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Deductive Fallacy Study Guide (page 2)

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Updated on Sep 19, 2011

Circular Reasoning

The conclusion of a valid deductive argument should follow logically from its premises, relying only on the information contained within them. But with the fallacy of circular reasoning, often called begging the question, someone assumes as truth the premise he or she is supposed to be proving. In all valid deductions, the conclusion—what someone is trying to prove—follows two premises; in an invalid circular-reasoning argument, the conclusion follows a single premise. In other words, a premise that's supposed to prove the truth of the conclusion is just the conclusion restated with a slight variation.

When a premise is left out, there is no argument. The person making the claim is simply telling you to believe that what he or she is telling you is true.

Examples

  1. "I told you to clean your room!" "Why?" "Because I said so!"
  2. "Why do you think the Yankees are the best team in baseball?" "Because they are."

How could these examples go from being invalid to valid, logical arguments? They need a second premise that supports, or gives reason for, the conclusion. Example 1 might add: "Your room is so messy that you can't find anything in it," or, "All of your laundry is on the floor, and it won't get washed until you clean it up and put it in the washer." Example 2 could add: "They have won the World Series 27 times," or, "They are the only team to sweep the World Series ten times."

Tip

Identify the most important words and phrases in an argument and ask yourself if they have more than one meaning. If they do, be sure you know which is the correct meaning for the situation.

Equivocation

Sometimes the fallacy of equivocation can be difficult to spot because both premises seem to be true, and often the conclusion seems to follow them. However, the meaning of the entire argument becomes invalid because either (1) a word is used twice, each time with a different meaning, or (2) a word with several meanings is used just once but gives the statement an ambiguous meaning. The ambiguity isn't due to grammar, but to the distinct meanings of the words.

Examples

My history professor said everyone who wrote a term paper favoring the separatists in the Philippines is sick. I guess if I'm sick, I can skip class today.

The word "sick" is used in the argument twice, each with a different meaning. The professor meant emotionally troubled, and the student thought he meant physically ill.

Hot dogs are better than nothing. Nothing is better than steak. Therefore, hot dogs are better than steak.

It is not hard to spot the logical fallacy here: The conclusion is obviously wrong although the premises are both true. There is an equivocation in the meaning of the word "nothing"; in the first premise, it means "not a thing," and in the second premise, it means "no other possible thing." Using a critical word with two different meanings makes the argument invalid.

The second way an argument becomes invalid due to equivocation is when a word, used only once, has several meanings. For example, "Save soap and waste paper." Here, the word waste could mean either the noun "garbage," or the verb "to use thoughtlessly." The equivocation of the word waste makes the meaning of the sentence unclear.

Equivocation can be confusing because it begins with truthful or reasonable premises, which you can agree with. Then, the meaning of a critical word is changed and an illogical or faulty conclusion is drawn. If you follow the argument, you could fall into the trap of agreeing with something you would never have otherwise accepted. The best way to handle this fallacy is to get information. Ask for clear definitions of any critical terms that could be used in different ways.

In Short

Not all deductive reasoning is reasonable. It may be flawed factually, meaning all or part of it is untrue. Or it may be flawed logically, and contain a fallacy. It is important to recognize logical fallacies so they do not persuade or mislead you. Some of the most common of these fallacies are slippery slope, false dilemma, circular reasoning, and equivocation.

Tip

People who use logical fallacies imply that we're ignorant and incapable of seeing through their deception. Prove them wrong! Learn to spot each kind.

Skill Building Until Next Time

  • Find a newspaper or magazine article that contains quotes from one or more politicians. Do any of them use logical fallacies in their arguments? If so, which fallacies, and how?
  • Think of an extravagant purchase you would like to make. Devise two arguments for buying the item, using both false dilemma and circular reasoning fallacies.

Exercises for this concept can be found at Deductive Fallacy Practice Exercises.

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