Critical Thinking - Bellevue College

Critical Thinking

Mark Storey

Bellevue College

Copyright (c) 2013 Mark Storey

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with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.

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Contents

Part 1

Chapter 1: Thinking Critically about the Logic of Arguments .. 3

Chapter 2: Deduction and Induction ¡­¡­¡­¡­ ¡­¡­¡­¡­¡­¡­. 10

Chapter 3: Evaluating Deductive Arguments ¡­¡­¡­¡­¡­...¡­. 16

Chapter 4: Evaluating Inductive Arguments ¡­¡­¡­¡­..¡­¡­¡­ 24

Chapter 5: Deductive Soundness and Inductive Cogency ¡­.¡­. 29

Chapter 6: The Counterexample Method ¡­¡­¡­¡­¡­¡­¡­¡­... 33

Part 2

Chapter 7: Fallacies ¡­¡­¡­¡­¡­¡­¡­.¡­¡­¡­¡­.¡­¡­¡­¡­¡­. 43

Chapter 8: Arguments from Analogy ¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­ 75

Part 3

Chapter 9: Categorical Patterns¡­.¡­¡­.¡­¡­¡­¡­.¡­¡­¡­¡­¡­ 86

Chapter 10: Propositional Patterns¡­¡­..¡­.¡­¡­¡­¡­...¡­¡­¡­ 116

Part 4

Chapter 11: Causal Arguments....¡­¡­..¡­¡­¡­¡­.¡­¡­¡­....¡­. 143

Chapter 12: Hypotheses.¡­.¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­.¡­ 159

Chapter 13: Definitions and Analyses...¡­¡­¡­¡­¡­¡­¡­...¡­... 179

Chapter 14: Probability¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­¡­.¡­¡­¡­199

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Chapter 1: Thinking Critically about the Logic of Arguments

Logic and critical thinking together make up the systematic study of reasoning, and reasoning is

what we do when we draw a conclusion on the basis of other claims. In other words, reasoning is

used when you infer one claim on the basis of another. For example, if you see a great deal of

snow falling from the sky outside your bedroom window one morning, you can reasonably

conclude that it¡¯s probably cold outside. Or, if you see a man smiling broadly, you can

reasonably conclude that he is at least somewhat happy. In both cases, you are reasoning from

evidence to a conclusion.

We use reasoning all the time, but sometimes we make a mess out of it. Whether a line of

reasoning is good or not is definitely more than ¡°just a matter of opinion.¡± Surely the reasoning

in the following arguments is not compelling:

* My four-year-old niece says that the planet Mars is smaller than Jupiter. It must thereby be the

case that Mars is smaller than Jupiter.

* Some women are baseball fans. And some mothers are baseball fans. Thus, all women are

mothers.

* An earthquake occurred in San Francisco five minutes after the senator¡¯s speech there. Thus

that senator¡¯s voice causes natural disasters.

But the reasoning in the next set of arguments is better, yes?

* All bears are mammals. Grizzlies are bears. Thus grizzlies are mammals.

* If Jimmy Carter was the U.S. President, then he was a politician. Carter was indeed the U.S.

President. Thus, Carter was a politician.

* It has rained in Seattle, Washington every year for the past 100 years. Thus it will probably

rain there next year.

Some examples of reasoning are clearly better than others. The study of logic and critical

thinking are designed to make us better at recognizing good from bad lines of argumentation.

An argument consists of one or more statements, called premises, offered as reason to believe

that a further statement, called the conclusion, is true. Technically speaking, premises and

conclusions should be made up of statements. A statement is a sentence that declares something

to be true or false. They are thus sometimes called declarative sentences. A sentence is a

grammatically correct string of words, and there are many kinds of sentences other than

statements. Questions (e.g., ¡°What is your name?¡±), commands (e.g., ¡°Turn to page three¡±), and

exclamations (e.g., ¡°Ouch!¡±) are all grammatically correct sentences that are not statements.

They are not statements because it makes no sense to say they are true or false. (¡°What is your

name?¡± ¡°That¡¯s true!¡± This would be a ridiculous mini-conversation.) Statements will always be

true or false, never both, and never neither. We may disagree on whether a given statement is

true (e.g., ¡°God exists¡±), or we may not be able to determine whether a statement is true or false

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(e.g., ¡°There is a mountain on Pluto exactly 1000 meters tall, plus or minus 2 centimeters¡±), yet

the statement is objectively true or false (but not both) nonetheless.

In this course, the words ¡°statement¡± and ¡°sentence¡± can¡ªin many contexts¡ªbe used

interchangeably. This is so because all statements are sentences (although not all sentences are

statements). So we can refer to ¡°Bellevue is in Washington¡± as both a statement (because it

declares something to be true) and a sentence (because it is a grammatically correct sequence of

words conveying a meaning).

An argument can have any number of premises, but technically speaking there is one conclusion

per argument. Thus, an argument splits into two distinct parts:

1. One or more premises offer evidence for the truth of the conclusion.

2. The conclusion is supported by the premise or premises.

Here is an argument:

All dogs are mammals.

No mammals are birds.

Thus, no dogs are birds.

The conclusion seems well supported by the two premises. However, things are not so good in

the following argument:

Some cats are animals.

Some animals are fish.

Hence, some cats are not fish.

In both examples above, the arguments contained two premises and one conclusion, but in the

second argument immediately above, the premises by themselves do not offer good reason to

believe the conclusion¡ªeven if though the premises are true!

Sometimes the conclusion of an argument can be used as a premise of a following argument,

making a chain of arguments. Still, to be precise, each argument or specific line of inference

contains one and only one conclusion, although each may contain varying number of premises.

For instance:

1. All dogs are mammals.

2. All mammals are animals.

3. Thus, all dogs are animals.

4. Scooby-Doo is a dog.

5. Thus, Scooby-Doo is an animal.

6. No animals are plants.

7. All trees are plants.

8. Thus, Scooby-Doo is not a tree.

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Whew! Here the first argument in the chain has lines 1 and 2 as premises, and has line 3 as its

conclusion. The second argument then uses line 3 as a premise and uses it with line 4 to conclude

in line 5 that Scooby-Doo is a dog. The third argument then uses line 5 as a premise, hooks it up

with lines 6 and 7, and uses the trio together to infer line 8 as the final conclusion.

**Practice Problems: Types of Sentences

Are the following statements or not?

1. George Carlin is presently president of the USA.

2. Chocolate is a popular flavor of ice cream in the USA.

3. Sally Brown, come on down!

4. Washington State is south of Oregon.

5. Bob believes that Washington State is south of Oregon.

6. College students are morally obliged to believe that Washington State is south of Oregon.

7. Who in Oregon is rooting for the Huskies?

8. It is prudent for Duck fans not to wear green when going to a Husky game in Seattle.

9. Green is an Oregon Ducks color, while purple is a Washington Huskies color.

10. The Huskies are my favorite college football team!

11. Go Cougars!

12. The Ducks will never win the Apple Cup.

13. Huskies

14. Ducks vs. Cougars

15. The Ducks will play the Cougars tonight.

16. Slap a ham on Omaha, pals!

17. Dennis and Edna sinned.

18. Rats live on no evil star.

19. Tarzan raised Desi Arnaz¡¯ rat.

20. Go deliver a dare, vile dog.

Answers:

1. statement

2. statement

3. not a statement

4. statement

5. statement

6. statement

7. not a statement

8. statement

9. statement

10. statement

11. not a statement

12. statement

13. not a statement

14. not a statement

15. statement

16. not a statement

17. statement

18. statement

19. statement

20. not a statement

Indicator Words

Before determining whether an argument is good or bad, we need to recognize its structure. We

need, that is, to know which claims are premises and which one is the conclusion. Indicator

words or phrases can help us out here.

A conclusion indicator is a word or phrase that, when used in the context of an argument, signals

that a conclusion is about to be given or was just given. In the two examples above, ¡°Thus¡± and

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