Logic and Logical Fallacies
Critical Thinking!!
Induction
• Most common type of argument
• Can never be 100% sure that it is
true because a new piece of
evidence might come along
• Conclusions are based on many
pieces of evidence (data or facts or
statistics)
• Involves the inductive leap to a
conclusion
• Is a feature of the scientific method
Deduction and
deductive reasoning
• You can be sure that correct
deductive reasoning is guaranteed to
be true
• Begins with a premise that is usually
scientific or legal (like the U.S.
Constitution) – something which
everybody agrees is true
• Argues that a particular case “fits”
the premise
Syllogism is the form for
deductive reasoning
• Major premise (All people have
hearts)
• Minor premise (Andres is a
person)
• _______________________________
• Conclusion (Andres has a heart)
What makes Induction
wrong?
• Not enough evidence (jumping
to conclusions or hasty
generalization)
• Evidence must be true;
statistics, for example, must be
correct. The MATH must be
correct.
What makes Deduction
wrong?
• Major premise is wrong (All
teachers love coffee)
• Minor premise doesn’t fit the
case (My mother hates coffee)
• It’s not set up to cross-cancel
Toulmin Logic
• Looks just like a syllogism in
deductive reasoning except upside
down
• Conclusion is first (Claim)
• Minor premise next (Data)
• Major premise (Warrant)
• Also, you normally use qualifying
words like “might” or “possibly”
Post hoc fallacy (false
cause)
• One assumes that because one
event happened after another event,
the earlier event caused the later
one.
• Example: Because I didn’t wear my
lucky shirt, I didn’t get an A on the
midterm
• Example: The volcano erupted in Iceland
and then the airline went bankrupt, so
the volcano caused the bankruptcy.
Superstitions are one
type of Post hoc fallacy
• I left my umbrella at home;
therefore it rained.
• I broke a mirror; now I just got a
ticket (bad luck).
False analogy
• Comparison in which a surface
similarity masks a significant
difference
• Example: We know that we can’t ban
cigarettes because the Prohibition
period (when alcohol was banned) just
led to more crime
• We lost the war in Vietnam because the
U.S. can’t fight countries having civil
wars; therefore we will lose in Iraq and
in Afghanistan.
Hasty generalization
• By far the most common logical
fallacy
• Not enough evidence to support
a conclusion
• Example: Every paper I saw
showed an A on the midterm, so
everybody in the class must have
gotten A’s.
Slippery slope
• We assume that one small
action will trigger a huge
negative result
• If you don’t pass the next quiz,
then you’ll flunk the course, and
then you’ll flunk out of De Anza
and then you’ll never get rich.
Bandwagon
• An argument that depends on
going along with the crowd on
the false assumption that a
large group of people will know
the truth.
• Since everybody I know wants to
go to Berkeley, it must be the
school for me.
Ad hominem
• Personal attack on someone who
disagrees with you, used instead of
actually countering that person’s
argument
• My opponent can’t be trusted to be
governor because he/ she has been
divorced
• My opponent has changed her hair style
five times this month; will she change
her opinions just as often?
Either or fallacy
• Reduces a complicated decision
to only two options
• It’s me or the dog. Which do you
want?
• You can either work at Great
America or you can take math
during summer school. Those are
your choices.
False authority
• This logical fallacy occurs when a
person (usually a famous person)
endorses a product. If the celebrity
has no expertise on the product,
then the ad is guilty of “false
authority.”
Red herring
• A deliberate attempt to change
the subject or divert the
argument from the real question
at issue to some side-point.
• For instance, “The survivor who
won the first season should not be
jailed for cheating on his income
tax. I think that the lawyers didn’t
like it that he appeared nude on
the show.”
Straw Man
• Straw Man Argument: A subtype of the red
herring, this fallacy includes any lame
attempt to "prove" an argument by
overstating, exaggerating, or oversimplifying the arguments of the opposing
side. The name comes from the idea of a
boxer or fighter who meticulously fashions
a false opponent out of straw, like a
scarecrow, and then easily knocks it over
in the ring before his admiring audience.
Begging the question
• Begging the Question is a
fallacy in which the premises
include the claim that the
conclusion is true or (directly or
indirectly) assume that the
conclusion is true.
• Example: If such actions were not
illegal, then they would not be
prohibited by the law.
_______________?
I just can’t decide between being
an optometrist and being an
accountant. I just go back and
forth and see the advantages
and disadvantages of each.
Here I am, 18 years old, and
don’t even know what my
career is going to be.
________________?
I washed my car last week and
then it rained, so it’s clear that
washing one’s car causes it to
rain.
__________________?
• Everybody in our class is going
to the Asian-American comedy
club Friday night, so you should,
too.
________________?
• Ms. Patton shows a film clip
almost every class, so she must
be an expert on film.
_________________?
I know that in the US, for New
Year’s people have parties,
drink too much, and kiss
somebody at midnight, so
obviously that is what must
happen on Feb. 4.
_______________?
• The automotive equivalent of a
really hot librarian.
_______________?
• I know I can quit smoking. No
problem. As soon as I figured
out that eating fries was making
me gain weight, I quit and lost 5
pounds. So I’ll just do it with
smoking.
___________________?
I have a photo of Martin Luther
King, Jr. with Malcolm X, so Dr.
King must have been connected
to the extremist positions of
Malcolm X and Elijah
Mohammed.
______________?
Every time my daughter throws a ball
with her right hand, she can hit the
target. However, every time she
throws with her left hand, she
misses. She must be right-handed.
This is NOT a logical fallacy. It’s an
example of one type of reasoning.
____________________?
•
According to the Constitution,
people who live in the U.S. have
freedom of speech.
• Tina and Joseph live in the U.S.
• Conclusion: Tina and Joseph have
freedom of speech to promote their
candidacies.
• This is not a logical fallacy. It is a type
of logical reasoning.
___________?
Beyonce is so beautiful when she
advertises this great new
L’Oreal lipstick, so I’m going to
buy it because if she likes it, it
must be fabulous.
_________________?
First you throw your recyclable
can in the trash, then you take a
fifteen-minute shower, and
before you know it, the North
and South Pole ice has melted
and Foster City and Redwood
City are under water.
___________?
• The belief in God is universal.
After all, everyone believes in
God.
______________?
“I should not pay a fine for
reckless driving. There are many
other people on the street who
are dangerous criminals and
rapists, and the police should be
chasing them, not harassing a
decent tax-paying citizen like
me.”
______________?
• Mr. Nguyen thinks that
capitalism is good because
everybody earns whatever
wealth they have, but this is
clearly false because many
people just inherit their
fortunes.
__________________?
• Here are some people from our
class going on the class hike.
Wow – those people are cool, so
everybody in the class must be
cool.
__________________?
First, you’ll go out on one date
with a white person and then
you’ll fall in love and before you
know it, you’ll have Hapa
babies!
______________________?
People in our class are going to
UCLA or Cal. They must be
really smart.
______________?
Students in our class are
creative.
Poe is in our class
__________________________
Poe is creative.
The above is not a logical fallacy;
it is a ________.
____________?
Rodney: This doctor. Is he
good?
Thick: Yup. Best in the business.
He even worked on
Schwarzenegger.