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Critical Thinking: Chapter 2 Two Kinds of Reasoning
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Page 1: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Critical Thinking: Chapter 2

Two Kinds of Reasoning

Page 2: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Valid Argument A valid argument has this characteristic:

On the assumption that the premises are true, it is impossible for the conclusion to be false.

Example: [Premise] Every philosopher is a good mechanic, and [premise] Emily is a philosopher. So, [conclusion] Emily is a good mechanic.

Page 3: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Valid Argument Example: [Premise] Every philosopher

is a good mechanic, and [premise] Emily is a philosopher. So, [conclusion] Emily is a good mechanic.

These premises, if true, guarantee that the conclusion is true.

Page 4: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Valid Argument Example: [Premise] Every philosopher

is a good mechanic, and [premise] Emily is a philosopher. So, [conclusion] Emily is a good mechanic.

But these premises are not true, so the argument is valid, but unsound, which makes it not a good argument.

Page 5: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Deductive Argument A deductive argument is either valid (or

intended to be so by its author). Example: [conclusion] Sheila’s clarinet is

French. [Premise] It’s a Leblanc, and [premise] all Leblanc instruments are made in France.

This is a valid argument and if the premises are true, it is also a sound argument, which would make it a good argument.

Page 6: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Sound Argument A sound argument is a valid argument whose

premises are true. Example: [Premise] Some pesticides are toxic

for humans, and [premise] anything that is toxic for humans is unsafe for most humans to consume. Therefore, [conclusion] some pesticides are unsafe for most humans to consume.

Page 7: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Invalid Argument An argument is invalid when its

conclusion does not necessarily follow from the premises.

Example: [Premise] Every year as far back as I can remember my roses have developed mildew in the spring. [Conclusion] Therefore, my roses will develop mildew this spring, too.

Page 8: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Invalid Argument Example: [Premise] Every year as far back as

I can remember my roses have developed mildew in the spring. [Conclusion] Therefore, my roses will develop mildew this spring, too.

This argument doesn’t qualify as valid (or sound) because it is possible that the conclusion is false even assuming the premise is true. Nevertheless, this is not a bad argument. In fact, it is a strong argument.

Page 9: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Strong Argument A strong argument has this

characteristic: On the assumption that the premises are true, the conclusion is probably true (or unlikely to be false).

Example: The roses will probably get mildew again this spring.

Page 10: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Inductive argument An inductive argument is an invalid

argument whose premises are intended to provide some support, but less than conclusive support for the conclusion. (if it was conclusive support it would be valid rather than strong and deductive rather than inductive).

Page 11: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Inductive argument Example: [Premise] The ensemble played an

encore at last year’s concert, and [premise] I’m pretty sure they played one the year before as well. So [conclusion] they will most likely play an encore at this year’s concert as well.

Notice it is not conclusive, but probably true. This means that this is a strong argument, and therefore a good argument.

Page 12: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

A Good Argument A good argument justifies acceptance of

the conclusion.

Page 13: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

A Good Argument A good argument can be valid and

sound, or invalid but strong.

Page 14: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Premise Indicators Since… Because… For… In view of… This is implied by

Page 15: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Conclusion Indicators Thus Therefore… Hence… This shows that… This suggests that… Consequently… So… Accordingly… This implies that… This proves that…

Page 16: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

True or False? When diagramming an argument, one

should include claims that contradict the conclusion.

Page 17: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

True or False? When diagramming an argument, one

should include claims that contradict the conclusion.

True. This will help you figure out whether it is a good argument or not.

Page 18: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

True or False? When evaluating an argument, one

should always take into consideration the existence and merit of any unstated premises.

Page 19: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

True or False? When evaluating an argument, one

should always take into consideration the existence and merit of any unstated premises.

True. Many times unstated premises are what makes an argument either valid and sound, or strong.

Page 20: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Unstated Premises When evaluating an argument with

unstated premises, an appropriate tactic is to find a claim that would make the argument valid or strong and evaluate the argument as if this claim had been included.

Page 21: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Identify the following passage as containing an argument, two arguments, or no argument; if it contains an argument, identify the conclusion(s); and, if it contains two arguments, indicate which argument is the principal argument.

Page 22: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Bamboo can grow up to four feet a day, but only after it is well established. This can take from three to five years, depending on the type of bamboo.

Page 23: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Bamboo can grow up to four feet a day, but only after it is well established. This can take from three to five years, depending on the type of bamboo.

Answer: No argument.

Page 24: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

The Burnhams have invited the performers home for a reception following the recital. But it would be wise to let them know if you plan to attend, because space is limited.

Page 25: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

The Burnhams have invited the performers home for a reception following the recital. But it would be wise to let them know if you plan to attend, because space is limited.

Answer: Argument; conclusion: It would be wise to let them know if you plan to attend.

Page 26: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Feldspar works at a restaurant at night and teaches during the day. I’d have to bet he’s tired most of the time, and that’s a good reason for thinking he won’t do well in school this term.

Page 27: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Feldspar works at a restaurant at night and teaches during the day. I’d have to bet he’s tired most of the time, and that’s a good reason for thinking he won’t do well in school this term.

Answer: Two arguments present; conclusions: He’s tired most of the time, and He won’t do well in school this term; He won’t do well . . . is the conclusion of the principal argument.

Page 28: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

The market for Jackson Pollock paintings has collapsed virtually overnight. Reason: A lot of them were bought during the 1980s, and 1990s. Investors figure that 1980s prices were too high.

Page 29: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

The market for Jackson Pollock paintings has collapsed virtually overnight. Reason: A lot of them were bought during the 1980s, and 1990s. Investors figure that 1980s prices were too high.

Answer: No argument.

Page 30: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

It is a very nice clock, but as you can see, it doesn’t really go very well on that wall. For one thing, it’s too large for the space. For another, it’s red, and the wall is green. The best thing you could do with it, I’m afraid, is take it back. Walmart is good about giving refunds.

Page 31: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Answer: Two arguments present; conclusions: It doesn’t really go very well on that wall, and the best thing you could do with it, I’m afraid, is take it back; The best thing you could do . . . is the conclusion of the principal argument.

Page 32: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Hey, what IS that stuff you’re cooking, anyway? It smells like fishFish! What do you mean, ‘fish’? That’s a pot roast I’m cooking.Oh . . . say, you don’t mind if I open a window, do you? No, it’s not the fish—uh, roast; it just seems sorta warm in here.

Page 33: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Answer: No argument; in the last passage the speaker is explaining—actually, pretending to explain—why he or she wants to open a window, not giving an argument that a window should be opened.

Page 34: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Imagine yourself naked, without weapons, and running after a deer. If you were to catch this deer how would you eat it? Humans are not equipped with canine teeth in order to eat meat without tools. A carnivore’s teeth are long and sharp, and its jaws move up and down. Humans, by contrast, use their molars to crush and grind their food. Have you ever noticed that so many Americans are overweight and unhealthy? That’s because they eat meat.—From a student paper

Page 35: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Answer: Argument; the conclusion is that meat isn’t an appropriate or healthy diet for humans.

Page 36: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Is Bill Clinton’s behavior prior to his becoming President relevant to how he should be judged in office? Yes: 22%; No: 71%—From a telephone poll of 800 adult Americans taken for Time/CNN by Yankelovich Partners, Inc.

Page 37: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Is Bill Clinton’s behavior prior to his becoming President relevant to how he should be judged in office? Yes: 22%; No: 71%—From a telephone poll of 800 adult Americans taken for Time/CNN by Yankelovich Partners, Inc.

Answer: No argument.

Page 38: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

If you don’t mow your lawn at least once a week, what happens is that when you do mow it, it’ll turn brown later.

Page 39: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

If you don’t mow your lawn at least once a week, what happens is that when you do mow it, it’ll turn brown later.

Answer: An argument in most contexts, for the unstated conclusion that you ought to mow your lawn at least once a week.

Page 40: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a general principle that, assuming it is true, makes the following into a relatively strong inductive argument:

Sydney is ten; therefore she likes horses.

Page 41: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a general principle that, assuming it is true, makes the following into a relatively strong inductive argument:

Sydney is ten; therefore she likes horses.

Answer: "Ten-year-olds" like horses.

Page 42: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle

Her home is in Santa Cruz, so I doubt it has depreciated in the last 5 years.

Page 43: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle

Her home is in Santa Cruz, so I doubt it has depreciated in the last 5 years.

Answer: Home prices in Santa Cruz have depreciated in the last 5 years.

Page 44: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle Monica may have trouble sleeping

tonight, since she drank tea at 10 pm.

Page 45: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle Monica may have trouble sleeping

tonight, since she drank tea at 10 pm.

Answer: Drinking caffeinated tea late in the day tends to keep people awake.

Page 46: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle I’d bet the TV doesn’t work right. Josh

was messing with it.

Page 47: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle I’d bet the TV doesn’t work right. Josh

was messing with it.

Answer: When people mess with TVs, the TVs usually don’t work right.

Page 48: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle Deborah loves American Idol. It’s a good bet

she watches Dancing with the Stars, too.

Page 49: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle Deborah loves American Idol. It’s a good bet

she watches Dancing with the Stars, too.

Answer: People who like American Idol generally watch shows like Dancing with the Stars, too.

Page 50: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle Mr. Zing has a background in

psychology. He is bound to make a good chairman.

Page 51: Critical Thinking: Chapter 2 Two Kinds of Reasoning.

Supply a General Principle Mr. Zing has a background in

psychology. He is bound to make a good chairman.

Answer: People versed in psychology tent to make good administrators.