Deductive Fallacy Study Guide (page 2)
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:
- Slippery Slope
- False Dilemma
- Circular Reasoning
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!
Always check an argument for a chain of consequences. When someone says "If this … then that," make sure the chains are reasonable.
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?
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.
- "I told you to clean your room!" "Why?" "Because I said so!"
- "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."
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.
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.
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.
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.
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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