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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.

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Critical Thinking: Chapter 2

Two Kinds of Reasoning


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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.


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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.


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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.


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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.


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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.


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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.


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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.


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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.


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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).


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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.


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A Good Argument

  • A good argument justifies acceptance of the conclusion.


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A Good Argument

  • A good argument can be valid and sound, or invalid but strong.


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Premise Indicators

  • Since…

  • Because…

  • For…

  • In view of…

  • This is implied by


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Conclusion Indicators

  • Thus

  • Therefore…

  • Hence…

  • This shows that…

  • This suggests that…

  • Consequently…

  • So…

  • Accordingly…

  • This implies that…

  • This proves that…


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True or False?

  • When diagramming an argument, one should include claims that contradict the conclusion.


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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.


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True or False?

  • When evaluating an argument, one should always take into consideration the existence and merit of any unstated premises.


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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.


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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.


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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.


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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.


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  • 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.


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  • 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.


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  • 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.


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  • Answer: Two arguments present; conclusions: 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.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.


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  • H 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.ey, 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.


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  • 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


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  • I relevant to how he should be judged in office? Yes: 22%; No: 71%f 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.


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  • I relevant to how he should be judged in office? Yes: 22%; No: 71%f 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 mowyour lawn at least once a week.


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Supply a General Principle the following into a relatively strong inductive argument:

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


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Supply a General Principle the following into a relatively strong inductive argument:

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.


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Supply a General Principle the following into a relatively strong inductive argument:

  • Monica may have trouble sleeping tonight, since she drank tea at 10 pm.


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Supply a General Principle the following into a relatively strong inductive argument:

  • 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.


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Supply a General Principle the following into a relatively strong inductive argument:

  • I’d bet the TV doesn’t work right. Josh was messing with it.


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Supply a General Principle the following into a relatively strong inductive argument:

  • 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.


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Supply a General Principle the following into a relatively strong inductive argument:

  • Deborah loves American Idol. It’s a good bet she watches Dancing with the Stars, too.


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Supply a General Principle the following into a relatively strong inductive argument:

  • 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.


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Supply a General Principle the following into a relatively strong inductive argument:

  • Mr. Zing has a background in psychology. He is bound to make a good chairman.


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Supply a General Principle the following into a relatively strong inductive argument:

  • 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.