Be sure you understand that just because a syllogism is a fallacy does not mean that the conclusion is false. Consider the following fallacy.
• Premise: When it rains, the ground gets wet. • Premise: The ground is wet. • Conclusion: Therefore, it must have rained.
This is a fallacy because the conclusion does not follow logically from the two premises. And yet, common sense tells you that most of the time, the conclusion would be true. After all, if you woke up in the morning and walked outside and saw that the ground was wet, wouldn't you assume that it had rained during the night?
Deduction of a fallacy is especially important in legal matters. Think about the situation described in Example 1. How would you feel if you were accused of a crime, given a polygraph test in which you truthfully proclaimed your innocence, and then told that the polygraph indicated that you lied?
Do you see why the type of fallacy in Example 1 is called affirming the consequent?
• Premise: When people lie, their heart rate increases and they sweat. • Premise: This man has an increased heart rate and is sweating. • Conclusion: Therefore, he is lying.
The second part of the first premise is called the consequent. The second premise is affirming that the consequent is true.
Did you know that Crater Lake has no tributaries; instead it relies on rain and snowfall to keep its water supply. Despite the fact that Crater Lake has no tributaries, it is the deepest lake in the United States.
If you plan on making a trip to Crater Lake you might want to check out Crater Lake Lodges to learn about some of the fun things to do at Crater Lake and book a reservation.
To help see that this argument is a fallacy, you can write it as a syllogism.
• Premise: When a large meteor hits Earth, it forms a large crater. • Premise: The diameter of Crater Lake in Oregon is about 5 miles. • Conclusion: So, Crater Lake must have been formed by a huge meteor.
In this form, you can see that the logic is not correct. This is another example of affirming the consequent. In this particular case, the reasoning is not only incorrect, the conclusion is actually false. Crater Lake was formed about 7700 years ago when an ancient volcano collapsed. It is not the result of a meteor striking Earth.
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