Transcript Logic and Logical Fallacies Critical Thinking!! Induction • Most common type of argument • Can never be 100% sure that it is true because.

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 (Jeremy is a
person)
• _______________________________
• Conclusion (Jeremy 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 loves tea.)
• 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: Steve Jobs refused to give any
money to De Anza College; then he died.
Better give money to De Anza!!
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.”
_______________?
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
Chinese and Vietnamese New
Year’s must be the
same.
_______________?
• 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 President
Obama shaking hands with
President Mubarak of Egypt, so
President Obama must be in
favor of a repressive
government, too.
______________?
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.
• Andy lives in the U.S.
• Conclusion: Andy has freedom of
speech.
• This is not a logical fallacy. It is a type
of logical reasoning.
___________?
Beyonce is so hot 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 first four weeks of Calculus
have been easy, so I am sure
that I’m going to get an A.
__________________?
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.
Jasmine is in our class
__________________________
Jasmine 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.