Legal Argumentation 3
Henry Prakken
April 4, 2013
The structure of arguments:basic elements
(Basic) arguments have: Premises (grounds) A conclusion A reasoning step from the premises to
the conclusion
Conclusion
Premise 1 Premise n…..
therefore
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
Argument schemes: general form
But also critical questions Negative answers are counterarguments
Premise 1, … , Premise nTherefore (presumably), conclusion
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 arguments
Causal relations Lowering income tax will increase
consumption Not an argument:
Income tax is lowered
Consumption will increase
Causal relations Lowering income tax will increase
consumption But a statement:
Lowering income taxwill increase consumption
Using causal generalisations in
arguments
Income tax is lowered
Consumption will increase
Using causal generalisations in
arguments
Income tax is lowered
Consumption will increase
Lowering income taxwill increase consumption
Using causal generalisations in
arguments
Income tax is lowered
Consumption will increase
The same happened in Germany
Lowering income taxwill increase consumption
‘forward’ use of causal generalisations
Income tax is lowered
Consumption will increase
Lowering income taxwill increase consumption
‘backward’ use of causal generalisations
Consumption has increased
Income tax was lowered
Lowering income taxwill increase consumption
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 has been observed 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 brings about 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
Making spam a criminal offence reduces spam
Reduction of spam is good
We should not make spam a
criminal offence
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
Making spam a criminal offence reduces spam
Reduction of spam is good
We should make spam civilly
unlawful
Making spam civilly unlawful reduces spam
Reduction of spam is good
Arguments from consequences (generalised to causal chains)
Critical questions: Does A also have bad (good) consequences? Are there other ways to bring about G? ...
Action A brings about G1, which brings about ….… which brings about Gn Gn is good (bad)Therefore (presumably), A should (not) be done
Causal chains Toppling the Hussein regime will pave the
way for democracy in Iraq Democracy in Iraq will advance the cause
of democracy elsewhere in the Middle East Advancing the cause of democracy will
diminish the risk of anti-American violence Diminishing the risk of anti-American
violence is good Therefore, we should topple the Hussein
regime
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 VTherefore (presumably), A should (not) be done
Example (arguments pro and con an action)
We should save DNA of all citizens
Saving DNA of all citizens leads to
solving more crimes
Solving more crimes promotes
security
We should not save DNA of all
citizens
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
Saving DNA of all citizens leads to
solving more crimes
Solving more crimes promotes
security
We should have more police
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 likelihood that such promotion or deomotion will occur
Determine the relative importance of the values at stake
Then weigh the pros and cons of all proposals But how?
Expected-utility arguments The expected utility of an action is (roughly)
the degree of goodness of badness (= utility) of the action’s consequences multiplied with the likelihood that these consequences will occur
A1,.., An are all my possible actionsA1 has the highest expected utility of A1, …, AnTherefore, A should be done
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
Better classification: arguments are deductive or presumptive (defeasible)
Evaluating arguments Does it instantiate an acceptable
argument scheme? Have all its counterarguments been
refuted? Are its premises acceptable? If presumptive: 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?
Can be indirect
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?
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