Critical Thinking: Chapter 2
Two Kinds of Reasoning
Valid Argument
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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
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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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These premises, if true, guarantee that
the conclusion is true.
Valid Argument
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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
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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
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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
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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
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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
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A strong argument has this
characteristic: On the assumption that
the premises are true, the conclusion is
probably true (or unlikely to be false).
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Example: The roses will probably get
mildew again this spring.
Inductive argument
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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
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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
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A good argument justifies acceptance of
the conclusion.
A Good Argument
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A good argument can be valid and
sound, or invalid but strong.
Premise Indicators
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Since…
Because…
For…
In view of…
This is implied by
Conclusion Indicators
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Thus
Therefore…
Hence…
This shows that…
This suggests that…
Consequently…
So…
Accordingly…
This implies that…
This proves that…
True or False?
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When diagramming an argument, one
should include claims that contradict the
conclusion.
True or False?
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When diagramming an argument, one
should include claims that contradict the
conclusion.
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True. This will help you figure out
whether it is a good argument or not.
True or False?
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When evaluating an argument, one
should always take into consideration
the existence and merit of any unstated
premises.
True or False?
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When evaluating an argument, one
should always take into consideration
the existence and merit of any unstated
premises.
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True. Many times unstated premises are
what makes an argument either valid
and sound, or strong.
Unstated Premises
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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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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.
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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.
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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.
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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.
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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.
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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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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.
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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.
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Answer: No 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: 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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Hey, what IS that stuff you’re
cooking, anyway? It smells like
fish
Fish! 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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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.
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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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Answer: Argument; the
conclusion is that meat isn’t
an appropriate or healthy diet
for humans.
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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.
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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.
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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.
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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.
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Supply a general principle that, assuming it is true,
makes the following into a relatively strong
inductive argument:
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Sydney is ten; therefore she likes horses.
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Supply a general principle that, assuming it is true,
makes the following into a relatively strong
inductive argument:
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Sydney is ten; therefore she likes horses.
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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.
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Answer: Home prices in Santa Cruz have
depreciated in the last 5 years.
Supply a General Principle
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Monica may have trouble sleeping
tonight, since she drank tea at 10 pm.
Supply a General Principle
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Monica may have trouble sleeping
tonight, since she drank tea at 10 pm.
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Answer: Drinking caffeinated tea late
in the day tends to keep people awake.
Supply a General Principle
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I’d bet the TV doesn’t work right. Josh
was messing with it.
Supply a General Principle
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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
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Deborah loves American Idol. It’s a good bet
she watches Dancing with the Stars, too.
Supply a General Principle
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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
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Mr. Zing has a background in
psychology. He is bound to make a
good chairman.
Supply a General Principle
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Mr. Zing has a background in
psychology. He is bound to make a
good chairman.
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Answer: People versed in psychology
tent to make good administrators.