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Lecture 6 - Logical Fallacies I

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Lecture 6 - Logical Fallacies I
Ling 21, Lecture 6:
Logical Fallacies - I
Two weeks ago, we studied about…
• Various patterns of deductive arguments.
– Modus ponens
– Categorical syllogisms
– Argument by elimination
– Argument based on mathematics
– Argument from definition
– *Denying the antecedent
– *Affirming the consequent
*These are faulty patterns of deductive reasoning!
Two weeks ago, we also studied about
• Various patterns of inductive arguments
– Inductive generalization
– Predictive argument
– Argument from authority
– Causal argument
– Statistical argument
– Argument from analogy
Finally, we also studied about …
• Deductive validity and soundness
• Inductive strength and cogency
Deductive
Inductive
Valid
Invalid
Strong
Weak
(all are
(all are
unsound
uncogent)
Sound Unsound
Cogent Uncogent
YOU MUST KNOW THESE
THINGS
TO GET THROUGH THIS
COURSE!!!
This week, we will learn about …
Logical Fallacies
What you should get from Ch. 5
• You should understand that a logical fallacy is
an argument that contains a mistake of
reasoning. Further, you should note that . . .
• fallacies are divided into two broad categories:
– Fallacies of relevance, and
– Fallacies of insufficient evidence
Logical Fallacy
• A logical fallacy – or fallacy for short – is an argument
that contains a mistake in reasoning.
• Fallacies of relevance are mistakes in reasoning that
occur because the premises are logically irrelevant to the
conclusion.
• Fallacies of insufficient evidence are mistakes in
reasoning that occur because the premises, though
logically relevant to the conclusion, fail to provide
sufficient evidence to support the conclusion.
The Concept of Relevance
• A statement is relevant to another when it
provides at least some evidence or reason for
thinking that the second statement is true or
false.
• A statement can be either
– Positively relevant
– Negatively relevant
– Logically irrelevant
Positive Relevance
• A statement is positively relevant to another
statement if it counts in favor of that statement.
– Labradors are dogs. Dogs are domestic animals, So
Labradors are domestic animals.
– Most SJSU students live off-campus. Annie is an
SJSU student. So probably Annie lives off-campus.
– Chris is a woman. Therefore, Chris enjoys knitting.
• Each of the premises is positively relevant to the
conclusion.
2 Important Points about Relevance
• A statement can be relevant to another
statement even if the first statement is
completely false.
– Dogs are cats. Cats are felines. So dogs are felines.
• Whether a statement is relevant to another
usually depends on the context in which the
statement is made.
– A) All dogs have five legs. B) Rover is a dog. So C)
Rover has five legs.
– A is positively relevant to C only because of B
Negative Relevance
• A statement is negatively relevant to another
if it counts against that statement.
– Marty is a high- school senior. So Marty likely has
a Ph.D.
– Althea is two years old. So Althea probably goes to
college.
Logical Irrelevance
• A statement is logically irrelevant to another
statement if it counts neither for nor against
that statement.
– The earth revolves around the sun. Therefore,
marijuana should be legalized.
– Last night I dreamed that the Yankees will win the
pennant. Therefore, the Yankees will win the
pennant.
Exercise 5.1, p. 121
Fallacies of Relevance
• Occur when an arguer offers reasons that are logically
irrelevant to his or her conclusion
–
–
–
–
–
–
–
–
–
–
–
Personal Attack (Ad Hominen)
Attacking the Motive
Look Who’s Talking (Tu Quoque, /tu kwoʊkwɛ/ )
Two Wrongs Make a Right
Scare Tactics
Appeal to Pity
Bandwagon Argument
Straw Man
Red Herring
Equivocation
Begging the Question
Personal Attack (Ad Hominem)
• Rejects someone’s argument or claim by attacking the
person rather than the person’s argument or claim.
a) X is a bad person.
b) Therefore, X’s argument must be bad.
Example: Hugh Hefner, founder of playboy magazine, has
argued against censorship of pornography. But Hefner is an
immature, self-indulgent millionaire who never outgrew the
adolescent fantasies of his youth. His argument, therefore, is
worthless.
Hugh Hefner is a bad person.
Therefore, Hugh Hefner’s argument must be bad.
Personal Attack (Ad Hominem)
•
The fallacy of personal attack occurs only if
1) An arguer rejects another person’s argument or
claim,
AND
2) The arguer attacks the person who offers the
argument or claim, rather than considering the
merits of that argument or claim.
Personal Attack (Ad Hominem)
• Not all personal attacks are fallacies!!!
– Millions of innocent people died in Stalin’s ruthless
ideological purges. Clearly Stalin was one of the most
brutal dictators of the twentieth century.
– Ms Fibber has testified that she saw my client rob the
Bank. But Ms Fibber has twice been convicted of
perjury. In addition, you’ve heard her own mother
testify that she is a pathological liar. Therefore, you
should not believe Ms. Fibber’s testimony against my
client.
In these cases, the personal attacks are relevant to the
conclusion so no fallacy is committed.
Attacking the Motive
•
An arguer criticizes a person’s motivation for offering a
particular argument or claim, rather than examining the
worth of the argument or claim itself.
a) X is biased or has questionable motives.
b) X’s argument or claim should be rejected.
Examples:
Professor Smith has argued in favor of academic tenure. But
why should we even listen to him? As a tenured professor,
of course he supports tenure.
Senator Pork supports the stimulus package. Representing a
state that will get a new bridge, of course he supports it.
BUT ….
• ‘Burton Wexler, spokesperson for the American Tobacco
Growers Association, has argued that there is no credible
scientific evidence that cigarette smoking causes cancer.
Given Wexler’s obvious bias in the matter, his arguments
should be taken with a grain of salt.’
• This argument reflects a common sense assumption that
the arguments put forward by Mr. Wexler need to be
scrutinized with particular care. It is not a fallacy of
attacking the motive.
Look Who’s Talking (Tu Quoque /tu kwoʊkwɛ/ )
• An arguer rejects another person’s argument or claim
because that person fails to practice what he or she
preaches.
a) X fails to follow his or her own advice.
b) Therefore, X’s claim or argument
should be rejected.
Examples:
Doctor: You should quit smoking.
Patient: Look who’s talking! I’ll quit when you quit.
Parent: I don’t want you to smoke marajuana.
Son:
But you told me that you did when you
were my age.
BUT ….
• Jim: Our neighbor Joe gave me a hard time
yesterday about washing my car during this
drought emergency.
Patti: Well, he’s right. But I wish that hypocrite
would follow his own advice. Just last week I saw
him watering his lawn in the middle of the
afternoon.
• Patti is not rejecting any argument by the
neighbor, so no fallacy is committed.
Two Wrongs Make a Right
•
An arguer attempts to justify a wrongful act by claiming
that some other act is just as bad or worse.
a) Others are committing worse or equally bad acts.
b) Therefore my wrongful act is justified.
Examples:
I don’t feel guilty about cheating on Dr. Boyer’s tests. Half
the class cheats on his tests.
Why pick on me, officer? Nobody comes to a complete
stop at that stop sign.
Mom: Kaia, stop hitting your sister.
Kaia: Well, she pinched me.
BUT ….
• Are these cases of ‘2 Wrongs Make a Right?’
– Umpire: Why did you throw at the batter’s head?
Pitcher: Because he threw at three of our players. I have
an obligation to protect my teammates if you guys don’t.
– Jeff Dahmer murdered seventeen men in cold blood.
Therefore, Jeff Dahmer should be put to death.
They commit the fallacy of ‘2WMR’ only if the
justification is insufficient to warrant the apparent
wrong-doing – debatable!
Scare Tactics
• An arguer threatens harm to the reader / listener
and this threat is irrelevant to the truth of the
arguer’s conclusion.
– Diplomat to diplomat: I’m sure you’ll agree that we are
the rightful rulers of the San Marcos Islands. It would
be regrettable if we had to send armed forces to
demonstrate the validity of our claim.
– Gun lobbyist to politician: This gun-control bill is wrong
for America, and any politician who supports it will
discover how wrong they were at the next election.
BUT ….
a) Parent to teen: If you come home late one more time,
your allowance will be cut.
b) President John Kennedy to Soviet Premier Nikita
Krushchev: If you don’t remove your nuclear missiles
from Cuba, we will have no choice but to remove them
by force. If we use force to remove the missiles, that
may provoke an all-out nuclear war. Neither of us wants
a nuclear war. Therefore, you should remove your
missiles from Cuba. (paraphrase)
a) = statement, not an argument; b) = not a fallacy; premises
are logically relevant to conclusion
Appeal to Pity
• An arguer attempts to evoke feelings of pity or
compassion, when such feelings are not logically relevant
to the arguer’s conclusion.
– Student to professor: I know I missed half your classes and
failed all my exams, but I had a really tough semester. First
my pet boa constrictor died. Then my girlfriend told me she
wants a sex-change operation. With all I went through this
semester, I don’t think I really deserved an F. Any chance
you might cut me some slack and change my grade?
– Parent to football coach: I admit that my son Billy can’t run,
pass, kick, catch, block or tackle, but he deserved to make
the football team. If he doesn’t make the team, he’s going
to be an emotional wreck, and he may even drop out of
school.
BUT ….
• What about these arguments?
Mother to daughter: Nana was asking about you the other day.
She’s so lonely and depressed since Grandpa passed away,
and her Alzheimer’s seems to get worse every day. She’s
done so much for you over the years. Don’t you think you
should pay her a visit?
High school softball coach: Girls, this state championship is the
biggest game of your lives. This is what you’ve been working
for all year. Your parents are counting on you, your school is
counting on you, and your community is counting on you.
Make them proud! Play like the champions you are!
Here the emotional appeals are appropriate and relevant to
the arguers’ purposes; hence no fallacy is committed.
Bandwagon Argument
• An argument plays on a person’s desire to be popular,
accepted, or valued, rather than appealing to logically
relevant reasons or evidence.
a) Most (or a select group of) people believe or do X.
b) Therefore, you should believe or do X.
Examples:
– All the really cool kids in your fraternity smoke
cigarettes. Therefore, you should, too.
– There must be something to astrology. Millions of
Americans can’t be wrong.
BUT ….
• All the villagers I’ve talked to say that the water is
safe to drink. Therefore, the water probably is
safe to drink.
• Lots of my friends recommend the Back Street
Deli, so it’s probably a good place to eat.
• In these bandwagon appeals, the premises are
relevant to the conclusion, so the arguments are
not fallacious.
Straw Man
• An arguer distorts an opponent’s argument or claim in
order to make it easier to attack
A) X’s view is false or unjustified [but where X’s
been unfairly characterized].
B) Therefore, X’s view should be rejected.
view has
• Examples:
Pete has argued that the NY Yankees are a better baseball team
than the Atlanta Braves. But the Braves aren’t a bad team. They
have a great pitching staff, and they consistently finish at or
near the top of their division, Obviously, Pete doesn’t know
what he’s talking about.
Senator Biddle has argued that we should outlaw violent
pornography. Obviously the senator favors complete
governmental censorship of books, magazines, and films.
Frankly, I’m shocked that such a view should be expressed on
the floor of the U.S. senate. It runs counter to everything this
great nation stands for.
Red Herring
• An arguer tries to sidetrack his or her audience by raising an
irrelevant issue and then claims that the original issue has
effectively been settled by the irrelevant diversion.
Examples:
Many people criticize Thomas Jefferson for being an owner of slaves.
But Jefferson was one of our greatest presidents, and his
Declaration of Independence is one of the most eloquent pleas for
freedom and democracy ever written. Clearly these criticisms are
unwarranted.
Critics have accused my administration of doing to little to save the
family farm. These critics forget that I grew up on a farm. I know
what it’s like to get up at the crack of dawn to milk the cows. I
know what it’s like to work in the field all day in the blazing sun.
Family farms are what made this country great, and those who
criticize my farm policies simply don’t know what they’re talking
about.
BUT ….
Political opponent: Congressman Crookley, now
that you have been convicted of bribery,
extortion, and grand theft auto, isn’t it high time
that you resigned from office?
Rep. Crookley: How ‘bout those Yankees? A tengame lead at the All-Star break!
Simply changing or evading the subject without
denying the charge or pretending to refute it is
not a fallacy.
Equivocation
• A key word is used in two or more senses in the same
argument and the apparent success of the argument
depends on the shift in meaning.
– Any law can be repealed by the proper legal
authority. The law of gravity is a law. Therefore, the
law of gravity can be repealed by the proper legal
authority.
When the two senses of ‘law’ (laws regulating human
conduct vs. uniformities of nature) are made explicit,
it is apparent that the premises don’t support the
conclusion, hence a fallacious argument!
Begging the Question
• An arguer states or assumes as a premise the very thing he
or she is trying to prove as a conclusion.
• Two common ways to beg the question
– Restating the conclusion in slightly different words.
Capital punishment is morally wrong because it is ethically
impermissible to inflict death as punishment for a crime.
– Circular reasoning
B: God wrote the bible.
N: How do you know?
B: Because it says so in the Bible and what the Bible says is
true.
N: How to you know what the Bible says is true?
B: Because God wrote the Bible.
Let’s practice recognizing some of these
fallacies!
According to the song, the pinball wizard
is deaf, dumb, and blind. Dumb people
aren't very smart. So, the pinball
wizard isn't very smart.
Based on your reading of this chapter, what
fallacy does this argument commit?
According to the song, the pinball wizard is deaf,
dumb, and blind. Dumb people aren't very smart. So,
the pinball wizard isn't very smart.
The fallacy of equivocation.
The arguer uses the word "dumb" in two different
senses.
In the first sentence, "dumb" means "unable to
speak." In the second sentence, it means
"unintelligent."
Consequently, although the argument may
superficially appear to be valid, the premises
do not support the conclusion.
I'm trying hard to understand this guy who identifies
himself as a security supervisor and criticizes the police
officers in this area. I can only come up with two
solutions. One, he is either a member of the criminal
element, or two, he is a frustrated security guard who
can never make it as a police officer and figures he can
take cheap shots at cops through the newspaper.
(adapted from a newspaper call-in column)
Based on your reading of this chapter, what
fallacy does this caller commit?
I'm trying hard to understand this guy who identifies himself as a
security supervisor and criticizes the police officers in this area. I
can only come up with two solutions. One, he is either a member
of the criminal element, or two, he is a frustrated security guard
who can never make it as a police officer and figures he can take
cheap shots at cops through the newspaper. (adapted from a
newspaper call-in column)
The fallacy of personal attack.
The caller never responds to the previous caller's
arguments. Instead, he simply attacks his or her
character.
By criticizing the previous caller's motives, the
arguer also commits the fallacy of attacking the
motive.
The Red Cross is worried about the treatment of the suspected
terrorists held by the U.S. at Guantanamo Bay, Cuba. What do
they want the U.S. to do with them, put them on the beaches of
Florida for a vacation or take them skiing in the Rockies? Come
on, let's worry about the Americans. (adapted from a newspaper
call-in column)
Based on your reading of this chapter, what
fallacy does this argument commit?
The Red Cross is worried about the treatment of the
suspected terrorists held by the U.S. at Guantanamo Bay,
Cuba. What do they want the U.S. to do with them, put them
on the beaches of Florida for a vacation or take them skiing in
the Rockies? Come on, let's worry about the Americans.
(adapted from a newspaper call-in column)
The fallacy of straw man.
The Red Cross, of course, is not suggesting that
the detainees be treated as vacationers. The
caller is misrepresenting the Red Cross's
argument in order to make it appear ridiculous.
Barbara Youngblood, a member of the Wilkes-Barre (Pa.) School
Board for twenty-three years, had six relatives on the school district
payroll before she was voted out of office in 2003. When
questioned, she offered the following justification for nepotism in
public education:
"Every board member is pushing somebody for a job -- friends' kids,
neighbors' kids. . . . This happens not only in the School District.
People have relatives working in the same company. It's an
everyday happening. Is that a sin?" (Wilkes-Barre Times Leader,
November 17, 2002)
Based on your reading of this chapter, what
fallacy does
Youngblood commit?
Barbara Youngblood, a member of the Wilkes-Barre (Pa.) School Board
for twenty-three years, had six relatives on the school district payroll
before she was voted out of office in 2003. When questioned, she
offered the following justification for nepotism in public education:
"Every board member is pushing somebody for a job -- friends' kids,
neighbors' kids. . . . This happens not only in the School District.
People have relatives working in the same company. It's an everyday
happening. Is that a sin?" (Wilkes-Barre Times Leader, November 17,
2002)
Bandwagon argument.
The speaker attempts to justify nepotism--a practice that creates
clear conflicts of interest and often results in the hiring of lessqualified applicants--simply by noting that it is widely practiced.
Paul: My philosophy teacher said that it's impossible to prove
that our memories are sometimes reliable. It's just something
we have to take on faith.
Lisa: That's baloney. I can remember countless times when I
recalled information correctly. Isn't that proof enough?
Based on your reading of this chapter, what
fallacy does Lisa commit?
Paul: My philosophy teacher said that it's impossible to prove
that our memories are sometimes reliable. It's just something
we have to take on faith.
Lisa: That's baloney. I can remember countless times when I
recalled information correctly. Isn't that proof enough?
Begging the question.
Lisa is trying to prove that our memories are sometimes
reliable. Yet in saying that she remembers times when her
memory was accurate, she is assuming what she attempts
to prove.
X
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