Legal Argumentation 2
Henry Prakken
March 28, 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
therefore
Premise 1
…..
Premise n
Three types of support
P
Cumulative (all premises
needed for conclusion)
E is expert on P
E says that P
The offer was written
Alternative (one premise
suffices for conclusion)
The offer was made
in a letter
The offer was made
in an email
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
Implicit premises
The offer was written
The offer was made
in a letter
Implicit premises
The offer was written
The offer was made
in a letter
If an offer is made in
a letter or email then
it is written
Manslaughter
Intent
Killed
Art. 287 CC
Recklessness
Collision
Police radars are a
reliable source of
information on speed
Victim died
Caused by
collision
Report
coroner
Report
coroner
Drove 180 where max 80
Police radar
Witness:
“collision”
Computer
log file
Police report:
“collision”
Overview of course
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Week 1:
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Basic structure of arguments
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Week 2:
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Combinations of premises
implicit premises
Multi-step arguments
Arguments and counterarguments
Argument schemes (1)
Week 3:
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Argument schemes (2)
Evaluating argument
Legal reasoning: three stages
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Determining the facts of the case
Classifying the facts under the
conditions of a legal rule
Applying the rule
Legal reasoning is considering
arguments pro and con

Determining the facts of the case
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Classifying the facts under the conditions of a legal
rule
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conflicting sources of evidence
generalisations with exceptions
…
Interpretation rules can have exceptions
conflicting interpretations
…
Applying the rule
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Conflicting rules
Reasons not to apply the rule
…
Example
“Vehicles are not allowed in the park”
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Evidence problems
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General terms
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Exceptions
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Purpose of the rule
Principle
Legal reasoning is adversarial
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Legal cases involve clashes of interest
Dispute
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study constructing and attacking arguments
Arguments and
counterarguments
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
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
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
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 so
E2 is oncologist
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
Attack on premise is often attack
on intermediate conclusion
Smoking increases
the chance of cancer
E1 says so
Fragment 2
experts
examination
report
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
Attack on inference step
Smoking increases
the chance of cancer
E1 says so
Smoking does not increase
the chance of cancer
E1 is oncologist
E2 says so
E2 is biased
E2 is paid by
Marlboro
Experts are often biased
towards who pays them
E2 is oncologist
Argument schemes and
Critical questions
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
Role of critical questions
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Critical questions are pointers to
counterarguments
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Some point to attacks on a premise
Some point to attacks on a conclusion
Some point to attacks on an inference
Legal rule application (simplistic)
IF conditions THEN legal consequence
conditions
So, legal consequence
Legal rule application:
critical questions
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Is the rule valid?
Is the rule applicable to this case?
Must the rule be applied?
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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?
Reasoning with generalisations
Guilt
P
If P then normally Q
So (presumably), Q
Fleas
If flees then normally guilt
People who flee from a crime scene normally have consciousness of guilt
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Critical questions:
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How strong is the connection?
Is there an exception?
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Illegal immigrant?
Client of prostitute?
…
How are generalisations justified?
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Scientific research (induction) Very reliable
Experts
Commonsense
Individual opinions
Prejudice?
Very unreliable
Inducing generalisations (1)
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
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Critical questions:
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Was the sample large enough?
was the selection of test cases biased?
Inducing generalisations (2)
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
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Critical questions:
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Was the size of the sample large enough?
was the sample selection biased?
Expert testimony
E is expert on D
E says that P
P is within D
Therefore (presumably), P is the case
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Critical questions:
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Is E biased?
Is P consistent with what other experts say?
Is P consistent with known evidence?
Witness testimony
Witness W says P
Therefore (presumably), P
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Critical questions:
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Is W sincere?
Is W’s memory OK?
Were W’s senses OK?
Analogy (normative)
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
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Critical questions:
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Are there also relevant differences between the
cases?
Are there conflicting precedents?
Analogy (factual)
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
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Critical questions:
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Are there also relevant differences between the
cases?
Are there conflicting similar cases?
Temporal persistence
(Forward)
P is true at T1 and T2 is later than T1
Therefore (presumably), P is
still true at T2
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Critical questions:
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Was P known to be false between T1 and
T2?
Is the gap between T1 and T2 too long?
Temporal persistence
(Backward)
P is true at T2 and T1 is earlier than T2
Therefore (presumably), P was already
true at T1
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Critical questions:
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Was P known to be false between T1 and
T2?
Is the gap between T1 and T2 too long?
S murdered V
d.m.p.
V murdered in
house at 4:45
If V murdered in L at T and
S was in L at T, then
(presumably)
S murdered V
X in 4:45
forw
temp pers
S entered 4:30
S entered 4:30
S in 4:45
aggr
S in 4:45
backw
temp pers
S left 5:00
testimony
testimony
W1: “S entered 4:30”
W2: “S left 5:00”
35
Next week
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Argument schemes (2)
Evaluating arguments