Summary We can't prove everything. We must take some claims as given or we'd
never get started. But when should we accept a claim someone puts forward without
proof, and when should we suspend judgment?
We don't have hard and fast rules, but we can formulate some guidelines.
Most important is experience: We can accept a claim that from experience we know
is true; we can reject a claim that we know from experience is false. But we need to
be sure that it is from our experience and not a faulty memory or a deduction. And
there are some times when we can reject what we seem to know from experience
because it contradicts other claims that we know are true and explain a lot.
We are inclined to accept claims from people we trust who know what they're
talking about, and to accept claims from respected authorities, though we can give
102 CHAPTER 5 Is That True?
too much deference to an authority. But it's wrong to think a claim is false because
of the source. We can argue badly by rejecting anything that a particular person or
group says. Worse is when we reject an argument because of who said it. Arguments
are good or bad regardless of who made them.
Key Words accept bad appeal to authority
reject bad appeal to common belief
suspend judgment mistaking the person for the claim
personal experience mistaking the person for the argument
arguing backwards refuting an argument
phony refutation
Further Study Courses in psychology deal with the reliability of witnesses and the
nature of memory. Courses in journalism or communications discuss the reliability
of various sources in the media and bias in the media. A short course on how to use
the library is offered at most schools in order to help you find your way through
reference sources.
A book about the psychology of why people believe claims for bad reasons is
How We Know What Isn't So, by Thomas Gilovich, The Free Press.
You can look up the Federal Trade Commission's guidelines against deceptive
pricing in advertising at <http://www.ftc.gov/bcp/guides/decptprc.htm>.
Writing Lesson 5
Write an argument in outline form either for or against the following:
No unmanned spacecraft landed on Mars; the photos are faked.
• Just list the premises and the conclusion. Nothing more.
• Your argument should be at most one page long.
• Check whether your instructor has chosen a different topic for
this assignment.
You know whether you believe this claim. But why do you believe it or doubt
it? Make your argument based on the criteria we studied in Chapter 5.
What if you're unsure? You write pro and con lists, yet you can't make up
your mind. You're really in doubt. Then write the best argument you can for why
someone should suspend judgment on the claim. That's not a cop-out; sometimes
suspending judgment is the most mature, reasonable choice to take. But you should
have good reasons for suspending judgment.
To give you an idea of what to do, here are arguments by Tom and Suzy on
other topics.
103
104 Writing Lesson 5
Tom Wyzyczy
Critical Thinking
Section 4
Writing Lesson 5
Issue: Elvis is still alive.
Definition: By "Elvis" I understand Elvis Presley.
Premises:
Elvis Presley was reported to have died a number of years ago.
All the reputable press agencies reported his death.
Many people went to his funeral, which was broadcast live.
His doctor signed his death certificate, according to news reports.
There have been reports that Elvis is alive.
No such report has been in the mainstream media, only in tabloids.
No physical evidence that he is alive has ever been produced.
No one would have anything to gain by faking his death.
If Elvis were alive, he would have much to gain by making that known
to the public.
Conclusion: Elvis is not alive.
Good. "But it could be better, first, split the third premise into two (A an B). I
don't know if it was broadcast live, yet I can accept part A.
Second, the sentence C is too vague— what's "mainstream media"? What counts as
a "tabloid" ? You should cite real sources if you want someone to accept your argument.
And premise D is dubious: Any of his heirs had lots to gain.
Finally, you take for granted that the reader knows why some of your premises are
glue, a premise or premises Uniting it to the conclusion, you're still leaving too much
unstated. "Don't rely so much on the other person making your argument for you.
Review Chapters 3 and 4.
Still, I think you have the idea from Chapter 5 and won't be suckered by the
conspiracy theorists.
Writing Lesson 5 105
Suzy Queue
Critical Thinking
Section 2
Writing Lesson 5
Issue: The CIA started the cocaine epidemic in the ghettos in order to
control and pacify African-Americans.
Premises:
The CIA has lied to us a lot in the past.
Riots in the past in the ghettos have been a serious problem in the U.S.
The government wants to control African-Americans, so they won't make any
trouble.
tose
African-American people in the ghetto had too much to loose to start.
Many people in the ghettos believe that the CIA introduced cocaine to the U.S.
It was reported on national news that the CIA was involved with drug running
from Latin America.
Conclusion: The CIA started the cocaine epidemic in the ghettos in order to
control and pacify African-Americans.
At Best you've given reason to suspend judgment, You haven't given me any reason
to believe the claim is true, only that it isn't obviously false.
Some of your premises are way too vague ("national news," "serious problem"). And I can't see how they link to the conclusion. Are you suggesting that if the CIA lied to us
in the past, that makes it highly probable that they introduced cocaine into theghettos?
Ihat's pretty weak. And big deal that a lot of people in the ghettos believe the CIA
introduced cocaine there. A lot of people think the moon doesn't rise or that it rises in
the West— that doesn't make it true. Are they authorities?
Review the criteria in Chapter 5.
Review Chapters 1-5
Let's review what we've done.
We began by saying we would study attempts to convince. But that was too
broad, so we restricted ourselves to convincing through arguments: collections of
claims used to show a particular claim is true.
We said a claim was any declarative sentence that we can view as true or false.
But to use that definition took practice. We learned to recognize sentences that
posed as claims but were ambiguous or too vague for us to deliberate. Definitions
were one way to clear up confusions. And we differentiated among claims, noting
that unstated standards could make a claim objective or subjective, and the need for
standards for most prescriptive claims.
We saw that there are three tests for an argum
ent to be good: There should be
good reasons to believe the premises, and we looked at criteria for that. And the
premises should be more plausible than the conclusion. But even if the premises are
plausible, it might not be enough to convince. The conclusion should follow from
the premises. We decided that means the argument must be either valid or strong.
Often there's a gap between the premises and conclusion. We needed a guide
for when it's reasonable to repair an argument and when an argument is unrepairable.
We based the guide on the assumptions we need in order to deliberate with someone.
Along the way, we also saw various types of bad arguments that are common
mistakes in reasoning.
You should now be able to analyze an attempt to convince.
Steps in evaluating an argument
• Is it an argument?
• What's the conclusion?
• What are the premises?
• Are any further premises needed?
• Is it valid? If not, where is it on the scale from very strong to weak?
• Is it a good argument?
• Can it be repaired?
107
108 REVIEW CHAPTERS 1-5
You'll get a lot more practice in analyzing arguments in the following chapters.
The review exercises here are designed to make sure you know the definitions. You
can't apply ideas you only half-remember.
Steps in understanding a definition
• Know what the words mean and be able to recall the definition.
• Know an example of the definition.
• Know an example of something that doesn't fit the definition.
• Practice classifying with the exercises.
• Relate the definition to other concepts you've learned.
The last step is crucial in putting this material together. You may have learned
the definition of "valid" and know how to recognize whether an argument is valid,
but you don't really understand that definition until you know how it relates to other
terms, such as "strong" and "good argument."
Review Exercises for Chapters 1-5
1. What is an argument?
2. What is a claim?
3. a. What is an objective claim?
b. Give an example of an objective claim.
c. Give an example of a subjective claim.
4. Can a vague sentence be a claim? Explain.
5. a. What is a prescriptive claim?
b. Give an example.
c. What standard, if any, is presupposed by your example?
6. Is a definition a claim? Explain.
7. a. What is a persuasive definition?
b. Give an example.
8. What is the drawing the line fallacy?
9. What three tests must an argument pass for it to be good?
10. a. What is a valid argument?
b. Give an example of a valid argument that is good.
c. Give an example of a valid argument that is bad.
11. a. What does it mean to say an argument is strong?
b. Give an example of a strong argument that is good.
c. Give an example of a strong argument that is bad.
REVIEW EXERCISES for Chapters 1-5 109
12. Is every weak argument bad? Give an explanation or example.
13. How do you show an argument is weak?
14. If a strong argument has eight true premises and one false premise, should we accept
the conclusion? Explain.
15. If an argument is bad, what does that tell us about its conclusion?
16. Is every valid or strong argument with true premises good? Give an explanation
or example.
17. Should we always prefer valid arguments to strong arguments? Give an explanation
or example.
18. State the Principle of Rational Discussion.
19. What is the mark of irrationality?
20. State the Guide to Repairing Arguments.
21. List the circumstances in which we shouldn't repair an argument.
22. a. What is an indicator word?
b. Is an indicator word part of a claim?
23. What is our most reliable source of information about the world?
24. What three choices can we make about whether to believe a claim?
25. Give five criteria for accepting an unsupported claim.
26. Give two criteria for rejecting an unsupported claim.
27. When should we suspend judgment on a claim?
28. What does it mean to say that someone is arguing backwards?
29. What does it mean to say that someone is mistaking the person for the argument?
30. When are we justified in rejecting a claim because of who said it?
31. When are we justified in rejecting an argument because of who said it?
32. What is a phony refutation?
The STRUCTURE
of ARGUMENTS
6 Compound Claims
A. Consider the Alternatives
1. Compound claims and "or" claims 113
2. The contradictory of a claim 114
• Exercises for Sections A. 1 and A.2 115
3. Reasoning with "or" claims 116
4. False dilemmas 118
• Exercises for Section A 119
B. Conditionals
1. Conditionals and their contradictories 120
•Exercises for Section B . l 122
2. Necessary and sufficient conditions 124
• Exercises for Section B.2 125
3. Valid and weak forms of arguments using conditionals 127
• Exercises for Section B.3 131
4. Reasoning in a chain and the slippery slope 132
5. Reasoning from hypotheses 133
Summary 134
• Exercises for Chapter 6 135
A. Consider the Alternatives
1. Compound claims and "or" claims
Some words can link two or more claims together to make a new, compound claim
whose truth-value depends on the truth-values of the claims that are part of it. For
example, suppose your neighbor says,
"I'll return your lawn mower or I'll buy you a new one."
Has he promised to return your lawn mower? No. Has he promised to buy you a
new lawn mower? No. He's promised to do one or the other. We have one claim,
not two.
Compound claim A compound claim is one composed of other claims,
but which has to be viewed as just one claim.
113
114 CHAPTER 6 Compound Claims
In this chapter we'll look at different kinds of compound claims and see how to
reason with them. One word that can link two claims to make a compound is "or":
Either a Democrat will win the election or a Republican will win.
Either some birds don't fly or penguins aren't birds.
Columbus landed in South Carolina or on some island near there.
Each is just one claim, though made up of two claims. The last one, for instance,
contains:
Columbus landed in South Carolina.
Columbus landed on some island near South Carolina.
Alternatives Alternatives are the claims that are the parts of an "or" claim.
But not every sentence with two or more claims is compound. For example,
Dr. E is a professor because he teaches critical thinking.
This is an argument, not a claim: the word "because" is an indicator word.
2. The contradictory of a claim
Because a compound claim is made up of other claims, it's easy to get confused
about how to say it's false.
Contradictory of a claim The contradictory of a claim is one that has
the opposite truth-
value in all possible circumstances. Sometimes a
contradictory is called the negation of a claim.
The contradictory of "Spot is a doberman" is "Spot is not a doberman." But the
contradictory of "Spot will never learn to fetch" is "Spot will learn how to fetch,"
and "not" doesn't appear in it.
Claim Contradictory
Spot is barking. Spot isn't barking.
Dick isn't a student. Dick is a student.
Suzy will go to the movies Suzy won't go to the movies
or she will stay home. and she won't stay home.
Tom or Suzy will pick up Neither Tom nor Suzy will pick up
Manuel for class today. Manuel for class today.
EXERCISES for Sections A. 1 and A.2 115
In order to discuss the forms of compound claims, we'll use the letters A, B, C,
D , . . . to stand for any claims, and "not A" to stand for the contradictory of a claim.
Contradictory of an or claim A or B has contradictory not A and not B.
For example, the following fits into this form:
Either Lee will pick up Manuel, or Manuel won't come home for dinner.
contradictory:
Lee won't pick up Manuel, and Manuel will come home for dinner.
We can also use neither A nor B for the contradictory of A or B.
Using "and" to join two claims creates a compound, but it's simpler to consider
each claim independently. For example,
Pigs can catch colds, and they can pass colds on to humans.
When is this true? Exactly when both "Pigs can catch colds" is true and "Pigs can
pass colds on to humans" is true. So in an argument we'd have to treat each of those
claims separately anyway. It's the same with "but."
Pigs can catch colds, but dogs can't.
This is true when both parts are true. So we might as well view each claim
independently, as if the sentence is just a list of claims. "But" works the same as
"and" in an argument—it's just a stylistic variation.
Contradictory of an and claim A and B has contradictory not A or not B.
Pigs can catch colds, but dogs can't.
contradictory: Pigs can't catch colds, or dogs can catch colds.
Exercises for Sections A.l and A.2
1. What is a compound claim?
2. What do we call the parts of an "or" claim?
3. What is the contradictory of a claim?
4. How do you say the contradictory of "A or B"?
5. How do you say the contradictory of "A and B"?
6. Why can we take both A and B to be premises when someone says "A and B"?