Identifying Faulty Reasoning
Identifying Faulty Reasoning
Identifying Faulty Reasoning
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Copyright © by McDougal Littell, a division of Houghton Mifflin Company<br />
LESSON<br />
5<br />
CRITICAL THINKING<br />
<strong>Identifying</strong> <strong>Faulty</strong> <strong>Reasoning</strong><br />
<strong>Reasoning</strong> is the process of using data to come to a conclusion. <strong>Faulty</strong> reasoning<br />
occurs when the conclusion is not supported by the data. It can be the result of not<br />
enough information, coming to a conclusion that is not really supported by the data,<br />
or adding personal opinions that are not part of the data into the conclusion.<br />
Overgeneralization “The elm tree on my block has Dutch elm disease.<br />
So does the one on your block. That means that all elm trees have Dutch elm<br />
disease.” This statement L5/06.02 sounds silly because you know that there are elm trees<br />
McDougal<br />
that do not have Dutch Problem elm Solving: disease. Critical It is an Thinking example of an overgeneralization,<br />
or drawing a conclusion 1/26/04 with KA<br />
too little data.<br />
Illogical Conclusion Another type of faulty reasoning is making an illogical<br />
conclusion, or inferring something that is not based on the data. “It rained all last<br />
week and now I have a cold. Rainy weather must cause colds.” This statement is<br />
an example of an illogical conclusion. Colds are caused by virus infections, which<br />
have nothing to do with rain.<br />
Personal Bias A third type of faulty reasoning, personal bias, occurs when a<br />
conclusion is not based on data, but on personal opinion. “Wooden baseball bats hit<br />
the ball farther than aluminum bats, because I can hit farther with a wooden bat.”<br />
Unless you have collected data on the use of both types of bat by different batters,<br />
you cannot support the conclusion that a wooden bat hits the ball farther with data.<br />
The conclusion is only your opinion.<br />
PROBLEM SOLVING AND CRITICAL THINKING 13<br />
<strong>Identifying</strong> <strong>Faulty</strong> <strong>Reasoning</strong>
PRACTICE IDENTIFYING FAULTY REASONING<br />
In each of these cases, determine what information is provided and identify the<br />
type of faulty reasoning that leads to an unsupported conclusion. Each type of faulty<br />
reasoning described on the previous page – overgeneralization, illogical conclusion,<br />
and personal bias – is illustrated.<br />
Shells were found that resemble those of sea creatures. However, they were found<br />
among rocks in mountains far from any ocean. From this evidence, I have determined<br />
that many of the animals that are found in the sea today once lived on land and then<br />
migrated to the sea.<br />
1. What type of faulty reasoning is illustrated in this example?<br />
2. Why is it faulty?<br />
I watched the birds at my bird feeder for two weeks and recorded what they ate.<br />
I have concluded that birds eat seeds, because every bird that I saw either ate seeds<br />
from the feeder or seeds that fell on the ground. I did not see any birds eating other<br />
types of food. Birds definitely eat seeds.<br />
3. What type of faulty reasoning is illustrated in this example?<br />
4. Why is it faulty?<br />
If you look at the illustration, L5/06.03you<br />
can see the difference between dogs and cats.<br />
McDougal<br />
Dogs are more active and Problem they like Solving: to Critical play Thinking games like fetching a stick. Cats lie in<br />
1/26/04 BD<br />
the sun all day and don’t do anything fun. Based on this data, it is clear that dogs<br />
are better pets than cats.<br />
5. What type of faulty reasoning is illustrated in this example?<br />
6. Why is it faulty?<br />
14 PROBLEM SOLVING AND CRITICAL THINKING<br />
Copyright © by McDougal Littell, a division of Houghton Mifflin Company
Copyright © by McDougal Littell, a division of Houghton Mifflin Company<br />
The Hawaiian islands were made by eruptions from undersea volcanoes.<br />
The Aleutian islands were also made by eruptions from undersea volcanoes.<br />
Therefore, all islands were made by eruptions from undersea volcanoes.<br />
7. What type of faulty reasoning is illustrated in this example?<br />
8. Why is it faulty?<br />
My little sister’s friend has lice in her hair. That’s because she goes to a city<br />
school where everybody has lice. The lice jump from one kid’s head to the<br />
next. I told my sister to stay away from that girl. She is not clean and goes<br />
to school with other dirty kids. My sister had better not get lice from her.<br />
9. What type of faulty reasoning is illustrated in this example?<br />
10. Why is it faulty?<br />
It would be hard to feed a mouse to a snake. Pet snakes need to eat live<br />
mice just like snakes in the wild do. However, the thought of watching a<br />
nasty snake eat a cute little mouse is sickening. Snakes should be given<br />
vegetables or ground meat instead.<br />
11. What type of faulty reasoning is illustrated in this example?<br />
12. Why is it faulty?<br />
PROBLEM SOLVING AND CRITICAL THINKING 15
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