Transcript LOGIC AND CRITICAL THINKING

LOGIC
AND
CRITICAL THINKING
by
Karey Perkins
RHETORICAL APPEALS:
USING LOGOS, ETHOS, PATHOS
• LOGOS = LOGIC and REASON= Soundness of facts,
evidence, statistics, and reasoning; soundness of
authority’s statements outside self; well-documented
evidence
• ETHOS = Credibility and reliability of writer
him/herself; character and reputation of the author
• PATHOS = EMOTION = Appeal to needs, values, and
attitudes; uses the emotional power of language
LOGOS: Reason, Facts
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20 tests conducted over a period of 5 years, no pollutants
were released into the atmosphere. The world will soon have
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exploring ways to use the process in cars, houses, airplanes,
and almost anything else that needs power. ACME batteries
will be refitted with small dihydro-cesium reactors. Once the
entire world is powered by ACME’s generators, we can all
relax and enjoy a much easier life.
ETHOS: Credibility of Source
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PATHOS: Emotional Appeals
CESIUM-BASED REACTOR KILLS!
A baby turtle breaks free from the leathery shell of its egg,
catching its first glimpse of its first sunrise. It pauses a
moment to rest, unaware of the danger that lies so close to it.
As the tide comes in, approaching the nest, it also approaches
a small pile of metal: cesium. The water draws closer and
closer, the turtle unsuspecting of the danger. Finally, the water
touches the cesium.
The nest is torn to bits in the resulting explosion, destroying
even more of an endangered species.
Why does this happen? One name: Acme.
(Examples from: The Art of Rhetoric: Learning How to Use the Three Main Rhetorical Styles. Available at:
(http://www.rpi.edu/dept/llc/webclass/web/project1/group4/ ))
Inductive and Deductive Reasoning
• Inductive Reasoning:
• Reasons from specific to general
• Notices many facts and comes to a general conclusion
• No certainty possible
• Deductive Reasoning:
• Reasons from general to specific
• Starts with a hypothesis and inserts a fact and comes to
a conclusion based on hypothesis
• Certainty can be possible if valid and true syllogism (say
some people)
INDUCTIVE REASONING
• Reasoning from sensory observation of specific
facts/evidence to general conclusion
• With inductive reasoning, there can NEVER be certainty,
because only ONE example can modify or refute the
conclusion. (This example can come from any future event, or
events in remote places and times we are not able to
observe.)
• Based on an accumulation of many facts (one fact = “x”):
• Observation of:
xxxxxxxxxxxxxxxxxxxxxxxx
Equals
General Statement about X’s in future
Examples of inductive reasoning
• If I jump off the building, I will fall to the ground.
(More specifically: Every time something with
weight and mass is released from any height, it falls
to the ground. Therefore, all things fall to the ground
(law of gravity).
• Based on an evaluation and observation of multiple
studies of adolescents who value peers more than
parents, Judith Rich Harris concludes that peers
matter more than parents in this age group.
• Whenever it snows or rains, it will eventually stop
snowing or raining.
Faulty Inductive Reasoning
• Today I saw an 80 year old lady driving 40 miles an
hour on 285, a 95 year old man going 50 mph on
400, and a 70 year old guy going 20 mph on Haynes
Bridge Road. Conclusion: Old people drive slow.
(This is a “stereotype,” or logical fallacy of inductive
origin.)
• Jesse Helms said in the Mexican Foreign Affairs
Subcommittee (to prevent immigration from
Mexico): “All Latins are volatile.” (This is a
“stereotype,” or logical fallacy of inductive origin.)
Deductive Reasoning
• Reasoning from general tenets and premises
to specific conclusions.
• With deductive reasoning, there CAN be
certainty.
• Syllogisms follow this format:
• Major Premise (general truth about life/humans)
• Minor Premise (specific fact that falls under the truth)
• Conclusion (a conclusion that can be drawn about the
specific fact based on the first generalization)
Syllogism Examples
Correct Syllogism:
• Major Premise: All mammals are warm-blooded animals.
• Minor Premise: No lizards are warm-blooded animals.
• Conclusion: Therefore, no lizards are mammals.
Correct Syllogism:
• Major Premise: All humans are mortal.
• Minor Premise: All Greeks are human.
• Conclusion: Therefore, all Greeks are mortal.
Descartes’ Syllogism (correct)
• Major Premise: Existence has be true if one is thinking.
• Minor Premise: I am thinking.
• Conclusion: I think, therefore, I am.
Syllogisms can be
• Valid or Invalid (reasoning in
incorrect order)
AND
• True or False (reasoning from a
faulty major premise)
Examples of Faulty Syllogisms
FALSE Syllogism (not TRUE -- false major premise)
– Major Premise: Blondes have more fun
– Minor Premise: Mary is blonde; Jane is brunette
– Conclusion: Mary has more fun than Jane.
INVALID Syllogism (not VALID – order of reasoning is
incorrect):
– Major Premise: All dogs eat meat
– Minor Premise: Bob (a human) eats meat
– Conclusion: Bob is a dog.
Corrections
Syllogism One:
The first faulty syllogism proceeds from a FALSE
major premise and therefore can be thrown out
entirely.
Syllogism Two:
– Major Premise: All dogs eat meat
– Minor Premise: Rover is a dog.
– Conclusion: Therefore, Rover eats meat.
Valid or invalid? True or False?
Example One:
• Major Premise: When it snows the streets get wet.
• Minor Premise: The streets are getting wet.
• Conclusion: Therefore, it is snowing.
Example Two:
• Major Premise: If you buy a Ferrari, you will instantly be
popular.
• Minor Premise: Ed just bought a Ferrari.
• Conclusion: Ed will achieve instant popularity.
Example Three:
• Major Premise: When the battery is dead, the car will not start.
• Minor Premise: The car will not start.
• Conclusion: Therefore, the battery is dead.
Corrections: Valid and True
Example One:
• Major Premise: When it snows, the streets get wet.
• Minor Premise: It is snowing.
• Conclusion: Therefore, the streets are getting one.
Example Two:
• Example Two proceeds from the beginning from a FALSE
major premise (Ferraris give instant popularity) and therefore
can be thrown out entirely.
Example Three:
• Major Premise: When the battery is dead, the car will not
start.
• Minor Premise: The battery is dead.
• Conclusion: Therefore, the car will not start.
Some types of syllogisms
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Modus Ponens
Modus Tollens
Hypothetical Syllogism
Disjunctive Syllogism
Modus Ponens
1. If A then B
2. A
3. Therefore, B
Examples:
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If it’s spring, then the birds are chirping
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It’s spring.
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The birds are chirping.
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If a world government doesn’t evolve soon, then wars will continue to occur
A world government isn’t going to evolve soon.
Wars will continue to occur
Modus Tollens
1. If A then B
2. Not B
3. Not A
Example:
1. If it’s spring then the birds are chirping
2. The birds aren’t chirping
3. Therefore, it isn’t spring.
Hypothetical Syllogism
1. If A then B
2. If B then C
3. If A then C
Example:
1.
2.
3.
If we successfully develop nuclear fusion power, then power will
become plentiful and cheap.
If power becomes cheap and plentiful, then the economy will flourish.
If we successfully develop nuclear fusion power, then the economy will
flourish.
Disjunctive Syllogism
1. A or B
2. Not A
3. B
Example:
1. Either Romney won in 2012 or Obama did.
2. Romney didn’t win.
3. Obama did win.
LOGICAL FALLACIES TO AVOID
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Circular Reasoning/Begging the Question: Promising support for your claim but
providing none
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Bandwagon/: Inviting readers to accept a claim because everyone else does or
because the prestigious group does
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Argument from authority: Inviting readers to accept a claim because because the
claim is put forth by someone in a position of authority – though the authority is
invalid.
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Slippery Slope: forecasting a series of events (usually disastrous) that will befall
one if the first stated step is taken.
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Straw Man: Inserting a false or unrelated premise into an argument, and then
proving the false or unrelated premise wrong as a claim that the initial argument is
wrong.
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Appeal to Fear: scaring the reader to your point of view
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Appeal to Pity: substituting emotions for reasoning.
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Appeal to Force: abandoning reason and using or threatening strong arm methods
by means of the political or physical power of the enforcer; “might makes right”
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Red Herring: introducing an irrelevant issue intended to distract readers from the
relevant issues – going off on a tangent.
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Self Contradiction: using two premises that can’t be simultaneously true
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Ad Hominem: (attacking the man) attacking the qualities of the people holding an
opposing view rather than the substance of the view itself.
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Guilt by Association: kind of “ad hominem” attack implying that an individual’s
arguments, ideas, or opinions lack merit because of that person’s activities,
interests or associates
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False Cause or “Post Hoc” Fallacy: (from Latin: post hoc, ergo propter hoc,
meaning “after this, therefore because of this”): assuming that because A
preceded B, then A must have caused B.
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False Analogy: Assuming that because two things are alike in similar ways, they
must be alike in other ways
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Either/Or Fallacy (False Dilemma): assuming that a complicated question has only
two answers—one good, and one bad, both good, or both bad.
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Hasty Generalization: making a claim on the basis of inadequate evidence.
Generalizing about something on the basis of too little evidence.
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Stereotyping is a type of Hasty Generalization applied to a group of people.
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Sweeping Generalization: making an insupportable statement, often using
absolute statements such as all, always, never, and no one.
• Non Sequitur: (Latin: It does not follow): linking two or more ideas that in
fact have no logical connection; the statement does not follow logically
from what has just been said
• Card Stacking/Special Pleading: Ignores evidence on the other side of the
question; only selecting those items that will build the best (or worst)
possible case.
• Oversimplification/Reductive Fallacy: Oversimplifying (reducing) the
relation between causes and effects
• Quibbling: Nitpicking at insignificant or possible errors in someone else’s
basically valid and sensible argument
• Language Fallacies:
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Emotional, biased, or slanted language
Equivocation
Ambiguity or obfuscation
Euphemism or PC
Doublespeak
Pretentious language
Bureaucratic language
Jargon
What is logically wrong with the following
statements?
• The Bible is true because it says so.
• Boxing is dangerous because it is an
unsafe sport.
Circular Reasoning/Begging the Question
• A.
Therefore, A.
• promising support/evidence/reasoning for your claim but
providing none
•
Promising:
Claim
•
Delivering:
Claim
(Support)
Support
??
• Billy Joe is honest; therefore, he will get a
good job
Non Sequitur
• (Latin for “It does not follow”): linking two or
more ideas that in fact have no logical
connection
• Missing is the unstated assumption that
“honest people get good jobs” which is not
always true.
??
• That candidate smoked pot when he was in
college; therefore, he should not be president
of the United States.
• That candidate had an affair; therefore, he
should not be president of the United States.
Ad Hominem
“Attacking the Man”
• attacking the personal qualities of the people
holding an opposing view rather than the
substance of the view itself
• In this case, the candidate may well be an
excellent president and policy maker even
though we may prefer not to have him as our
husband.
??
• Jane is friends with Bob who was caught
embezzling from the company; therefore, she
should be fired.
Guilt by Association
• A kind of “ad hominem” attack implying that
an individual’s arguments, ideas, or opinions
lack merit because of that person’s activities,
interests or associates
??
• Professor Perkins, if I don’t make an “A” in your class
I won’t get my HOPE scholarship next semester.
• A plaintiff’s attorney brings his injured client, who is
seeking compensatory damages, into the courtroom
and plays up the injury in front of the jury (“A Civil
Action”)
Appeal to Pity
• substituting emotions for reasoning; an appeal
to emotion, in which the altruism and mercy
of the audience are appealed to
??
• When Lyndon Johnson was running for president, his
team ran a commercial of a little girl in a field holding
a daisy, then an atomic bomb went off in the
background.
http://www.youtube.com/watch?v=ExjDzDsgbww
• The Bush campaign ran a 2004 commercial showing
images from 9/11. (It was immediately withdrawn
due to the families of victims complaining).
Appeal to Fear
• Tries to scare the reader to your point of view
??
• Mom, I should be able to go to Underground Atlanta
to see the free Weezer concert that lasts til 1 AM –
all my friends’ moms are letting them go.
• Despite the cost, impending gas crisis, and the harm
to the environment, it is desirable to own an SUV
because successful people do.
• More people in America drink Budweiser than any
other beer; therefore, it must be the best beer and
you should buy it.
Bandwagon
• Inviting readers to accept a claim because
everyone else does or because a prestigious
group does
??
• Television commercials that advertise their
product using famous stars, for example, Ray
Charles advertising Coke.
• Matt Damon should write an expose about
the CIA since knows all about it; he was in the
Bourne trilogy.
False Authority
• Citing the opinion of an expert who has no
real claim to expertise on the topic
??
• You should go on the “Lose Weight Overnight”
diet plan because Mary Smith and Tom Jones
lost 100 pounds total in three months on it.
Card Stacking
Special Pleading
• Ignores evidence on the other side of the
question
• Only selecting those items that will build the
best (or worst) possible case.
??
• The new mayor took office last January and
crime in the streets has already increased 25
percent.
• A new weather satellite was launched last
week and it has been raining ever since.
• After that black cat crossed my path this
morning, I got into a car accident. That cat
was bad luck.
Post Hoc Fallacy
False Cause
• (from Latin: post hoc, ergo propter hoc,
meaning “after this, therefore because of
this”): assuming that because A preceded B,
then A must have caused B.
??
• Banning pesticides to improve the environment is
as impractical and unfeasible as banning the
automobile.
• Education cannot prepare men and women for
marriage. Trying to educate them for marriage is
like trying to teach them to swim without allowing
them to go into the water. It can’t be done.
False Analogy
• using a comparison in which two things are
alike in one way to justify that they will be
alike in another way. However, the similarities
are irrelevant to the claim the analogy is
intended to support.
1. A is like B.
2. B has property P.
3. Therefore, A has property P.
??
• America: love it or leave it.
• We have only two choices: ban nuclear
weapons or destroy the earth.
• Either we go to war against Iran, or the United
States will suffer another terrorist attack.
Either/Or Fallacy
and False Dilemma
• Stating that only two alternatives exist when
in fact there are more than two
??
• Teenagers are reckless drivers
• America is the best place to live (stated
by one who has rarely visited other
countries)
• “All Latins are volatile people”
Jesse Helms to the Mexican Foreign Affairs
Subcommittee (to prevent immigration from Mexico)
Hasty Generalization
• Generalizing or making a claim on the basis of
inadequate evidence.
??
• Why spend money on solving the problem of
global warming when we ought to be doing
something about the terrorists?
Red Herring
• introducing an irrelevant issue intended to
distract readers from the relevant issues –
going off on a tangent:
1. Topic A is under discussion.
2. Topic B is introduced under the guise of being
relevant to topic A (when topic B is actually not
relevant to topic A).
3. Topic A is abandoned.
??
• If we make handguns illegal, the state will gain too
much power and eventually put us all in
concentration camps for the slightest infraction.
• If we legalize marijuana, eventually everyone will
start smoking it regularly (at 4:20) and then they’ll
want to go on to more kinds of drugs – soon heroin
will become legal and most of America will become
heroin addicts.
Slippery Slope
• forecasting a series of events (usually
disastrous) that will befall one if the first
stated step is taken
??
• Only when nuclear weapons have finally
destroyed us will we be convinced of the
need to control them.
Self Contradiction
• using two premises that can’t be
simultaneously true
??
• After Will said that we should put more money into
health and education, Warren responded by saying
that he was surprised that Will hates our country so
much that he wants to leave it defenseless by cutting
military spending.
Straw Man
• Inserting a false or unrelated premise into an
argument, and then proving the false or
unrelated premise wrong as a claim that the
initial argument is wrong:
1. Person A has position X.
2. Person B presents position Y (which is a distorted or
completed unrelated version of X).
3. Person B attacks position Y.
4. Therefore X is false/incorrect/flawed.
??
• “I am firm, you are stubborn, he is
pigheaded.” – Bertrand Russell
• “Friendly fire” – military term for killing your
own team by mistake
• “Collateral damage” – military term for people
who are inadvertently killed or property
inadvertently destroyed in warfare
Language Fallacies
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Emotional or biased language
Equivocation
Ambiguity or obfuscation
Euphemism or PC language
Doublespeak
Pretentious language
Bureaucratic language
Jargon
Impediments to Logic,
Good Reasoning, and Critical Thinking
• LOYALTY – loyalty to our own community or group; see its beliefs as more
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favorable than others
PROVINCIALISM – narrowing our view to identify with ideas and interests of
only our own group
THE HERD INSTINCT – keeping our beliefs and actions within society’s
(or our community’s) boundaries
BACKGROUND BELIEFS
PREJUDICE AND STEREOTYPING
SCAPEGOATING – problems blamed on a person/group
PARTISAN MINDSET – “us” vs. “them”
SUPERSTITIOUS BELIEFS
WISHFUL THINKING AND SELF-DECEPTION
RATIONALIZATION – ignore evidence to justify actions
SUPPRESSION – avoiding the anxiety of stressful thoughts
DENIAL – change our view of the facts of the situation to be more positive
in spite of evidence
• LACK OF A SENSE OF BALANCE OR PRUDENCE
SUCCESSFUL ARGUMENTS
persuade the reader by means of:
• Clear premises and conclusions
• Evidence, not unsupported claims and appeals to
emotions
• Consideration of the other side
• Consideration of audience to whom argument is
given
• Use appropriate tone and diction
• Avoidance of logical fallacies
EVALUATING EVIDENCE
IN AN ARGUMENT
• Is the evidence:
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SUFFICIENT
REPRESENTATIVE
RELEVANT
ACCURATE
FAIR AND BALANCED
Argument: The Toulmin Model
• Claim: Main point or central message;
thesis statement
• Support: Data, evidence, reasons, details
• Warrant: Underlying assumptions implied
but not stated. Reader infers
assumptions.
Warrants are based on:
– Authority: respect for credibility and trustworthiness of source
– Substance: reliability of facts and evidence
– Motivation: values and beliefs of audience and writer
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“The object of reasoning is to find out, from the consideration of what we already know, something
else which we do not know.” -- Charles Sanders Peirce
“A simple person believes every word he hears; a clever one understands the need for truth.” –
Proverbs 14:15
“Those who do not remember the past are condemned to relive it.” --- George Santayana
(One government official to another): “Congratulations Dave! I don’t think I’ve read a more
beautifully evasive and subtlely misleading statement in all my years in government.” – cartoon in
the New Yorker by Stevenson
“Read not to contradict and confute, nor to believe and take for granted … but to weigh and
consider.” – Francis Bacon
“Ignorance of reality provides no protection from it.” – Harold Gordon
“Every man is encompassed by a cloud of comforting convictions, which move with him like flies on
a summer day.” – Bertrand Russell
“Arguments, like men, are often pretenders.” – Plato
“There are lies, damn lies, and statistics.” – Benjamin Disraeli
“Every dogma has it’s day.” – Abraham Rotstein
“Nothing is so firmly believed as what we least know.” – Montaigne
“Populus vult decipi.” (The people want to be deceived.) – Ancient Roman saying
“Logic is only the beginning of wisdom.” -- Spock, Star Trek V, The Undiscovered Country
"If I release a hammer on a positive gravity planet, I do not have to watch it to know that it will
fall." – Spock, Star Trek TV seriies, episode “The Court Martial”
“When an idea is wanting a word can always be found to take its place.” – Goethe
“He who defines the terms wins the argument.” – Chinese proverb
“There is no expedient to which a man will not resort to avoid the real labor of thinking.” – Sir
Joshua Reynolds
“There are no dull subjects. There only dull writers.” -- H. L. Mencken
“Advertising is legalized lying.” – H.G. Wells
Works Cited and Works Consulted
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Copi, Irving M. and Carl Cohen. Introduction to Logic, 10th ed.
Upper Saddle River, NJ: Prentice Hall, 1998.
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DeVry/Alpharetta Coll 147/149 Resource Notebook. General Education
Department. Alpharetta, GA: DeVry University, 2004.
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Hodges, John C. & Mary Whitten. Harbrace College
Handbook, 10th ed. New York: HBJ, 1986.
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Kahane, Howard and Nancy Cavender. Logic and
Contemporary Rhetoric: The Use of Reason in
Everyday Life. Belmont, CA: Wadsworth/Thomson
Learning, 2002.
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Troyka, Lynn Quitman. Simon & Schuster Handbook for
Writers, 5th ed. Upper Saddle River, NJ: Prentice
Hall, 1999.