To give you a better idea of what you're expected to do, I've included
Manuel's argument on a different issue on the following page.
78
Writing Lesson 4 79
Manuel Luis Andrade y Castillo de Pocas
Critical Thinking
Section 2
Writing Lesson 4
Issue: The chance of contracting AIDS through sexual contact can be
significantly reduced by using condoms.
Definition: "AIDS" means "Acquired Immunodeficiency Syndrome"
"significantly reduced" means by more than 50%
"using condoms" means using a condom in sexual intercourse rather
than having unprotected sex
Premises:
• AIDS can only be contracted by exchanging blood or semen.
• In unprotected sex there is a chance of exchanging blood or semen.
• Condoms are better than 90% effective in stopping blood and semen.*
• 90% is bigger than 50%.
• AIDS has never been known to have been contracted from sharing food,
using a dirty toilet seat, from touching, or from breathing in the same
room with someone who has AIDS.
• If you want to avoid contracting AIDS you should use a condom.
Conclusion: The chance of contracting AIDS through sexual contact can be
significantly reduced by using condoms.
*I'm not sure of the exact figure, but I know it's bigger than 90%.
Good. Your argument is indeed valid. (But it could easily be better, You don't need
"only" in A, which is what makes me uneasy in accepting that claim. And without a
reference to medical literature, I'm not going to accept B. But you don't need it. You
can delete it and your argument is just as good.
And the last claim, C, is really irrelevant— delete it. This isn't an editorial:
you're not trying to convince someone to do something;you're trying to convince them
an objective claim is true.
Cartoon Writing Lesson B
Here is a chance to reason as you might in your everyday life.
For each cartoon write the best argument you can that has as its conclusion
the claim that accompanies the cartoon. List only the premises and conclusion.
If you believe the best argument is only weak, explain why. Refer back to Cartoon
Writing Lesson A on p. 55 for suggestions about how to do this lesson.
Remember that with subjective claims, you may need to have a premise that
links actions to thoughts, beliefs, or feelings.
To give you a better idea of what you're expected to do, I've included Maria's
writing lesson for a different cartoon below the ones you're to do.
Spot ran away.
Dr. E shaved.
80
Cartoon Writing Lesson B 81
3.
The dog is trying to catch the Frisbee.
The mother is scolding her child for breaking the flower pot.
Spot is afraid of being punished.
Suzy hit Puff with the car.
82 Cartoon Writing Lesson B
Name Maria Schwartz Rodriguez Section 6.
In New Mexico cars are required to have only one license plate, in the rear.
1. Some of the cars don't have license plates in the front.
2. All of the cars have license plates in the back.
3. So probably the rear license plate is required, and no front plate is required in
New Mexico, since it is pretty unlikely all the front plates just fell off.
You've only proved part of the conclusion with your argument. How do you know
these are New Mexico cars?
first, this is a restaurant parking lot, so these, are normal cars, not cars for sale in
a used car lot, where of course many of them wouldn't have license plates.
Second, the restaurant is advertising New Mexico's best chile, and so it must be
in New Mexico. It would be absurd for a restaurant to advertise like that in another
state.
'Third, if it's in New Mexico, it's likely that most of the cars there are from 'Hew
Mexico— not certain, but likely.
Now you can use the argument you gave to get the conclusion. 'But you could
have gotten a much stronger argument using the following general claim:
It would be extremely unlikely for three drivers at the same time and place
to have lost their front plates and to risk a serious penalty for not having a
front plate.
Overall, this is pretty good, you're only using what you see, not making up a
story. But you're not using enough of what you see— remember to prove all of the
conclusion. Also, it's really good how you put in theglue, the last part of #3 that
shows how yougot from what you saw to the conclusion. But #3 is two claims, not
one, as you recognized by using that indicator word "since." Be sure to list each claim separately so you can judge the plausibility of each and see how it links to the others.
Is That True?
A. Evaluating Premises 83
B. Criteria for Accepting or Rejecting Claims
1. Personal experience 84
• Exercises for Sections A and B.l 86
2. Other sources 87
Summary: When to accept and when to reject 90
• Exercises for Section B 91
C. Advertising and the Internet
Advertising 94
The Internet 94
• Exercises for Section C 95
D. Common Mistakes in Evaluating Premises
1. Arguing backwards 96
2. Confusing possibility with plausibility 97
3. Bad appeals to authority 97
4. Mistaking the person for the claim 97
5. Mistaking the person for the argument 98
• Exercises for Section D 99
Summary 101
A. Evaluating Premises
Recall the tests that an argument must pass to be good:
There is good reason to believe the premises.
The premises are more plausible than the conclusion.
The argument is valid or strong.
In the last two chapters we looked at how to evaluate whether the conclusion follows
from the premises. Now we'll consider what is good reason to believe the premises.
But why simply believe a premise? Shouldn't every claim be backed up with
an argument? We can't do that. If we want a justification for every claim, we'd
have to go on forever. We'd never get started. Sometimes when someone makes a
claim we just have to decide if we believe it.
83
84 CHAPTER 5 Is That True?
Three choices we can make about whether to believe a claim
• Accept the claim as true.
• Reject the claim as false.
• Suspend judgment.
We needn't pretend to be all wise, nor force ourselves to make judgments.
Sometimes it's best to suspend judgment and evaluate the argument as well as we
can. If we find that it's valid or strong, we can then worry about whether the premise
is true. Rejecting a claim means to say that it is false.
not believe it believe it is false
lack of evidence evidence it is false
B. Criteria for Accepting or Rejecting Claims
There are no absolute rules for when to accept, when to reject, and when to suspend
judgment about a claim. It's a skill, weighing up the criteria in this section, as
presented in their order of importance.
1. Personal experience
What would you think of an adult who never trusted his own experience, who always
deferred to authority? He goes to a priest and asks him if it's daytime. He looks up
in an atlas whether his hometown is in Nevada. He asks his wife whether the room
they're standing in is painted white. You'd say he's crazy.
Our most reliable source of information about the world is our own experience.
We need to trust our own experience because that's the best we have.
Everything else is second-hand. Should you trust your buddy, your spouse, your
priest, your professor, the President, the dictator, when what they say contradicts
what you know from your own experience? That way lies demagoguery, religious
intolerance, and worse. Too often leaders have manipulated the populace: All
Muslims want the overthrow of the West? But what about my neighbor who's
Muslim and a city councilor? You have to forget your own experience to believe the
Big Lie. They repeat it over and over and over again until you begin to believe it,
even when your own experience says it isn't so.
Oh, we get the idea. Don't trust the politicians. No. It's a lot closer to home
than that. Every rumor, all the gossip you hear, compare it to what you know about
the person or situation. Don't repeat it. Be rational, not part of the humming crowd.
SECTION B Criteria for Accepting or Rejecting Claims 85
Still, there are times we shouldn't trust our own experience. Sometimes our
memory is not reliable. As Sgt. Carlson of the Las Vegas Police Department says,
"Eyewitnesses are terrible. You get a gun stuck in your face and you can't remember
anything." The police do line-ups, putting a suspect to be identified by a witness
among other people who look a bit similar. The police have to be very careful not to
say anything that may influence the witness, because memory is malleable.
The state of the world around us can also affect our observations and make our
personal experience unreliable. You could honestly say you were sure the other
driver didn't put on a turn signal, when it was the rain and distractions that made
you not notice.
But even then, there are times we're right not to trust our own experience.
You go to the circus and see a magician cut a lady in half. You saw it, so it has to
be true. Yet you don't believe it. Why? Because it contradicts too much else you
know about the world.
Or stranger still: Day, after day, after day we see the sun rise in the east and set
in the west, yet we say the sun isn't moving, the earth is. We don't accept our own
experience because there's a long story, a theory of how the earth turns on its axis
and revolves around the sun. And that story explains neatly and clearly so many
other phenomena, like the seasons and the movement of stars in the skies, that we
accept it. A convincing argument has been given for us to reject our own experience,
and that argument builds on other experiences of ours.
• We accept a claim if we know it is true from our own experience.
• We reject a claim if we know it is false from our own experience.
Exceptions
—We have good reason to doubt our memory or our perception.
— The claim contradicts other experiences of ours, and there is a
good argument (theory) against the claim.
But too often we remember what we deduced from our experience, not what we
actually experienced. Look at Tom's cartoon writing lesson on p. 56. He said he
saw the guy grab the purse. But he didn't see that; he inferred it.
86
CHAPTER 5 Is That True?
Exercises for Sections A and B.l
1. Why can't we require that every claim be backed up?
2. What three choices can we make about whether to believe a claim?
3. If the conclusion of a valid argument is false, why must one of the premises be false?
4. Give an example of a rumor or gossip you heard in your personal life recently that
you believed. Did you have good reason to believe it? Why?
5. We can tell that a rumor or gossip is coming up when someone says, "Guess what I
heard." Give five other phrases that alert us similarly.
6. Shouldn't you trust an encyclopedia over your own experience? Explain.
7. Give an example of a claim that someone made this week that you knew from your own
experience was false.
8. Give an example of a claim that you believed was true from memory, but really you
were making a deduction from your experience.
9. When is it reasonable for us to accept a claim that disagrees with our own experience?
Give an example (not from the text) of a claim that it is reasonable for you to accept
even though it seems false from your own experience.
10. Remember the last time this class met? Answer the following about your instructor.
a. Male or Female? f. Did he/she bring a backpack to class? Describe it.
b. Hair color? g. Did he/she use notes?
c. Eye color? h. Did he/she get to class early?
d. Approximate height? i. Did he/she wear a hat?
e. Approximate weight? j. Is he/she left-handed or right-handed?
11. Remember the last time this class met? Answer the following about the room.
b. How many doors? h. Chalkboard?
c. How many walls? i. Lectern?
d. Any pictures? j. Wastebasket?
e. How high is the ceiling? k. What kind of floor (concrete, tile, linoleum, carpet)?
f. How many chairs? 1. Did you get out of class early?
12. Which of your answers to Exercises 10 and 11 were from actual memory and which
were inferences?
13. List five ways that the physical conditions around us can affect our observations.
14. List five ways that your mental state could affect your observations.
15. Our personal observations are no better than .
16. What does a bad argument tell us about its conclusion?
17. If a strong argument has one false premise and thirteen true premises, what choice
should we make about whether to believe its conclusion?
SECTION B Criteria for Accepting or Rejecting Claims 87
2. Other sources
What about claims from other sources?
We can accept a claim made by someone we know and trust who is
an authority on this kind of claim.
Zoe tells Harry to stay away from the area of town around South 3rd. She's seen
people doing drugs there and knows two people who have been held up in that
neighbor-hood. He'll believe those premises and likely accept the conclusion that
follows from those (and other unstated) premises. It makes sense. Zoe is reliable,
and the claims she's making are the sort about which her knowledge would matter.
On the other hand, your mother tells you that you should major in business so
you can get ahead in life. Should you believe her? She can tell you about her
friends' children. But what are the chances of getting a good job with a degree in
business? It would be more reasonable to check with the local colleges where they
keep records on the hiring of graduates. Don't reject her claim. Suspend judgment
until you get more information.
Other authorities we don't know as well are sometimes reliable, too. For
example, the Surgeon General announces that smoking is bad for your health. She's
got no axe to grind. She's a physician. She's in a position to su
rvey the research on
the subject. It's reasonable to believe her.
We can accept a claim made by a reputable authority whom we can trust
as an expert on this kind of claim and who has no motive to mislead.
The doctor hired by the tobacco company says there's no proof that smoking
is addictive or causes lung cancer. Is he an expert on smoking-related diseases or a
pediatrician? It matters in deciding whether to trust his ability to interpret the
epidemiological data. And he has a motive to mislead, being paid by the tobacco
companies. There's no reason to accept his claim, and some motive to reject it.
And when the Surgeon General says that marijuana should not be legal, we
should ask what kind of authority she is on this subject. Is she a politician? What
kind of expertise does she have on matters of law and public policy? She's an
authority figure, but not an expert on this kind of claim. No reason to accept her
claim just because she said so.
Which authorities we trust and which we disregard change from era to era. It
was the lying by Presidents Nixon and Johnson that led many of us to distrust
pronouncements from the government. It was the Chicago police killing the Black
Panthers in their beds and calling it self-defense that convinced many of us not to
88 CHAPTER 5 Is That True?
accept what big city police say. I remember when I visited Denmark in 1965 as an
exchange student, they asked me who I thought killed President Kennedy. I said,
"Oswald." They asked me why I believed that. I said because the FBI said so. They
all shook their heads in sadness, right after they stopped laughing.
The moral is that some authorities are more trustworthy than others, even in
their own areas of expertise. Some may have motive to mislead. The more you tell
the truth, the more likely you are to be believed; but even one lie can ruin your
reputation for reliability.
What are you to do if the authorities disagree? Suspend judgment. Except that
you don't always have that option. If you're on a jury where two ballistics experts
disagree on whether the bullet that killed the victim came from the defendant's gun,
what should you do? You have to make a decision. Even if you think an authority
has the expertise to speak on a subject and has no motive to mislead, you'll still have