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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

A Good Argument A good argument justifies acceptance of

the conclusion.

A Good Argument A good argument can be valid and

sound, or invalid but strong.

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

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

True or False? When diagramming an argument, one

should include claims that contradict the conclusion.

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.

True or False? When evaluating an argument, one

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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.

Supply a General Principle

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

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.

Supply a General Principle Monica may have trouble sleeping

tonight, since she drank tea at 10 pm.

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.

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

was messing with it.

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.

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

she watches Dancing with the Stars, too.

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.

Supply a General Principle Mr. Zing has a background in

psychology. He is bound to make a good chairman.

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.