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Deductive Fallacy Study Guide

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Updated on Sep 19, 2011

Lesson Summary

Logical errors are, I think, of greater practical importance than many people believe; they enable their perpetrators to hold the comfortable opinion on every subject in turn.

Bertrand Russell, British philosopher, mathematician, and historian (1872–1970)

An argument with poor reasoning to support its conclusion is called a fallacy. In this lesson, you'll discover the relationship between deductive reasoning and how logic does or doesn't work. And you'll investigate four of the most common logical fallacies people use that make deductive reasoning fall apart.

Deductive Reasoning Study Guide explains what makes a valid deductive argument—two premises that are true and a conclusion that logically follows from them, without assuming anything not in those premises. A factual error, like a false premise or a conclusion that is not supported by the premises, makes an argument invalid. Moreover, an error in logic can make an argument invalid. This logical error is known as a fallacy.

There are a number of logical fallacies that occur in deductive arguments. Sometimes it's hard to recognize such fallacies, but it's important to so you're not misled or persuaded by someone's faulty logic. There are four major logical fallacies:

  1. Slippery Slope
  2. False Dilemma
  3. Circular Reasoning
  4. Equivocation

Slippery Slope

As you read in the last lesson, conditionals are premises that use "if … then" to lead to a conclusion. Example: If you oversleep, then you'll miss the bus. A slippery slope is a conditional that contains a logical fallacy. It doesn't explain how the first event leads to the second. Example: If you don't pay your electric bill, then you'll never be able to get a loan for a car.

Slippery slope makes an argument seem more severe; it assumes that one wrong could slide toward something else that is wrong. It leaves out what is "between" the two events, without saying why. In the previous example, there are many possible steps between event A, not paying an electric bill, and event B, not being able to get a car loan. It's true that not paying a bill on time would show up on your credit report, but just one late payment doesn't inescapably lead to having such a bad credit report that you can't get a loan for a car.

Other examples follow. Keep in mind the possible steps between event A and event B in each, and the likelihood, or unlikelihood, that B will ever be a result of A.

  • Don't let him help you with that. The next thing you know, he will be running your life.
  • You can never give anyone a break. If you do, they will walk all over you.
  • This week, you want to stay out past your cur-few. If I let you stay out, next week you'll be gone all night!

Tip

Always check an argument for a chain of consequences. When someone says "If this … then that," make sure the chains are reasonable.

False Dilemma

A false dilemma is an argument that presents a limited number of options (usually two), while in reality there are more. In other words, it gives a choice between one or another ("either-or") even though there are other choices that could be made. The false dilemma is commonly seen in black or white terms; it sets up one thing as all good and the other as all bad. Here's an example:

Stop wasting my time in this store! Either decide you can afford the stereo, or go without music in your room!

This argument contains a logical fallacy because it fails to recognize that there are many other options besides just buying one particular (expensive) stereo and going without music. You could, for instance, buy a less expensive stereo or even a radio. Or, you could borrow a stereo and have music in your room without making a purchase. There are many options beside the two presented as "either-or" in the argument.

Other common false dilemmas include:

      Love it or leave it.
      Either you're with us, or you're against us.
      Get better grades or you will never go to college.

False dilemmas are also common in politics. Many politicians would like you to believe that they, and their party, have all the right answers, and their opponents are not only wrong, but are ruining the country. They set up a choice between all good and all bad. For instance: "Price supports on agricultural production are part of the socialist agenda. My opponent in this race consistently votes for price supports on dairy and tobacco products. It is time to stop electing socialists to Congress. Should you vote for my opponent, who wants to lead our country on the path toward socialism, or should you vote for me and restore democracy?

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