Informal Logic (study version).ppt

7/27/2019 Informal Logic (study version).ppt

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Legal Technique and Logic

INFORMAL/PRACTICAL REASONING


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OBJECTIVES

Informal ReasoningConcept

Distinction from Formal Logic

Tolmiens Model of Reasoning

Essential Three Parts Claim, Support & WarrantOther Parts Backing, Rebuttal & Qualifier

Fallacies

Fallacies of Ambiguity

Fallacies of RelevanceFallacies of Insufficient Evidence

Practical Exercise / Case Analysis


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Concept of Informal Reasoning

Informal logic has yet to come together as a

clearly defined discipline, one organized

around some well-defined and agreed upon

systematic techniques that have a definite

structure and can be decisively applied by

users.


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It is the attempt to formulate, to test, toclarify, and to systematize concepts andprinciples for the interpretation, the

evaluation, and the sound practice ofreasoning.

Informal logic is an attempt to develop a logicthat can assess and analyze the argumentsthat occur in natural language discourse.


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Common Misconceptions

It is philosophical analysis too broad!

It is merely a study of informal fallacies too

restrictive! It is formal logic without the formality no, where

formalism illuminates, informal logic employs it!

It only mediates between formal logic and reasoning in

the natural language no, it tackles implications thatare not entailments, and other aspects of argumentanalysis and evaluation that are not amenable to suchformal treatment.


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Premise 1: If a man holds the hand of another man,

then he is gay.

Premise 2: Her husband is holding the hand of anotherman.

Conclusion: Her husband is gay.


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Formal logic is a step by step progression from premise toconclusion.

Informal logic assumes a body of knowledge and premisesthe argument from that body of knowledge. i.e. it proceeds

stepwise from stipulation to conclusion.

Formal logic tells us that this argument is valid, in virtue ofbeing an instance of a general schematic form.

Informal logic, on the other hand, could rather conclude

that this argument lacks relevance or is informallyfallacious or invalid, as it would be frowned upon in aspecific debate.


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Formal logic looks at the schematic structure of theargument and its validity.

Informal logic looks at the content of the propositionsin the argument, determining its validity given the

current set of facts.

All handsome people are actually angels from heaven. I amhandsome. Therefore, I am an angel from heaven. (valid but notsound)

All law students are graduates of a college course. No graduate ofa college course is below 15 years of age. Therefore, no lawstudent is below 15 years of age. (valid and sound)


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Toulmins Model of Reasoning


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A - Hey B! If youre not married before the

normal marrying age of 20, then you are

not normal.

B - Hmph! You are normal. I dont want to be

you. Therefore, I dont want to be normal.


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Why

Limited practical value of the absolutism of formallogic, and radical uncertainty of relativism

How

Identify the basic parts of the argument

What For

To identify and analyze sources by identifying the basicelements of the arguments being made

To test and critique the argument


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A is a married woman while B is single.

A - Hey B! If youre not married before thenormal marrying age of 20, then you are

not normal.

B - Hmph! You are normal. I dont want to be

you. Therefore, I dont want to be normal.


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A - Hey B! There are so many women here

who get married by the age of 20 that a

lot of people pester us to find ourselves

husbands . And since everyone is so keento marry us off, we are forced to re-

evaluate our lives if we go past that age

and are still single (like yourself).


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Claim the conclusion; the position beingargued for

Data the supporting evidence to back up theclaim

Warrant the principle, provision or chain ofreasoning that connects the data tothe claim


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Claimwe are forced to re-consider our lives ifwe go past the age of 20 and are still single.

Data there are so many women who getmarried by the age of 20; the majority, in fact.

Warrant you care about how the communityvalues the norm of women getting married bythe time they are 20.


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Qualifier a statement of how strong

your claim is by acknowledging its limits or

conditions, if there are any

Rebuttal exceptions to the claim; description

and rebuttal of counter-examples and

counter-arguments.

Backing support, justification, reasons to

back up the warrant.


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QualifierBs re-evaluation of her life is the

most important activity she can do to stop

being pressured.

Rebuttal There are people who are career-oriented and have planned beforehand to

marry late; thus, re-evaluation is not needed.

Backing Your parents will be ridiculed by your

community if you dont consider marrying

soon. Also, you will die an old maid.


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Informal Fallacies

These are patterns of reasoning that are obviously incorrect for

reasons other than a flaw in the structure of the argument.

Sometimes, even if the premises are true, it does not follow with

100% certainty that their conclusions is true. Even a good inductiveargument with true premises might have a false conclusion; that

the argument is a good one and that its premises are true only

establishes that its conclusion is probably true.

The study helps to determine when a valid argument that is not

sound is accepted as sound. Only when an argument is also sound is

it 'true' according to some fact of knowledge acceptable to all.


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B would not want to marry if she loves women.

B loves women.

Therefore, B would not want to marry.


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Arguments that make use of a double

meaning or ambiguity of language to mislead

or misrepresent the truth.

They contain ambiguous words or phrases,

whose meanings shift and change more or less

subtly in the course of the argument and thus

render it fallacious


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Fallacy of Equivocation

It is committed when a key word or phrase is used withtwo or more different meanings in the same argument.

Love of women could mean different things:- Eros, or romantic love

- Phileo, or love between friends or people whoshare a common interest, belief, etc.

The use of the word love subtly changes over thecourse of the argument.


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Fallacy of Amphiboly

This occurs when the arguer misinterprets a statement thatis ambiguous owing to some structural defect and proceedsto draw a conclusion based on this faulty interpretation.

Mrs. A said that she will make an effort to go aroundthe community to stress the need for the earlymarriage of women. It follows that the community hasa number of women who need to be encouraged to

marry early.

Differences from Equivocation: (1) source of ambiguity and(2) the mistake of the author vs. the intent of the author


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Fallacy of Division

The exact reverse of composition, division goes from wholeto parts. It is committed when the conclusion of anargument depends on the erroneous transference of acharacteristic from a whole onto parts.

Humanity is the most intelligent of all living things.

Every human is the most intelligent compared to allnon-human living thing.

Congress voted in favor of the bill. Therefore, eachmember of congress voted in favor of the bill.


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Fallacy of Composition

This occurs when it is argued that because the partshave a certain characteristic, it follows that the wholehas the characteristic, too, and the situation is such

that the characteristic in question cannot belegitimately transferred from the parts to the whole.

Every person in the community can sing beautifully.Therefore, the community can sing beautifully.

A prick of a needle is tolerable. Therefore, to bepricked by a million needles is likewise tolerable.


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Fallacy of Accent

This occurs when the arguer illegitimately stresses one or morewords in the given statement and then proceeds to draw aconclusion based on the resultant interpretation.

It arises from the mistaken interpretation of a statement but themistake is due to the ambiguity in the way the statement is spoken.

B said that(she) does not(want to marry) (her) (boyfriend)(within this week).

B could mean that: (1) Somebody else wants to marry herboyfriend, (2) She just wants things to stay as is, (3) She wants tomarry someone elses, (4) She wants to marry someone not herboyfriend, or (5) She wants to marry but at a later time.


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Fallacies of Relevance

These are attempts to prove a conclusion byoffering considerations that simply dont bear onits truth. Arguments that commit fallacies ofrelevance do not offer relevant supporting

evidence; the considerations that they offer insupport of their conclusion are irrelevant todetermining whether that conclusion is true.

The considerations offered by such are usuallypersuasive, however, even if they dont have anyevidential value.


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Ad Hominem / Genetic Fallacy

It attempts to discredit a point of view by discrediting the person or sourcethat holds it. The character of the person or source that holds a view,though, entails nothing about the truth of that view.

Bandwagon Fallacy It is committed by arguments that appeal to the growing popularity of an

idea as a reason for accepting it as true. The arguer takes the mere factthat an idea suddenly attracting adherents as a reason for the listener tojoin in with the trend and become an adherent of the idea himself.

Fallacists Fallacy It involves rejecting an idea as false simply because the argument offered

for it is fallacious. Having examined the case for a particular point of view,and found it wanting, it can be tempting to conclude that the point of viewis false. This, however, would be to go beyond the evidence.


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Gamblers Fallacy

It is assuming that short-term deviations from probabilitywill be corrected in the long-term.

Moralistic Fallacy It moves from statements about how things ought to be to

statements about how things are; it assumes that the worldis as it should be.

Naturalistic Fallacy Opposite of the moralistic fallacy, it moves from

descriptions of how things are to statements of how thingsought to be


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Irrelevant Appeals

These are attempts to sway the listener with information that,though persuasive, is irrelevant to the matter at hand.

Some of the types are appeals to: Antiquity or Tradition,Authority, Consequences, Force, Novelty, Pity, Emotion,Popularity, Poverty, and Wealth.

(consequences) B does not believe in marrying early;stating that if she did, then she would be depressed for

being single for so long.

(wealth) A told B that since Bs aunt is already rich, then it iseasier for the aunt to bear her being single compared to B.


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Red Herring Fallacy

It is a fallacy of distraction, and is committed

when a listener attempts to divert an arguer from

his argument by introducing another topic.

Weak Analogy Fallacy

An argument by analogy is only as strong as thecomparison on which it rests. It is committed

when the comparison is not strong enough.


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Fallacies of Insufficient Evidence

These are cases where the proof or evidence

provided in support of a claim are not enough

to justify it, and that such lack is disguised by

sweeping generalization or by withholding itslimitations altogether.


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Fallacy of Exclusion Leaving out evidence that would lead to a different conclusion is

called the fallacy of exclusion.

A tells B that since 2008, majority of the women in the community

have been marrying early. Therefore, early marriage is espoused by it.(excluded: from 1900 to 2007, women have married late)

Fallacy of Oversimplification

In this fallacy, in which the relationship of causation betweenevents is misconstrued by either exaggerating the effect or reducing

the effect to the point that it no longer reflects the genuine natureof that relationship.

B argues that since A has started to pester her to marry, she has beenhaving identity and self-esteem issues.


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Fallacy of Exclusion A hasty generalization bases a conclusion on too little

evidence.

A tells B that more women married early this year than lastyear. It follows that her advice to B is becoming more accepted.

Appeal to Ignorance

It asserts a proposition is true simply on the basis that it

has not been proved false or that it is false simply becauseit has not been proved true.

B tells A that a Mr. Right who is perfect for her does not existbecause his existence has not been proven.


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Slippery Slope Fallacy It asserts that some event must inevitably follow from

another without any argument for the inevitability of theevent in question.

B tells A that she cant follow As advice to marry early. If shedid, A would then pressure B to have 12 children. The next thingshe knows, A will dictate in what schools Bs future children willstudy in.


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Practical Exercises


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Practical Exercise

Claim Fernando Poe, Jr. is a Filipino citizenand is eligible to run for the presidency.

Data His father was covered by the enmasse Filipinization of 1902.

Warrant Section 1, Article IV of the 1987 ConstitutionQualifier The same holds true even for illegitimatechildren of a Filipino parent as long asfiliation is established.

Rebuttal There is no proof that his father was in the

Philippines during that time.Backing The prevailing jurisprudence is that

citizenship is inherited from Filipinoparent/s, not where you were born.


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References

Maurice A. Finocchiaro, Informal Logic and the Theory of Reasoning, University of Nevada, 1984

Ralph W. Johnson and J. Anthony Blair, Informal Logic: An Overview, University of Windsor, 2000

http://en.wikipedia.org/wiki/Informal_logic

http://plato.stanford.edu/entries/logic-informal/#Two

http://rstechnica.com/tech-policy/2010/11/getting -qed-part-1/

http://owlet.letu.edu/contenthtml/research/toulmin.html

http://www.iep.utm.edu/val-snd/http://www-rohan.sdsu.edu/~digger/305/toulmin_model.htm

http://logical-critical-thinking.com/logic/informal-logic/

http://www.artofmanliness.com/2011/05/26/classical-rhetoric-101-logical-fallacies/

http://online.santarosa.edu/homepage/bgear/FALLACYLIST.htm

http://www.logicalfallacies.info/

http://www.philosophypages.com/lg/e06a.htm

http://www.nizkor.org/features/fallacies/

Basic Instructions comic by Scott Meyer

Calvin and Hobbes comic by Bill Waterson

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