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Logical Fallacies in Deductive Reasoning Help

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Updated on Sep 28, 2011

Exercises for this lesson can be found at Ad Hominem, Red Herring, and Straw Man: Reasoning Skills Success Practice Exercises.

Introduction to Logical Fallacies in Deductive Reasoning

"Logic is the anatomy of thought."

–John Locke, English philosopher (1632–1704)

Lesson Summary

In this lesson about logical fallacies in deductive reasoning, you'll learn about fallacies that try to divert your attention from the main issue or to distort the issue so you're more likely to accept the argument. These fallacies include ad hominem, the red herring, and the straw man.

Imagine the following scenario: You have been renting your apartment for one year, and your landlord tells you that she is going to raise the rent $500 a month. One day, you run into another building tenant, Tina, in the hall. You tell her of your problem with the landlord. Tina gives you some advice. Later that week, you run into another tenant, Frank, who has heard about your predicament from Tina. Frank says to you, "Listen, I know this is none of my business, but if I were you, I wouldn't take Tina's advice about housing issues. She was evicted from her last apartment!"

Should you listen to Frank and ignore Tina's advice?

Since you haven't lived in the building for very long and don't know your neighbors very well, you have somewhat of a dilemma on your hands. Who do you trust? Who is more credible? You can't answer these questions because you are a fairly new tenant, but it is important that you realize that Frank has committed a logical fallacy. In this last lesson about logical fallacies in deductive reasoning, you'll learn about distracters and distorters—fallacies that aim to confuse the issues so that you more easily accept the conclusion of the argument. Ad hominem will be discussed first, followed by red herrings and the straw man.

Ad Hominem

What has Frank done wrong? Indeed, since Tina was evicted from her last apartment, how can she give you good advice? It would appear as if what Frank says makes a lot of sense.

Frank's argument may seem logical, but it's not. That's because Frank is not attacking Tina's advice; instead, he's simply attacking Tina. This kind of false reasoning is called ad hominem, which in Latin means, "to the man." Ad hominem fallacies attack the person making the claim rather than the claim itself.

An ad hominem fallacy can take a variety of forms. You can attack a person, as Frank does, for his or her personality or actions. You can also attack a person for his or her beliefs or affiliations. For example, you might say, "Don't listen to him. He's a liberal." Or you can attack a person for his or her nationality, ethnicity, appearance, occupation, or any other categorization. For example, imagine someone says to you:

"Of course he's wrong. Someone who dresses like that obviously doesn't have a clue about anything."

This is a clear-cut case of ad hominem.

Ad hominem aims to distract you from looking at the validity of the claim by destroying the credibility of the person making the claim. But the trouble with ad hominem is that it doesn't really take into account the issue of credibility. Just because Tina was evicted from her last apartment doesn't mean she can't give you good advice about how to deal with your landlord. In fact, because she's dealt with a fairly serious housing issue, she might be considered more of an expert than most. It all depends on what kind of advice you're looking for. Maybe Tina was a victim of circumstance. Whatever the case, Tina may still be in a position to give you good advice. If Frank wants to prove his point, he needs to attack Tina's actual argument about how to handle your landlord rather than to attack Tina herself.

To clarify when something is and isn't an ad hominem, read the following example:

  1. Don't listen to what Bob says about investments. That guy is the most money-grubbing creep I've ever met.
  2. I wouldn't listen to what Bob says about investments if I were you. He recently made his own investment decisions and lost all of his money in the stock market.

Are either of these ad hominem fallacies? Both? Neither?

You probably saw that argument A uses ad hominem quite shamelessly. So what if Bob is a "money-grubber"? That doesn't mean he can't have good advice about investments. In fact, if he's greedy, he may be quite knowledgeable about the kinds of investments that make the most money. Whether you like him or not is a separate matter from whether he has good advice or not. His "money-grubbing" nature should not really affect the credibility of his argument. Remember, credibility is based on freedom from bias and on expertise—not on appearance, personality, past behavior, or beliefs.

If, on the other hand, Bob has recently made investments and lost his money, his expertise in the matter of investments should be called into question. He has experience in investing, yes—but his experience shows that he may not be too knowledgeable about the subject. You should probably investigate further before deciding whether or not to listen to his advice. At any rate, at least argument B avoids the ad hominem fallacy.

Ad hominem fallacies can also work in reverse. That is, the argument can urge you to accept someone's argument based on who or what the person is rather than on the validity of the premises. For example:

Len says, "I agree with Rich. After all, he's a Lithuanian, too."

Does the fact that Len and Rich share the same nationality mean that Rich's argument—whatever it may be—is valid? Of course not.

Tip

Have you ever "assumed" something about a person just because he or she belonged to a certain group, listened to a certain type of music, or wore a certain type of clothes? It's easy to assume things about people that aren't true. Don't fall into that trap because not only does stereotyping lead to illogical thinking in arguments—but in daily life as well.

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