of premises implicit premises Multi-step arguments Week 2: Arguments and counterarguments Argument schemes (1) Week 3: Argument schemes (2) Evaluating argument

8 Legal reasoning: three stages Determining the facts of the case Classifying the facts under the conditions of a legal rule Applying the rule

9 Legal reasoning is considering arguments pro and con Determining the facts of the case conflicting sources of evidence generalisations with exceptions … Classifying the facts under the conditions of a legal rule Interpretation rules can have exceptions conflicting interpretations … Applying the rule Conflicting rules Reasons not to apply the rule …

10 Example Evidence problems General terms Exceptions Purpose of the rule Principle “Vehicles are not allowed in the park”

11 Legal reasoning is adversarial Legal cases involve clashes of interest Dispute study constructing and attacking arguments

12 Arguments and counterarguments

13 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 Except if deductive The reasoning step from premises to conclusion Except if deductive

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

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

16 Attack on conclusion Smoking increases the chance of cancer E1 says so E1 is oncologist Smoking does not increase the chance of cancer E2 says soE2 is oncologist

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

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

19 Attack on inference step Smoking increases the chance of cancer E1 says so E1 is oncologist Smoking does not increase the chance of cancer E2 says soE2 is oncologist E2 is paid by Marlboro Experts are often biased towards who pays them E2 is biased

20 Argument schemes and Critical questions

21 Argument schemes: general form But also critical questions Negative answers are counterarguments Premise 1, …, Premise n Therefore (presumably), conclusion

22 Role of critical questions Critical questions are pointers to counterarguments Some point to attacks on a premise Some point to attacks on a conclusion Some point to attacks on an inference

23 Legal rule application (simplistic) IF conditions THEN legal consequence conditions So, legal consequence

24 Legal rule application: critical questions Is the rule valid? Is the rule applicable to this case? Must the rule be applied? Is there a statutory exception? Does applying the rule violate its purpose? Does applying the rule have bad consequences? Is there a principle that overrides the rule?

25 Reasoning with generalisations Critical questions: How strong is the connection? Is there an exception? Illegal immigrant? Client of prostitute? … P If P then normally Q So (presumably), Q People who flee from a crime scene normally have consciousness of guilt Guilt FleasIf flees then normally guilt

26 How are generalisations justified? Scientific research (induction) Experts Commonsense Individual opinions Prejudice? Very reliable Very unreliable

27 Inducing generalisations (1) Critical questions: Was the sample large enough? was the selection of test cases biased? In the investigated cases most P’s were Q’s So (presumably), most P’s are Q’s In the investigated cases most experts were called in the initial stages of the proceedings Most experts are called in the initial stages of the proceedings

28 Inducing generalisations (2) Critical questions: Was the size of the sample large enough? was the sample selection biased? Almost all observed P’s were Q’s Therefore (presumably), If P then usually Q In 16 of 17 tests the ballpoint shot with this bow caused this type of eye injury A ballpoint shot with this type of bow will usually cause this type of eye injury

29 Expert testimony Critical questions: Is E biased? Is P consistent with what other experts say? Is P consistent with known evidence? E is expert on D E says that P P is within D Therefore (presumably), P is the case

30 Witness testimony Critical questions: Is W sincere? Is W’s memory OK? Were W’s senses OK? Witness W says P Therefore (presumably), P

31 Analogy (normative) Critical questions: Are there also relevant differences between the cases? Are there conflicting precedents? Relevantly similar cases should be decided in the same way This case is relevantly similar to that precedent Therefore (presumably), this case should have The same outcome as the precedent

32 Analogy (factual) Critical questions: Are there also relevant differences between the cases? Are there conflicting similar cases? Relevantly similar cases have the same outcome This case is relevantly similar to that case Therefore (presumably), this case will have The same outcome as that case

33 Temporal persistence (Forward) Critical questions: Was P known to be false between T1 and T2? Is the gap between T1 and T2 too long? P is true at T1 and T2 is later than T1 Therefore (presumably), P is still true at T2

34 Temporal persistence (Backward) Critical questions: Was P known to be false between T1 and T2? Is the gap between T1 and T2 too long? P is true at T2 and T1 is earlier than T2 Therefore (presumably), P was already true at T1

35 35 S murdered V V murdered in house at 4:45 S in 4:45 X in 4:45S in 4:45 S left 5:00 W2: “S left 5:00”W1: “S entered 4:30” S entered 4:30 aggr testimony forw temp pers backw temp pers d.m.p. If V murdered in L at T and S was in L at T, then (presumably) S murdered V

36 Next week Argument schemes (2) Evaluating arguments


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