Ten Common Logical Fallacies
An important way to evaluate the soundness of an argument is to examine its logic and, in particular, look for so-called logical fallacies that may lead writers’ reasoning astray. Here are ten of the most common logical fallacies.
1. Hasty generalization: We frequently make a judgment about someone after just meeting them. Or we conclude that a class is useless after attending a single session. All of these are generalizations based on insufficient evidence. Hasty generalizations might be true—the class might turn out to be useless—but you should always be wary of them, especially when talking about people.
Example
I've met three race drivers today and they all were rather aggressive. Clearly, race drivers are all aggressive.
2. Ad hominem: When arguments turn into shouting matches, they almost inevitably get personal. People shift away from the substance of an argument to attack the person making it.
Example (From the movie My Girl)
Vada: Let’s go to the church hall, they’re playing bingo
Thomas J: I told ya, I’d get in trouble!
Vada: Pacifist!
Thomas J: Am not!
Vada: Bed wetter!
3. Appeal to False authority: Finding support for a claim from an expert is a smart move, but sometimes the authority we cite isn’t really an expert on the subject.
Example
The TV show The West Wing stars Martin Sheen, who plays the President of the United States. The President has to know a lot about foreign policy; therefore, Martin Sheen is an excellent authority on how long the U.S. should be in Iraq.
4. Straw man: One way to sidetrack reason in an argument is to misrepresent or ignore the actual position of an opponent. We would then proceed to criticize that opponent based on that misrepresentation.
Step 1: Build the Straw Man: "Evolution is false! How could a mouse evolve into an elephant!?" (The person assumes that that’s the main position of evolution)
Step 2: Knock down the Straw Man by any means necessary: "How could a mouse evolve into an elephant? There would have to be billions of changes for that to occur, and nobody has ever seen speciation anyway!" (More assuming)
Step 3: Connect the original position to the Straw Man:"So it's silly...who has ever seen a mouse evolve into an elephant? Nobody!!" (More assuming)
Step 4: Claim to negate the opposing position by the connection in 3. "Therefore, evolution must be false!" (Conclusion based on false assumptions)
5. False analogy: Analogies can be powerful comparisons in argument, but they can also lead us astray. If A and B are nothing alike, it would be illogical to compare them to make a point.
Example
People are like dogs. They respond best to clear discipline.
6. Post hoc or false cause: Just because one thing follows another doesn’t mean that one causes the other. It might be coincidence.
Example
You used the telephone, and then it stopped working. You broke the phone.
7. Appeal to popularity: This argues that since it’s popular it must be true.
Example
We should see Rocky 27; it was the most watched movie last week.
8. Slippery Slope: You might start out reasonably enough, but soon you start sliding towards conclusions that simply don’t follow.
Example
Passing gun control laws will lead to more laws restricting our freedom, and pretty soon we’ll live in a communist society, and we won’t have any freedom at all.
9. Either/or Fallacy: In a black and white world, something is right or wrong, true or false, good or bad.
Example
America: love it or leave it.
10. Begging the question: This one is also called circular reasoning because it assumes the truth of the arguer’s conclusion without bothering to prove it.
Example
God exists because the Bible says so. The Bible was written by God.