Law Logic Summerschool 2012 Session 3.1.2: Argument schemes

Law Logic Summerschool 2012Session 3.1.2:Argument schemes Henry Prakken July 18, 2012

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

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

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 inference • Except if deductive

Evaluating arguments • Does each step instantiate an acceptable argument scheme? • Have all its counterarguments been refuted? • Are its premises acceptable? • If defeasible: what about attacks on inference or conclusion?

Contents • Defeasible argument schemes • Analysing the argumentation structure of a given text • (Software exercise)

Argument schemes and Critical questions

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

Logical account of argument schemes • Argument schemes are inference rules • Critical questions are pointers to counterarguments • Some point to premise attacks • Some point to rebutting attacks • Some point to undercutting attacks • If all critical questions point to premise attack, then the inference rule is deductive, otherwise it is defeasible

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

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 have bad consequences? • Is there a principle that overrides the rule?

Reasoning with generalisations Involved • 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 Fleas If fleas then normally involved People who flea from a crime scene are normally involved in the crime

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

Induction of generalisations • 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 Most experts are called in the initial stages of the proceedings In the investigated cases most experts were called in the initial stages of the proceedings

Inducing generalisations • Critical questions: • Is 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 A ballpoint shot with this type of bow will usually cause this type of eye injury In 16 of 17 tests the ballpoint shot with this bow caused this type of eye injury

Expert testimony(Walton 1996) • 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

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

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 > T1 Therefore (presumably), P is still true at T2

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 T1 and T2 < T1 Therefore (presumably), P was already true at T2

X murdered Y d.m.p. Y murdered in house at 4:45 V murdered in L at T & S was in L at T  S murdered V X in 4:45 aggregate X in 4:45{X in 4:30} X in 4:45{X in 5:00} backw temp pers forw temp pers X left 5:00 X in 4:30 aggregate X in 4:30{W1} X in 4:30{W2} testimony testimony testimony W2: “X in 4:30” W1: “X in 4:30” W3: “X left 5:00”

Causal relations Invading Iraq will cause Iraq to become democratic Not an argument: Iraq will become democratic Iraq is invaded

Causal relations Lowering income tax will increase consumption But a statement: If Iraq is invaded then Iraq will become democratic

Using causal generalisations in arguments Iraq will become democratic Iraq is invaded

Using causal generalisations in arguments Iraq will become democratic Iraq is invaded If Iraq is invaded then Iraq will become democratic

Using causal generalisations in arguments Iraq will become democratic Iraq is invaded If Iraq is invaded then Iraq will become democratic The same happened in Germany and Japan after WWII

‘Predictive’ use of causal generalisations Iraq will become democratic Iraq is invaded If Iraq is invaded then Iraq will become democratic

‘Explanatory’ use of causal generalisations Iraq was invaded Iraq has become democratic If Iraq is invaded then Iraq will become democratic

Causal explanation (Abduction) • Critical questions: • Could Q be caused by something else? • Does P cause something of which we know it is not the case? P causes Q Q so (presumably), P

Arguments from consequences • Critical questions: • Does A also have bad (good) consequences? • Are there other ways to bring about G? • ... Action A causes G, G is good (bad) Therefore (presumably), A should (not) be done

Example (arguments pro and con an action) We should make spam a criminal offence We should not make spam a criminal offence Reduction of spam is good Making spam a criminal offence reduces spam Making spam a criminal offence increases workload of police and judiciary Increased workload of police and judiciary is bad

Example (arguments pro alternative actions) We should make spam a criminal offence We should make spam civilly unlawful Making spam a criminal offence reduces spam Making spam civilly unlawful reduces spam Reduction of spam is good Reduction of spam is good

Refinement: promoting or demoting legal values • Critical questions: • Are there other ways to cause G? • Does A also cause something else that promotes or demotes other values? • ... Action A causes G, G promotes (demotes) legal value V Therefore (presumably), A should (not) be done

Example (arguments pro and con an action) We should save DNA of all citizens We should not save DNA of all citizens Solving more crimes promotes security Saving DNA of all citizens leads to solving more crimes Saving DNA of all citizens makes more private data publicly accessible Making more private data publicly available demotes privacy

Example (arguments pro alternative actions) We should save DNA of all citizens We should have more police Solving more crimes promotes security Saving DNA of all citizens leads to solving more crimes Having more police leads to solving more crimes Solving more crimes promotes security

Comparing action proposals • For every proposal that is based on acceptable premises: • List all legal values that it promotes or demotes • Determine the extent to which the proposal promotes or demotes the value • Determine the relative importance of the values at stake • Then weigh the pros and cons of all proposals • But how?

Evaluating arguments Can be indirect • Does each step instantiate an acceptable argument scheme? • Have all its counterarguments been refuted? • Are its premises acceptable? • If defeasible: what about attacks on inference or conclusion? • Argument schemes help in identifying sources of doubt in an argument. • (Has the search for counterarguments been thorough enough?)

Fallacies • There are conventional lists of fallacies • Affirming the consequent, authority, attacking the source, ... • But such arguments often make sense! • They are schemes for presumptive arguments • What is important is: can they be defended against attack?

Classification of arguments • Conventional classification: arguments are deductive, inductive or abductive • However: • Only applies to epistemic arguments • “inductive” is ambiguous • There are other types of arguments • Modern classification: arguments are deductive or defeasible • Then further classify defeasible arguments

Practicing with Araucaria • Go to • http://www.staff.science.uu.nl/~prakk101/argprom/argprom11.html • Download Araucaria • Download the text file of exercises

Law Logic Summerschool 2012 Session 3.1.2: Argument schemes