CRITICAL THINKING
5 Identifying Faulty Reasoning
Copyright © by McDougal Littell, a division of Houghton Mifflin Company
Reasoning is the process of using data to come to a conclusion. Faulty reasoning
occurs when the conclusion is not supported by the data. It can be the result of not
enough information, coming to a conclusion that is not really supported by the data,
or adding personal opinions that are not part of the data into the conclusion.
“The elm tree on my block has Dutch elm disease.
So does the one on your block. That means that all elm trees have Dutch elm
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disease.” This statement
sounds silly because you know that there are elm trees
McDougal
that do not have Dutch
elmSolving:
disease.Critical
It is anThinking
example of an overgeneralization,
Problem
or drawing a conclusion
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KA too little data.
Illogical Conclusion Another type of faulty reasoning is making an illogical
conclusion, or inferring something that is not based on the data. “It rained all last
week and now I have a cold. Rainy weather must cause colds.” This statement is
an example of an illogical conclusion. Colds are caused by virus infections, which
have nothing to do with rain.
Personal Bias A third type of faulty reasoning, personal bias, occurs when a
conclusion is not based on data, but on personal opinion. “Wooden baseball bats hit
the ball farther than aluminum bats, because I can hit farther with a wooden bat.”
Unless you have collected data on the use of both types of bat by different batters,
you cannot support the conclusion that a wooden bat hits the ball farther with data.
The conclusion is only your opinion.
Overgeneralization
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Identifying Faulty Reasoning
LESSON
PRACTICE IDENTIFYING FAULTY REASONING
In each of these cases, determine what information is provided and identify the
type of faulty reasoning that leads to an unsupported conclusion. Each type of faulty
reasoning described on the previous page – overgeneralization, illogical conclusion,
and personal bias – is illustrated.
Shells were found that resemble those of sea creatures. However, they were found
among rocks in mountains far from any ocean. From this evidence, I have determined
that many of the animals that are found in the sea today once lived on land and then
migrated to the sea.
1.
What type of faulty reasoning is illustrated in this example?
2.
Why is it faulty?
3.
What type of faulty reasoning is illustrated in this example?
4.
Why is it faulty?
If you look at the illustration,
you can see the difference between dogs and cats.
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Dogs are more active and McDougal
they
play
games like fetching a stick. Cats lie in
Problem like
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Critical
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the sun all day and don’t do anything fun. Based on this data, it is clear that dogs
are better pets than cats.
5.
What type of faulty reasoning is illustrated in this example?
6.
Why is it faulty?
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Copyright © by McDougal Littell, a division of Houghton Mifflin Company
I watched the birds at my bird feeder for two weeks and recorded what they ate.
I have concluded that birds eat seeds, because every bird that I saw either ate seeds
from the feeder or seeds that fell on the ground. I did not see any birds eating other
types of food. Birds definitely eat seeds.
The Hawaiian islands were made by eruptions from undersea volcanoes.
The Aleutian islands were also made by eruptions from undersea volcanoes.
Therefore, all islands were made by eruptions from undersea volcanoes.
7.
What type of faulty reasoning is illustrated in this example?
8.
Why is it faulty?
My little sister’s friend has lice in her hair. That’s because she goes to a city
school where everybody has lice. The lice jump from one kid’s head to the
next. I told my sister to stay away from that girl. She is not clean and goes
to school with other dirty kids. My sister had better not get lice from her.
9.
10.
What type of faulty reasoning is illustrated in this example?
Why is it faulty?
Copyright © by McDougal Littell, a division of Houghton Mifflin Company
It would be hard to feed a mouse to a snake. Pet snakes need to eat live
mice just like snakes in the wild do. However, the thought of watching a
nasty snake eat a cute little mouse is sickening. Snakes should be given
vegetables or ground meat instead.
11.
What type of faulty reasoning is illustrated in this example?
12.
Why is it faulty?
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