Legal Argumentation 3
Henry Prakken
April 4, 2013

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

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

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

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

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Lowering income tax will increase consumption
Not an argument :
Consumption will increase
Income tax is lowered

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Lowering income tax will increase consumption
But a statement :
Lowering income tax will increase consumption

Consumption will increase
Income tax is lowered

Consumption will increase
Income tax is lowered Lowering income tax will increase consumption

Consumption will increase
Income tax is lowered Lowering income tax will increase consumption
The same happened in Germany

Consumption will increase
Income tax is lowered Lowering income tax will increase consumption

Income tax was lowered
Consumption has increased Lowering income tax will increase consumption

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

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

Example (arguments pro and con an action)
We should make spam a criminal offence
We should not make spam a criminal offence
Making spam a criminal offence reduces spam
Reduction of spam is good
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
Reduction of spam is good
Making spam civilly unlawful reduces spam
Reduction of spam is good

Arguments from consequences
(generalised to causal chains)
Action A brings about G1, which brings about ….
… which brings about Gn
Gn is good (bad)
Therefore (presumably), A should (not) be done
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Critical questions:
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Does A also have bad (good) consequences?
Are there other ways to bring about G?
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...

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

Example (arguments pro and con an action)
We should save DNA of all citizens
We should not save
DNA of all citizens
Saving DNA of all citizens leads to solving more crimes
Solving more crimes promotes security
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
Saving DNA of all citizens leads to solving more crimes
Solving more crimes promotes security
Having more police leads to solving more crimes
Solving more crimes promotes security

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For every proposal that is based on acceptable premises:
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List all legal values that it promotes or demotes
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Determine the extent to which the proposal promotes or demotes the value
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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
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But how?

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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 actions
A1 has the highest expected utility of A1, …, An
Therefore, A should be done

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Conventional classification: arguments are deductive, inductive or abductive
However:
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Only applies to epistemic arguments
“inductive” is ambiguous
There are other types of arguments
Better classification: arguments are deductive or presumptive (defeasible)

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Can be indirect
Does it instantiate an acceptable argument scheme?
Have all its counterarguments been refuted?
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Are its premises acceptable?
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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?

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There are conventional lists of fallacies
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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?