Module 1 Lesson 6 - Read

Read: Logical Fallacies in Common Language 

Overview

This section will cover several logical fallacies in common language and how to identify them in arguments.

Logical Fallacies in Common Language

When evaluating deductive arguments, an argument can be validated by identifying its argument form as one of the logical laws such as modus ponens, modus tollens, hypothetical syllogism, or disjunctive syllogism. An argument can also be identified as invalid if it is committing the fallacy of the converse or the fallacy of the inverse, because the fallacy of the converse and the fallacy of the inverse are invalid argument forms. However, in everyday life, we often find ourselves in debates with friends or family members and entangled in all sorts of logical fallacies in common language. These fallacies are not so much about argument forms that are invalid as they are about factors outside of logic that influence the way we reason.
    
To begin, there’s the ad hominem fallacy. Consider the following dialogue:

Mike: Aliens are real.
Jennifer: But, you never graduated high school. So you don’t know what you’re talking about.

In this dialogue, Jennifer attacks Mike’s character rather than dealing with the argument that Mike may have provided. Here, the attack on Mike’s character is irrelevant, even if what is said about Mike is true.

Next, there’s the fallacy of appeal to ignorance. Consider the following dialogue:

Mike: There is no flying teapot in outer space.
Jennifer: It hasn’t been proven that there are no flying teapots in outer space. Therefore, there is a flying teapot in outer space.

In the appeal to ignorance, Jennifer attempts to say that something is true because it hasn’t been proven false. It’s a fallacy because the failure to disprove a claim doesn’t establish the truth of the claim. For instance, mathematicians may fail to disprove a conjecture, and the conjecture may still turn out to be false.
    
Sometimes an appeal to authority is made to argue a point. Consider this dialogue:

Mike: Men are smarter than women.
Jennifer: Why?
Mike: My professor told me.

Here, Mike is appealing to the authority of his professor to justify his claim that men are smarter than women. The appeal to authority is a fallacy since the professor may have equally competent colleagues who disagree with his claim, or the professor’s expertise may be irrelevant to the issue.
    
Sometimes, a person will not like the consequences of an argument and deny one or more of the premises. For example, consider the dialogue:

Mike: There is life outside our solar system.
Jennifer: Why?
Mike: If there is no life outside our solar system, then the universe would be a lonely place. Therefore, there is life outside our solar system. 

Here, Mike doesn’t like the consequences if there is no life outside the solar system. So, he denies the premise that there is no life outside the solar system. Another related fallacy is when a person affirms the premises of an argument because the person likes the consequences of the argument. For example, consider:

Mike: There is life outside our solar system.
Jennifer: Why?
Mike: If there is life outside our solar system, then the universe would not be a lonely place.

Here, Mike likes the consequences of there being life outside the solar system, so he affirms the premise that there is life outside the solar system. 

The two related fallacies just discussed are forms of appeal to consequence. 

 

Sometimes, a person will frame an argument as a dilemma between two choices, ignoring any other possible choices. Consider the dialogue: 

Mike: Either the factory was blown up by terrorists or it was blown up by the government.  

Jennifer: But, how do you know the government did it?
Mike: The report about terrorists was shown to be bogus.

Here, Mike frames the argument as a false dilemma between only two choices, the terrorists or the government, without allowing for the possibility that someone else did it.
    
Sometimes, a person will justify a conclusion by a premise that itself depends on the conclusion being true. This is called circular reasoning. Consider the dialogue:

Mike: The universe is not a lonely place because there is life outside our solar system.
Jennifer: Why is there life outside our solar system?
Mike: Because if there weren’t life outside our solar system, the universe would be a lonely place, but the universe is not a lonely place.

Here, Mike uses the conclusion that the universe is not a lonely place to support the premise that there is life outside the solar system, while at the same time using the premise to support the conclusion.
    
Sometimes, in response to an argument, a person will attack a weaker argument that is a distortion or misrepresentation of the original argument. Consider the dialogue:

Mike: There is no conclusive proof that there is life outside our solar system. So we should concede that we just don’t know the answer. 
Jennifer: So you’re saying that because we don’t have conclusive proof that there is life outside our solar system, we need to accept that people never witness UFOs. However, we have a ton of evidence that people do witness UFOs.

Here, Jennifer distorts Mike’s argument as an argument for the claim that people never witness UFOs. She then tries to refute the claim that people never witness UFOs. The distorted argument is a weaker argument than Mike’s original argument, and Jennifer tries to attack the distorted argument. This fallacy is called the straw man fallacy. Mike’s original claim is that we don’t know whether there is life outside the solar system or not. So even if Jennifer successfully refutes the distorted argument, she hasn’t refuted Mike’s original argument. She attacked a straw man.

 

Expand: Post Hoc and Correlation Implies Causation 

Discover

This section will look at two additional logical fallacies in common language: the post hoc fallacy and the fallacy of correlation implies causation. Both of these fallacies involve the notion of causation.

Let’s look at a fallacy where causation is inferred from a sequential occurrence of two events. This is called the post hoc fallacy. For example, consider the argument:

Premise: Today Jill woke up late. Then Jill had a bad day.
Conclusion: For Jill, waking up late causes a bad day.

Here, Jill notices that her bad day happened after she woke up late. From this, she infers that her waking up late caused her to have a bad day. She is seeing a causal relationship that may not be there. For instance, her bad day may have been caused by someone insulting her that day and her waking up late had nothing to do with it.
    
It’s a fallacy to infer causation based on one instance of a sequential occurrence of two events. It’s also a fallacy to infer causation based on multiple instances of a sequential occurrence of two events. For example, consider:

Premise: Every time Davon goes on the freeway, there’s a traffic jam.
Conclusion: His going on the freeway causes traffic jams.

 

Here, there are multiple instances where a traffic jam occurs after Davon goes on the freeway. He then infers that he causes the traffic jams. However, there may be another reason for the traffic jam. For instance, maybe he goes on the freeway at a specific time each day when it’s rush hour.
    
More generally, there may be a correlation between two things, and a person infers causation based on the correlation. This fallacy is called correlation implies causation. For example, consider the argument:

Premise: Anyone who regularly cooks fried food gets lung cancer.
Conclusion: Regularly cooking fried food causes lung cancer.

Here, there may be a correlation between regularly cooking fried food and lung cancer. However, it ignores the possibility that there may be a hidden cause. For instance, regularly cooking fried food may require using a deep fryer, and cooking with a deep fryer regularly may lead to inhaling harmful air pollutants that, over time, cause lung cancer.

 

 

Module 1 Lesson 6 of 6