Arguments and fallacies review.
DEDUCTIVE ARGUMENTS
An argument is deductive if the truth of its conclusion follows with absolute certainty from the truth of its
premises.
An argument is valid if and only if it is impossible for its conclusion to be false if its premises are true. In
other words, the only way to get the conclusion to be false is to make one of the premises false.
All swans are birds.
All birds are creatures that hatch from eggs.
All swans hatch from eggs.
INDUCTIVE
An argument is inductive if its conclusion is intended to follow from its premises with some degree of
probability (rather than with absolute certainty).
The deficit has contributed to inflation.
The deficit has hurt American exports.
The federal deficit must be reduced.
Notice that it is conceivable that the conclusion could be false even though the premises were both true.
As a result, this argument is inductive rather than deductive.
Argument Structure
1. One claim supports the conclusion.
Alex is feeling better. I hear him laughing a lot more.
We ran out of milk. That’s why I went to the store.
2. Any given claim can function as a premise in one argument and as a or conclusion in the
other.
C
All cats have dander.
Daisy is a cat.
Daisy has dander
C
Dander makes me sneeze.
Daisy has dander
Daisy makes me sneeze.
3. Arguments can have unstated (implicit) premises
Karen will do well in law school, because Karen did well in logic.
Unstated premise or assumption:
Karen did well in logic.
[unstated or implicit premise]
Conclusion: Karen will do well in law school
Karen did well in logic.
Doing well in logic means that you will do well in law school.
Conclusion: Karen will do well in law school
4. Arguments can have unstated Conclusions
If he copied that paragraph from an internet article, he committed plagiarism.
He copied that paragraph from an internet article …
Voila
5. Arguments can have Dependent Premises
PREMISES: Dependent vs. Independent
Independent
The premise stands on its own.
The premise supports the conclusion
even if the other premise is false.
Dependent Premises
More than one premise must
be true for the premises to
support the conclusion.
Smoking costs too much.
Smoking gives you heart disease.
Smoking gives you wrinkles.
Smoking gives you lung cancer.
C: You shouldn’t smoke
All cats have dander.
Daisy is a cat.
C: Daisy has dander
You can also say you have four independent
arguments for the same conclusion.
Smoking costs too much.
C: You shouldn’t smoke
Smoking increases your chance of heart disease.
C: You shouldn’t smoke
Smoking gives you wrinkles.
C: You shouldn’t smoke
 Smoking gives you lung cancer.
C: You shouldn’t smoke.
INDEPENDENT PREMISES
You should wash your hair more than once a week.
If you don’t wash your hair more than once a week you might get lice.
If you don’t wash your hair more than once a week it will look bad.
If you don’t wash your hair more than once a week it will smell terrible.
DEPENDENT PREMISES
If you are under four feet tall you cannot ride on the Ferris wheel.
Jack is under four feet tall.
Therefore, Jack can’t ride on the Ferris wheel.
Anyone without type A blood did not kill the boy.
Jack does not have type A blood.
Therefore, Jack did not kill the boy.
DEDUCTIVE ARGUMENTS
VALIDITY AND SOUNDNESS
An argument is VALID if, when ALL the premises are true, the conclusion needs to be true as well.
Karen is taller than Mike.
Mike is taller than Jeremy.
Karen is taller than Jeremy.
An argument is INVALID if it is possible for
1. All the premises to be true while
2. The conclusion is false
Karen is taller than Mike.
Mike is taller than Jeremy.
Jeremy is taller than Jack.
An argument is SOUND if and only if
1. If it is valid (the truth of the premises forces the truth of the conclusion) and
2. Its premises are true.
VALID, but UNSOUND
All insects have three legs.
A fly is an insect.
A fly has three legs.
VALID AND SOUND
All insects have six legs.
A fly is an insect.
A fly has three legs.
ARGUMENTS AND EXPLANATIONS
Argument
Wants us to believe the claim
Explanation_______________________
Wants us to know more about the subject
Gives us evidence (relevant reasons)
Answers the questions how? and/or why?
Purpose: To justify or prove
Purpose: To educate, to answer how?
and/or why?
Audience: Skeptical listener
Audience: Someone needing information
Examples:
I went to sleep early. I wanted to feel good for the soccer game.
I am a good swimmer. I won our swim club annual freestyle competition.
HOW TO DISTINGUISH BETWEEN ARGUMENTS AND EXPLANATIONS
1. Decide what the claim or conclusion is.
2. Decide whether the remaining claims are designed (1) to convince a skeptical listener or (2) to
teach someone more about the topic.
EXAMPLES:
I bought some boots at half price today. See, here’s the receipt.
I bought some boots at half price today. I want to keep my feet warm this winter.
That’s a crummy company to work for. That’s why I turned down the job.
That’s a crummy company to work for. They don’t give their employees health insurance.
Exercise: match the concepts.
You should stop smoking.
Smoking near children gives them asthma.
I always buy their soy milk. I like it better.
It’s OK to leave the shopping cart on the curb near the
grocery store. Everyone does it.
Argument
Deductive
Match:
Inductive
Valid
Invalid
Strong
Weak
Sound
Unsound
Cogent
Uncogent
Unsound
Uncogent
Argument type if an argument
Fallacy type if a fallacy
An explanation
MORE PSEUDOREASONING OR FALLACIES
AD HOMINEM
In all ad hominem fallacies we attack the speaker rather than the claim.
Personal Attack
We commit the fallacy of PERSONAL ATTACK AD HOMINEM when we try to discredit a claim by
personally attacking the speaker.
Jack said that bilingual education has been a failure, but what does he know? He’s totally
clueless.
Circumstantial Ad Hominem
We commit the fallacy of CIRCUMSTANTIAL AD HOMINEM when we try to discredit a claim by
attacking the speaker based upon their circumstances.
My brother says you always should give a 20% tip, but he works in the food industry.
Misplaced Burden of Proof
Someone commits the MISPLACED BURDEN OF PROOF fallacy when they are making a claim
they should have to prove, but they act like the burden of proof falls on the other person.
She: Souls exist.
He: Can you prove it?
She: Can you prove that they don’t?
STRAWMAN
We commit the STRAWMAN fallacy when, instead of attacking (or defending) the claim we are
supposed to address, we attack or defend a different claim altogether.
She: Let’s stop studying and take a half hour break.
He: No, if we play around all day we won’t do well on this project.
FALSE DILEMMA
We commit the fallacy of FALSE DILEMMA when we wrongly see a situation as having only two
alternatives, when it has more.
You either need to declare a major or drop out of school.
PERFECTIONISTIC FALLACY
We commit the PERFECTIONISTIC fallacy when we reject a reasonable proposal simply because it isn’t
perfect.
Nurse practitioners shouldn’t be allowed to give prescriptions at all because there will be some cases in
which they can’t because a doctor is needed.
LINE DRAWING FALLACY
We commit the fallacy of LINE DRAWING when we wrongly insist, in a case in which no clear line can
be drawn, that either a line must be drawn or we can’t claim that there is a difference between one
extreme and the other. In the line drawing fallacy, a claim is made about a concept in which no clear
line can be drawn. Then the claim is rejected because no clear line can be drawn.
There’s no such thing as eating too many cookies. Clearly there’s nothing wrong with one cookie. So
there must not be anything wrong with two cookies. Since we can’t tell exactly when there are too many
cookies, there can’t ever be too many cookies.
I’m tired of people saying that secondhand smoke causes cancer. I just don’t get it. When exactly does
smoke cause cancer? When you take your first breath in a smoky room? Your second? Your third? If
you can’t tell me exactly when the smoke causes cancer then it doesn’t cause cancer at all.
You can say that Harriet ate too much. When exactly did she eat too much? Was it when she put jelly
on her waffles? Was it when she ate the ice cream cone?
I don’t think you can say that I’m late, because you don’t say on the syllabus exactly how many minutes
you have to be late for it to count.
SLIPPERY SLOPE
Someone commits a SLIPPERY SLOPE fallacy when they assume that taking one step will take them
down one step after another until they are all the way down the slope.
My son wants me to give him five dollars for his class’s field trip. I’m not going to give him the five
dollars. I have five dollars and wouldn’t mind giving it to him, but if I give him five dollars for this field trip
I will have to give every class five dollars and that will cost over one hundred dollars.
Our economist thinks that we should raise taxes one percent. I am against it. If we raise taxes this year
we will have to raise them next year by the same amount, and I’m against more than a one percent rise
in taxes.
BEGGING THE QUESTION
Someone is BEGGING THE QUESTION when, instead of answering the question, the speaker just
repeats their position.
Kara: It’s wrong to take more than twenty napkins at Burger King.
Jack: Why?
Kara: Because it’s wrong.
Murderers don’t have a right to live. Why? Because they don’t have a right to live.
GENETIC FALLACY is kind of ad hominem.
Someone commits the GENETIC fallacy when they challenge a claim based on its history or source
(where the source isn’t an individual, in which case it would be ad hominem).
The FDA says that blue food coloring is safe to eat but I don’t trust anything the FDA says.
Identify the argument:
The new neighbors have a large dog carrier on their front porch. I saw a heavy duty leash among the
goods they moved in. The new neighbors must have a dog.
I saw a pink balloon on the front porch of the Johnson’s house. I heard a baby crying when I walked by
the Johnson’s house. Their daughter must have given birth.
Richard weighs more than Karen. Karen weighs more than Sean. Richard weighs more than Sean.
The federal deficit must be reduced. The deficit has contributed to inflation. The deficit has hurt
American exports.
Because the weather forecast was for rain, you should take your umbrella.
More than half of all mothers with children underage six have jobs, and the number is increasing.
Daycare is indispensable.
John won’t plead guilty to a misdemeanor. If he won’t plead guilty, then he will be tried on a felony
charge. John will be tried on a felony charge.
Whenever the winter winds start, my lips get chapped.
The winter winds will start in a month. My lips will get chapped in a month.
Bamboo can grow up to four feet a day, but only after it is well established. This can take from three to
five years, depending on the type of bamboo.