L E S S O N S U M M A R Y
Since it’s the evidence in a deductive argument that makes the con-clusion valid, it’s important to evaluate that evidence. This lesson will show you how to check premises for two key factors: credibility and reasonableness.
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Ty p e s o f E v i d e n c e
There are many different types of evidence that can be offered in support of a conclusion. One of the most basic distinctions to make is between premises that are fact, premises that are opinion, and premises that can be accepted only as tentative truths.
Before going any further, here’s a review of the difference between fact and opinion:
■ A fact is something known for certain to have happened, to be true, or to exist.
■ An opinion is something believed to have hap-pened, to be true, or to exist.
■ A tentative truth is a claim that may be a fact but that needs to be verified.
Whether they’re facts, opinions, or tentative truths, premises can come in the following forms:
■ Statistics or figures
■ Physical evidence (artifacts)
■ Things seen, felt, or heard (observations)
■ Statements from experts and expert witnesses
■ Reports of experiences
■ Ideas, feelings, or beliefs
Of course, some types of evidence seem more convincing than others. That is, people are often more likely to believe or be convinced by statistics than by someone’s opinion. But that doesn’t mean that all sta-tistics should automatically be accepted and that all opinions should be rejected. Because statistics can be manipulated and because opinions can be quite rea-sonable, all forms of evidence need to be examined for both credibility and reasonableness.
For example, the reasonableness of statistics can’t really be questioned, but their credibility must be ques-tioned. Similarly, any feeling or belief should be exam-ined for both credibility and reasonableness.
I s t h e E v i d e n c e C r e d i b l e ?
Whatever the type of evidence the arguer offers, the first thing that needs to be considered is the credibility of the arguer. Is the person making the argument credible?
Second, if the arguer offers evidence from other sources, the credibility of those sources needs to be questioned. If both the arguer and his or her sources are credible, then the argument can tentatively be accepted.
If not, the argument shouldn’t be accepted until it is examined further.
First, here’s a review of the criteria that deter-mine credibility. To be credible, a source must:
■ Be free of bias
In the case of an eyewitness account, the follow-ing must be considered:
■ The witness’s potential for bias
■ The environment
■ The physical and mental condition of the witness
■ The time between the event and recollection of the event
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Here is a short deductive argument. Read the fol-lowing passage carefully:
Current statistics show that 15% of children are obese. Childhood obesity increases the risk for developing high blood pressure, Type 2 diabetes, and coronary heart disease. In fact, 80% of children diagnosed with Type 2 diabetes are overweight.
Being obese also lowers children’s self-esteem and affects their relationships with their peers. This growing epidemic can be attributed to several fac-tors: genetics, lack of physical activity—children are spending more and more time in front of the tele-vision and the computer—and lack of nutritional education. If children were educated about nutrition and exercise, then obesity rates would decline sig-nificantly. That’s why we must pass a law that requires that nutrition and exercise education be part of the school curriculum for all students in grades K–12. Unfortunately, it’s too late for my 12-year-old brother; he’s already been diagnosed with Type 2 diabetes. But we must take measures to improve the health and well-being of future gener-ations to come.
*This and other statistics in the rest of the text are fictitious and meant to serve purely as examples.
First, identify the conclusion in this passage. What is the overall claim or point that the passage is trying to prove? Once you identify the conclusion, underline it.
You should have underlined the claim, “We must pass a law that requires that nutrition and exercise edu-cation be part of the school curriculum for all students
in grades K–12.” The phrase “That’s why” may have helped you identify this idea as the main claim. (If you had trouble, take a moment to review Lesson 7, “Work-ing with Arguments.”) The follow“Work-ing table lists the premises that support this conclusion. Note that not every sentence in this argument is a premise.
The arguer’s experience offers an important clue here about her credibility. Because of what happened to her brother, is she likely to be biased on the issue?
Absolutely. However, does this rule her out as a credible arguer? Not necessarily. Chances are that if her brother was diagnosed with diabetes due to poor nutritional habits, she knows more about the issue than the average person. In other words, her experience indicates that she has some level of expertise in the area. Thus, though there’s evidence of some bias, there’s also evidence of some expertise. Because there is both bias and expert-ise, the argument needs to be examined further before you can determine whether or not to accept it.
Is the arguer’s experience credible? Well, it can be assumed that she’s telling the truth about her brother being diagnosed. Is her opinion credible? That depends on her own credibility, which is still in question, and the reasonableness of that opinion, which is covered in the next section.
The next step is to consider the credibility of premises provided by the outside source; that is, the sta-tistics offered about childhood obesity. Notice that here the arguer doesn’t give a source for the figures that she provides. This should automatically raise a red flag. Because numbers can so easily be manipulated and misleading, it’s crucial to know the source of any figures offered in support of an argument.
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PREMISES THAT SUPPORT THE CONCLUSION
TYPE OF PREMISE PREMISE
Opinion If children were educated about nutrition and exercise, then obesity rates would decline significantly.
Statistics Current statistics show that 15% of children are obese. In fact, 80% of children diagnosed with Type 2 diabetes are overweight.
Practice
1. Which of the following sources for the statistic would you find most credible, and why?
a. Parents against Obesity
b. National Institute of Health Statistics c. The makers of SweetSnackPacks for Kids
Answer
The most credible source is b, the National Institute of Health Statistics. Of these three sources, the National Institute of Health Statistics is by far the least biased.
Parents against Obesity has a position on children’s nutritional education initiatives (for them), as do the makers of SweetSnackPacks for Kids (most likely against them).
I s t h e E v i d e n c e R e a s o n a b l e ?
Now that you’ve considered the credibility of the arguer and the evidence she’s offered, the next question you should ask is whether or not the evidence is reasonable.
This question relates mostly to evidence in the form of opinions and tentative truths.
Remember that reasonable means logical: accord-ing to conclusions drawn from evidence or common sense. So whenever evidence comes in the form of an opinion or tentative truth, you need to consider how reasonable that premise is. Read this opinion:
If children were educated about nutrition and exer-cise, then obesity rates would decline significantly.
Does this seem like a reasonable opinion to you? Why or why not?
However you feel about nutritional education programs, there is some sense to this opinion. After all, if children were educated about nutrition and exer-cise, it seems logical that they would eat healthier and exercise more, thereby reducing obesity rates. Common sense, right?
But this opinion isn’t a conclusion drawn from evidence. Look how much stronger this premise would be if it added evidence to common sense:
If children were educated about nutrition and exer-cise, then obesity rates would decline significantly.
For example, in Toledo, Ohio, all schools in 1999 implemented nutrition and exercise education pro-grams into the curriculum for grades K–12. As a result, obesity rates in children ages 6–11 dropped from 15% in 1999 to 10% in 2004.
Notice that this statistic is used to support the opinion, which is then used to support the conclusion. In other words, this premise is part of a chain of support.
Opinions, then, can be reasonable either because they’re based on common sense or because they’re drawn from evidence, like what happened in Toledo. Of course, if an opinion is reasonable on both accounts, it’s that much stronger as support for the conclusion.
Practice
Read the following opinions carefully. Are they rea-sonable? If so, is the reasonableness based on logic, common sense, or evidence?
2. You should quit smoking. The smoke in your lungs can’t be good for you.
3. You should quit smoking. The Surgeon General says that it causes lung cancer, emphysema, and shortness of breath.
4. Don’t listen to him. He’s a jerk.
5. Don’t listen to him. He gave me the same advice and it almost got me fired.
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Answers
2. Reasonable, based on common sense.
3. Reasonable, based on evidence; in this case, on an expert’s opinion.
4. Unreasonable. Because this is a deductive argu-ment where the premise is unreasonable, the whole argument should be rejected as unreasonable.
5. Reasonable, based on evidence; in this case, on experience.
Practice
6. Reread the argument from the last lesson:
He’s tall, so he must be a good basketball player.
All tall people are good basketball players.
Are the premises in this argument reasonable?
Why or why not?
Answer
No, the premises in this argument are not reasonable, and therefore, the conclusion is not reasonable, either.
Why not? Because common sense should tell you that you can’t make big generalizations like “All tall people are good basketball players.” You should beware of any premise that makes a claim about all or none. There is almost always an exception.
I n S h o r t
Premises can come in many forms, from statistics to feelings or opinions. When evaluating evidence, it’s necessary to examine credibility and reasonableness:
the credibility of the arguer, the credibility of any sources, and the reasonableness of each premise.
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■ As you hear deductive arguments throughout the day, pay attention to what type of evidence is offered in support of the conclusion. Statistics? Experiences? Opinions?
■ Consider the credibility of the people who present you with deductive arguments today. Could they be biased? What is their level of expertise? If they offer other sources to support their arguments, are those sources credible?
Skill Building until Next Time
Y
ou got laid off from your job two months ago. You’ve been looking for another job but haven’t had much luck. But the company you interviewed with yesterday just made you an offer. The pay isn’t that good, but you’re thinking about taking the job anyway; you need the money. Your friend, how-ever, tells you not to take it: “The pay is lousy, the hours are terrible, and there are no benefits,” he says. “Don’t do it.” Should you listen to your friend? Has he made a good argument? How can you tell?You already know what a deductive argument is. You know how to separate the conclusion from the evidence.
And you know how to evaluate the evidence. These are essential steps in analyzing a deductive argument. But in order to determine the overall strength of an argument, there are several other criteria to take into consideration.
Specifically, in a good deductive argument:
■ The conclusion and premises are clear and complete.
■ The conclusion and premises are free of excessive subtle persuasion.
■ The premises are credible and reasonable.
■ The premises are sufficient and substantive.
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