Chapter 3



Chapter 3

Lesson 5: Claims Must Be Supported By Evidence

In Chapter 3, many claims are made about the causes and effects of stress. Are these claims true? How would we determine this? Should we base our decision about whether a claim is true or false on our intuition--on a "feeling" we have about the claim? Although our intuitions may be good starting places, they can be based on irrational influences and, thus, we should not place too much trust in them. Should we base our decision on the fact that a claim is made by an authority? Again, although we need to consider seriously what an authority has to say, he or she may be mistaken. In the end, if we want to make an informed decision about a claim, we must consider the evidence for and against the claim.

Evidence consists of anything that allows us to evaluate the truth of a claim. Let's consider a very simple claim that probably everyone believes is true: "The sun will rise tomorrow morning." This claim is derived from the following evidence:

• As far back as we can remember, we have seen the sun rise each and every morning of our lives. This kind of evidence is referred to as "direct observations."

• If we asked them, elderly people would tell us that the sun has risen each and every morning of their lives. This kind of evidence would involve a "survey."

• No mention has ever been made in any historical document that the sun has ever failed to rise. It seems likely that something as significant as the sun not rising would have been recorded and reported. This kind of evidence is referred to as "documentary evidence."

From this, it seems reasonable to infer (to infer means to draw a conclusion based on evidence) that the sun will continue to rise each morning in the future. In other words, there are very good reasons to conclude that the sun will rise tomorrow. If someone claimed that he knew that the sun was not going to rise tomorrow, you would immediately ask him why he believed this claim (this is equivalent to asking him for his evidence). If he stated that he dreamed that this would happen and that his dreams often come true, most of you probably would be skeptical. To be skeptical means to doubt something until an examination of the evidence has been completed. If, after such an examination, you either accepted or rejected the claim, you no longer would be skeptical: you have made a decision.

Let's say that we wanted to explain why the sun has risen each morning in the past and why it will continue to rise in the future. Many explanations of this phenomenon have been given over the centuries. Almost all people now accept the following explanation:

The sun will rise tomorrow because a roughly cylindrical earth rotates once every 24 hours (approximately) and the sun shines on the part of the earth that is pointing towards it. As the earth rotates, the part of it illuminated by the sun changes. The sun only appears to be moving to those observing it from the surface of the earth.

What is the evidence for these claims? There are at least two pieces of evidence that most of you would find convincing:

• There exist many pictures and movies taken of a cylindrical earth.

• When the movies are examined, the rotation of the earth can be observed.

Some people, however, have believed that the earth is flat, not cylindrical; and they have developed a different explanation of the apparent rising of the sun. The late Charles K. Johnson--who was president of the International Flat Earth Research Society before his death in March, 2001--was such a person. In a story written by Robert J. Schadewald (1980) in Science Digest, Johnson stated: "The known, inhabited world is flat. Just as a guess, I'd say that the dome of heaven is about 4,000 miles away, and the stars are about as far as San Francisco is from Boston." Johnson believed that the North Pole is at the center of the flat earth and the South Pole does not exist: it actually is a wall of ice that rings the edges of the inhabited earth. The sun is about 32 miles in diameter and circles the earth in the vicinity of the equator. The apparent rising of the sun is an optical illusion created by its circling around the equator.

What was Johnson's evidence for these claims?

• Many biblical verses imply that the earth is flat. This is known as "authoritative evidence." Johnson believed that writings found in the Bible were superior to any other kind of evidence.

• Johnson reasoned that, if the earth is a sphere, then the surface of large bodies of water must be curved. When he and his wife looked at the surfaces of Lake Tahoe and the Salton Sea (a shallow salt lake in southern California), they could not see any curvature. Here, they were using "direct observations."

Johnson rejected the evidence that supports the cylindrical-earth theory. He believed that no one has ever traveled into space and that all space missions were faked. As for the scientific evidence in favor of a cylindrical earth, Johnson believed that it is part of an international conspiracy designed to keep secret the "truth" that the earth actually is flat. He provided little evidence for these claims.

Many of you probably are skeptical of Johnson's claim that the earth is flat. This skepticism should lead you to evaluate his evidence. You would have to provide reasons for rejecting his evidence if you are to conclude that his flat-earth claim is wrong. On the other hand, if you accept his evidence, thereby concluding that his claim is correct, you would have to provide reasons for rejecting the evidence of a cylindrical earth. In the end, you should be left with a claim that is supported by adequate evidence and good reasoning (logic). In the next section, we will look at the most fundamental requirement of good reasoning in science.

Evidence for a Claim Must Not Be Contradictory

If a person simultaneously makes two statements that contradict one another, the person's thinking is incoherent. The Law of Noncontradiction states that a claim cannot be both true and false at the same time. If a statement makes you stop and say, "huh?", it is likely that the statement contains a contradiction. For example, we violate the law of noncontradiction when we make statements such as the following:

• A newspaper headline that reads: Area Jobless Rate Up, But Still at Record Low (Paulos, 1998, p. 12)

• The circle has three sides.

• The television show was so sad that I couldn't stop laughing.

• You need to be cruel to be kind.

• He works very hard at being lazy.

• The room was very light and very dark.

• Even though I suffer from severe insomnia, I get more than enough sleep each night.

In some cases, a contradiction is made either as a joke or for some dramatic effect. In such cases, we would not think of it as an expression of poor reasoning. But when someone either is making a statement that is meant to be a clear description of a fact or is presenting an argument (an argument is a set of statements made up of reasons that support a particular conclusion), the individual must obey the law of noncontradiction. In psychology, we are interested in detecting contradictions of several different types. One important type involves contradictions among different pieces of evidence relevant to a claim.

As you learned above, a claim must have evidence in support of it if we are to accept it as provisionally true. The problem is that, for any particular claim, there usually is some evidence supporting it and other evidence failing to support it. For example, the claim that antidepressant medications reduce the symptoms of major depression has much evidence in favor of it but also some evidence against it. The majority of studies show an average improvement in the symptoms of depression after subjects have been on an antidepressant medication for several weeks; but some studies show no average improvement overall in the symptoms. Antidepressant medications either lead to an improvement in the symptoms of depression or they don't. One cannot claim that they do both because this would violate the law of noncontradiction. We will examine this apparent violation of the Law of Noncontradiction in a future lesson. As you will see there, contradictions involving evidence for a claim typically are resolved by looking for the influence of "extraneous" variables--unmeasured (and, therefore, unknown) variables that are having an effect on the phenomenon being studied.

Critical Thinking Questions

Question 5-1

Evaluate the evidence for the claim that the earth is cylindrical and the claim that the earth is flat. Which claim is more credible (a credible claim is one that is believable because it is supported by evidence and logic)? Why is the claim you chose more credible?

Suggested Answer

Question 5-2

(a) What is an "opinion"? Argue against the following claim: "One person's opinion is as good as another's."

Suggested Answer

(b) If one were to accept the claim in Question 5-2(a), what would be the implications for our understanding of reality?

Suggested Answer

Question 5-3

Describe the evidence for and against the claim that "life changes produce stress." Since the evidence relevant to this claim is contradictory (that is, the evidence is not all "for" or "against" the truth of the claim), how would you decide what to believe regarding this claim? (Note: in order to answer this question, you will need to read the Application for Chapter 3 in the textbook.)

Suggested Answer

Question 5-4

Explain how each of the following statements or arguments violates the law of noncontradiction:

(a) Sara said: "It's certainly true that every claim made is simply an opinion. Thus, we can never know for certain if a particular claim is true or false."

Suggested Answer

(b) Tom said: "Men are better thinkers than women are. For example, all the greatest philosophers were men: Socrates, Plato, Aristotle, Descartes, Kant, Wittgenstein, and so on. And today, men are much more likely than women to be successful at science, engineering, and any other profession that requires good thinking skills. This doesn't mean, however, that women are intellectually inferior to men. For example, women do just as well as men on IQ tests."

Suggested Answer

(c) Gwen said: "I do better on tests when I don't study for them than when I do study for them."

Suggested Answer

(d) Phil said: "I think we need to celebrate all cultures. Each culture that has ever existed has something important to teach us and, so, it is essential to understand them from their own perspectives. Even when their values and customs seem strange or even immoral to us, it is very important not to judge them from the perspective of our own culture. Thus, I always try to understand people from other cultures in a nonjudgmental way. I feel very strongly about this. For too long, Western culture has ignored or oppressed those from other cultures because they saw them as immoral or inferior. This attitude towards other cultures shows that, fundamentally, Western culture is corrupt and evil. It would be good if Western culture had never existed"

Suggested Answer

Bibliography and References

Asimov, I. (Fall, 1989). The relativity of wrong. Skeptical Inquirer, 14(1), 35-44. Retrieved March 24, 2002, from

Nidus Information Services. (2001). What is depression? Retrieved March 14, 2002, from

Paulos, J. A. (1998). Once upon a number: The hidden mathematical logic of stories. New York: Basic.

Porter, B. F. (2002). The voice of reason: Fundamentals of critical thinking. New York: Oxford University Press.

Schadewald, R. J. (1980). The Flat-out Truth: Earth Orbits? Moon Landings? A Fraud! Says This Prophet. Reprinted from Science Digest. Retrieved March 14, 2002, from

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