Transcript Need for Logic of Argumentation

ARGUMENTATION METHODS OF
ARGUMENT RECONSTRUCTION
Douglas Walton
CRRAR
FMAR, Konstanz, Sept. 21, 2012
Argumentation Schemes
• The current problem with trying to evaluate plausible reasoning
using systems of weights with numerical values is that there can be
many ways of doing this.
• Argumentation takes the approach of using forms of argument that
have now been identified that represent different kinds of plausible
reasoning called argumentation schemes.
• Argumentation schemes sometimes appear similar to the forms of
arguments we are familiar with in deductive logic, like modus
ponens or syllogistic argument forms.
• But the most common ones (a) represent defeasible, plausible
arguments that depend on common understanding of the way
things can normally be expected to go in a kind of case familiar to
speaker and hearer, and (b) depend on warrants that are
generalizations that hold only subject to exceptions.
Matching Critical Questions
• Each argumentation scheme has a matching set of
critical questions, and the scheme, used in conjunction
with the questions, is the device for identifying,
analyzing and evaluating the argument.
• Defeasible argumentation schemes with critical
questions are used in the formal systems for modeling
legal argumentation ASPIC+ and Carneades.
• Over 60 schemes are set out in (Walton, Reed and
Macagno, 2008), and some of them have been used in
empirical studies to identify arguments in everyday in
political discourse and legal texts (argument mining).
Argument from Expert Opinion
• Major Premise: Source E is an expert in field F
containing proposition A.
• Minor Premise: E asserts that proposition A
(in field F) is true (false).
• Conclusion: A may plausibly be taken to be
true (false).
Six Basic Critical Questions
• Expertise Question: How credible is E as an expert
source?
• Field Question: Is E an expert in the field F that A is in?
• Opinion Question: What did E assert that implies A?
• Trustworthiness Question: Is E personally reliable as a
source?
• Consistency Question: Is A consistent with what other
experts assert?
• Backup Evidence Question: Is E’s assertion based on
evidence?
Forensics of Fine Art Example
• According to Martin Kemp, a professor at Oxford
who is an expert on the forensics of fine art a
portrait of a young woman in Renaissance dress,
originally sold for about $20,000 in 1998, is an
undiscovered masterwork of Leonardo da Vinci
possibly worth more than $100 million.
• The fine details of the drawing were examined by
Kemp’s expert eye. He found tiny marks and
techniques in the painting consistent with the
works of Leonardo.
Expert Kemp
• Martin Kemp is Emeritus Professor of the History
of Art at Oxford University.He was trained in
Natural Sciences and Art History at Cambridge
University and the Courtauld Institute, London.
He was British Academy Wolfson Research
Professor (1993-98).
• He has taught at the universities of Glasgow and
St. Andrews. He has held visiting posts in
Princeton, New York, North Carolina and Los
Angeles.
Carbon Dating Evidence
• The young woman in the portrait is identified
as Bianca Sforza, the daughter of da Vinci's
patron Ludovico Sforza, the Duke of Milan.
• Carbon dating showed the vellum, or animal
skin on which the portrait was painted, is
consistent with the time of da Vinci's work.
• However forgers often use old material to try
to deceive art experts.
Corroborating Evidence
• Another expert, Pascal Cotte, uses a special
camera to examine the painting and identifies
left-handed brushstrokes of a kind consistent
with the painting technique of Leonardo.
• Cotte found evidence of knife marks on the
vellum, suggesting someone might have cut the
portrait out as a single page of a book.
• A book is found in the national Library of Poland,
the history of the Sforza family, and Cotte
matched three holes in the portrait with the
stitching in the book. It is a perfect match.
Expert Cotte
• Pascal Cotte is an engineer who developed a
tool that enables the in-depth study of fine art
paintings to reveal the true pigments for
viewing and analysis without touching or
damaging the paintings.
• He is the founder of Lumiere Technologies, a
company in Paris that offers this service to
museums to authenticate, restore and
reproduce masterpieces of fine art.
Scheme with Implicit Premise
• Major Premise: Source E is an expert in subject
domain S containing proposition A.
• Minor Premise: E asserts that proposition A (in
domain S) is true (false).
• Conditional Premise: If source E is an expert in a
subject domain S containing proposition A, and E
asserts that proposition A is true (false), then A
may plausibly be taken to be true (false).
• Conclusion: A may plausibly be taken to be true
(false).
Explaining the Fallacy
• Because this version of the scheme has the
modus ponens form, some might think argument
from expert opinion can be modeled as
deductively valid (closed; monotonic).
• This would be a great error, according to the
analysis of the argumentum ad verecundiam
fallacy given in (Walton, 1997, ch. 8).
• On that analysis, “fallacious cases occur where
what is basically a presumptive and defeasible
form of argument is presented in an absolutistic
and final manner in a dialogue” (p. 230).
The Trouble with Experts
Experts are often wrong.
In the recent stock meltdown,
we discovered that some of our most
important experts – our financial gurus - didn’t
know much at all.
Dutch artist John Myatt used house paint and KY
jelly to forge the works of Great Masters. He
managed to fool top art critics and museums for
eight years before he was finally caught.
Pollock’s Red Light Example
• Pollock (1995, 41) offered the following argument as an example.
•
•
•
Premise 1: This object looks red to me.
Premise 2: When an object looks red, then (normally, but
subject to exceptions) it is red.
Conclusion: This object is red.
• This argument is defeasible, since even objects that are not red look
red when illuminated by a red light.
• Nevertheless, it is an argument that can be justifiably held to hold
tentatively, and give a reason to accept its conclusion, if there is no
evidence that the situation is exceptional.
Argument from Perception
• Rather than seeing the argumentation in Pollock’s red
light example as an inductive form of reasoning, it can
be represented using the scheme for argument from
perception (Walton, Reed and Macagno, 2008, 345).
• MAJOR PREMISE: To have a φ image (an image of a
perceptible property) is a prima facie reason to believe
that the circumstances exemplify φ.
• MINOR PREMISE: Person P has a φ image (an image of a
perceptible property)
• CONCLUSION: It is reasonable to believe that φ is the
case
Critical Questions
• CQ1: Are there circumstances giving a reason
to think that P’s having a φ image may not be
a reliable indicator of φ?
• CQ2: Are there other images that P has that
suggest that P’s having a φ image may not be
a reliable indicator of φ?
• CQ3: What test has been made, if any, that
would confirm that P’s having a φ image is a
reliable indicator of φ?
CQ1 Modeled as an Exception
CQ1: Are there circumstances giving a reason
to think that P’s having a φ image may not be a
reliable indicator of φ?
How Carneades Evaluates It
This object is red.
This object looks red to me.
+ARGUMENT
FROM
PERCEPTION
When an object looks red then it is red.
−
This object is illuminated by a red light.
This object is red.
The circumstances are such that looking
red is not a reliable indicator of being red.
+
This object looks red to me.
+ARGUMENT
FROM
PERCEPTION
When an object looks red then it is red.
−
This object is illuminated by a red light.
+
The circumstances are such that looking
red is not a reliable indicator of being red.
Precedent and Analogy
• The basis for deciding whether one case is a precedent for another
in law is that it is based on an underlying argument from analogy
(Walton, D., Similarity, Precedent and Argument from Analogy
Artificial Intelligence and Law, 18 (3), 2010, 217-246) .
• But how is a case where a man sued a company because there was
a decomposed snail in his beer bottle similar to a case where a man
tried to sue because of a defective Buick automobile?
• My answer: there is something about the common sequence of
events that makes the one case similar to the other.
• First the plaintiff bought some product that he assumed was the
normal product he expected, and he thought therefore that the
product was reasonably safe to use. Then something in the product
turned out to be defective, and when he used the product this
defect caused some harm that impacted badly on his health.
Script Common to Both Cases
Scheme for Argument from Analogy
• In the basic scheme for argument from analogy, a
similarity between two cases where a proposition A
holds in the one case can shift a weight of evidence
from one case to the other.
• The following basic scheme (Walton, Reed and
Macagno, 2008, 315) represents this version of the
structure of argument from analogy.
•
Similarity Premise: Generally, case C1 is similar to
case C2.
•
Base Premise: A is true (false) in case C1.
•
Conclusion: A is true (false) in case C2.
Script as the Basis of the Similarity
Premise in Argument from Analogy
CQs for Argument from Analogy
• CQ1: Are there particular respects in which C1 and
C2 are different that would tend to undermine the
force of the similarity cited?
• CQ2: Are there particular respects in which C1 and
C2 are similar that would tend to further support
the force of the similarity cited?
• CQ3: Is A the right conclusion to be drawn in C1?
• CQ4: Is there some other case C3 that is also
similar to C1, but in which some conclusion other
than A should be drawn?
Analogy Scheme with Implicit Premise
• Similarity Premise: Case C1 is similar to case
C2.
• Base Premise: A is true (false) in case C1.
• Implicit Conditional Premise: If case C1 is
similar to case C2, and A is true (false) in case
C1, then A is true (false) in case C2 [material
conditional: no exceptions].
• Conclusion: A is true (false) in case C2.
Is it Deductive?
• Once again, as in the case of argument from
expert opinion, the argument from analogy
becomes a fallacious argument when construed
in this closed (nonmonotonic) way.
• Argument from analogy has to be defeasible and
open to case-based critical questioning and
counter-arguments as new information comes in.
• Otherwise it is ossified, fixed in an unduly
restrictive (closed) way that makes it not open to
refutation.
Case-Based Reasoning
• CATO (Aleven, 1997) is based on factors representing
respects in which a case is similar to or different from
another one (questions CQ1 and CQ2 ).
• This can happen in case-based reasoning when some
factors support the argument while others detract from it.
• In case-based reasoning argument from analogy is a
defeasible form of argument in which further evidence can
be introduced that can go against or even defeat the
argument.
• To weigh the arguments on each side, we have to consider
the factors on each side, and determine which factors are
more “on-point”, or relevant.
Version 2 and Modus Ponens
• If you look at version 2 you can see that the
argument has something like a modus ponens
structure as an inference. It looks like the form
called defeasible modus ponens.
• Verheij (2001, p. 232) proposed that defeasible
argumentation schemes fit a form of argument
he called modus non excipiens: as a rule, if P then
Q; P; it is not the case that there is an exception
to the rule that if P then Q; therefore Q.
DMP
• This form of argument can be used for evaluating
defeasible inferences like the Tweety argument: If
Tweety is a bird, Tweety flies; Tweety is a bird;
therefore Tweety flies.
• This form of argument was called defeasible
modus ponens (DMP) by Walton (2002).
• An example (Copi and Cohen, 1998, p. 363) also
illustrates DMP: if he has a good lawyer then he
will be acquitted; he has a good lawyer; therefore
he will be acquitted.
Binary DMP Form
• The binary DMP form of argument has the following structure,
where A1, A2,…, An is a set of assumptions, E1, E2,…, En is set of
exceptions, and B is a proposition.
[(A1, A2,…, An) & (E1, E2,…, En)] => B
A1, A2,…, An
E1, E2,…, En
------------------------B
• There are two different kinds of prerequisites, the assumptions,
including the ordinary premises, and the exceptions, that have to be
met for the conclusion to be defeasibly inferred.
Research on Other Schemes
• Even though in this paper evidence has been presented
to show that the scheme for argument from expert
opinion has this DMP form, it remains to be seen how
many of the other schemes share it.
• Some of them, for example the scheme for argument
from lack of evidence, also called the argument from
ignorance, does not have this form (Caminada, 2008).
• It has a defeasible modus tollens form of argument.
Also, some of the other schemes are more complex,
and apparently need a separate study.
Conclusion
• It has also been argued that the general logical
DMP form is not itself an argumentation scheme,
but is better seen as a general logical category of
reasoning into which the schemes fit.
• Prakken (2010) also noted that some of the
arguments categorized as argumentation
schemes in the argumentation literature do not
really seem to be schemes in narrower sense
applicable to many of the other schemes.
• Instead, they appear to be more general
categories of reasoning.
References
Caminada, M. (2008). On the Issue of Contraposition of Defeasible Rules.
Computational Models of Argument: Proceedings of COMMA 2008. Ed. P. Besnard, S.
Doutre and A. Hunter. Amsterdam: IOS Press, 109-115.
Gordon, T. F. (2010). The Carneades Argumentation Support System, Dialectics,
Dialogue and Argumentation, ed. C. Reed and C. W. Tindale, London: College
Publications.
Prakken, H. (2010).On the Nature of Argument Schemes. Dialectics, Dialogue and
Argumentation, ed. Chris Reed and Christopher W. Tindale, London: College
Publications, 167-185.
Reed, C. and Walton, D. (2003). Diagramming, Argumentation Schemes and Critical
Questions, Anyone Who Has a View: Theoretical Contributions to the Study of
Argumentation, ed. F. H. van Eemeren, J. A. Blair, C. A. Willard and A. Snoek
Henkemans. Dordrecht: Kluwer, 195-211.
Verheij, B. (2001). Legal Decision Making as Dialectical Theory Construction with
Argumentation Schemes, The 8th International Conference on Artificial Intelligence
and Law: Proceedings of the Conference, New York Association for Computing
Machinery, 225-236. Available at http://www.ai.rug.nl/~verheij/publications.htm.
Walton, D. Appeal to Expert Opinion. University Park: Penn State Press, 1997.
Walton D. and Gordon, T. F. (2005). Critical Questions in Computational Models of
Legal Argument, Argumentation in Artificial Intelligence and Law, IAAIL Workshop
Series, ed. Dunne, P. E. and T. J. M. Bench-Capon. Nijmegen: Wolf Legal Publishers,
103-111.
Walton, D., Reed, C. and Macagno, F. (2008). Argumentation Schemes, Cambridge:
Cambridge University Press.