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What is a Logical Fallacy, Exactly?

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The word "fallacy" comes from the Latin "fallacia" which means "deception, deceit, trick, artifice," however, a more specific meaning in logic (a logical fallacy) that dates back to the 1550s means "false syllogism, invalid argumentation."

An Error in Reasoning

One of the earliest academic discussions of logical fallacies comes from the book Elementary Lessons in Logic : Deductive and Inductive, published by MacMillian and Co. in 1872 where the modern definition of logical fallacies is used: "the modes in which, by neglecting the rules of logic, we often fall into erroneous reasoning." Today, this basic definition is still used, and often abbreviated to just "an error in reasoning." It is not a factual error.

The Difference Between Logical Fallacies and Cognitive Biases

In the early 1970s, two behavioral researchers, Daniel Kahneman and Amos Tversky pioneered the field of behavioral economics through their work with cognitive biases and heuristics, which like logical fallacies, deal with errors in reasoning. The main difference, however, is that logical fallacies require an argument whereas cognitive biases and heuristics (mental shortcuts) refer to our default pattern of thinking. Sometimes there is crossover. Logical fallacies can be the result of a cognitive bias, but having biases (which we all do) does not mean that we have to commit logical fallacies. Consider the bandwagon effect, a cognitive bias that demonstrates the tendency to believe things because many other people believe them. This cognitive bias can be found in the logical fallacy, appeal to popularity.

Everybody is doing X.

Therefore, X must be the right thing to do.

The cognitive bias is the main reason we commit this fallacy. However, if we just started working at a soup kitchen because all of our friends were working there, this wouldn't be a logical fallacy, although the bandwagon effect would be behind our behavior. The appeal to popularity is a fallacy because it applies to an argument.

I would say that more often than not, cognitive biases do not lead to logical fallacies. This is because cognitive biases are largely unconscious processes that bypass reason, and the mere exercise of consciously evaluating an argument often causes us to counteract the bias.

Factual Errors are Not Logical Fallacies

To illustrate this point, let's consider the availability heuristic, a cognitive bias that describes the tendency for one to overestimate the likelihood of more salient events, usually the result of how recent the memories are or how unusual or emotionally charged they may be. This bias can be demonstrated in believing that you are more likely to die in a plane crash than an automobile accident because of all the plane crashes you see in the news. As a result of this bias, one might argue:

Plane crashes kill more people than automobile accidents. Therefore, it is safer to drive in a car than fly in a plane.

This is not fallacious; it's factually incorrect. If it were true that plane crashes kill more people than automobile accidents, the conclusion would be reasonable. The argument itself does not contain flawed reasoning; it contains incorrect information. While we can say the reasoning behind the argument was fallacious, there is no logical fallacy present in the argument. Similarly, if I told you that the sun was about 30 miles from the earth and the size of a football stadium, I would not be committing a fallacy—but I would be a moron. Factual errors are not fallacies.

Logical Fallacies Can Be Committed by the Arguer or Audience

In this book, I will be using the term "fallacious" in the following ways, all of which support the primary purpose of this book—to promote better reasoning.

Fallacious Arguments. Arguments that are fallacious contain one or more non-factual errors in their form.  

Just as a woman has the right to get a tattoo, she has the right to get an abortion. (Weak analogy)

Fallacious Reasoning. When an individual is using erroneous thinking (including bypassing reason) in evaluating or creating an argument, claim, proposition, or belief. This is where cognitive biases frequently play a role.

I was pro-abortion before, but now that this speaker made me cry by showing me a photo of an aborted fetus, I am against abortion. (Appeal to emotion)

Fallacious Tactics. Deliberately trying to get your opponent or audience to use fallacious reasoning in accepting the truth claims of your argument.

Look at this photo of an aborted fetus. How can you tell me that you still are pro-choice? (Appeal to emotion)

Note that fallacious tactics are not a deficiency in reasoning (morality, perhaps) on the part of the arguer, although people who fall victim to these tactics do demonstrate fallacious reasoning. These tactics are still labeled logical fallacies, but the arguer would not be held responsible for committing a logical fallacy. When charities run ads, they don't bombard us with data and moral arguments; they show us a photo of a suffering child who needs our help. These charities know what they are doing. They are not lacking in reason; quite the opposite, in fact. They are using effective persuasion techniques.

Logical Fallacies are Deceptive

Another characteristic of logical fallacies is that they are not always easy to spot, especially to the untrained mind. Yet they often elude our critical faculties, making them persuasive for all the wrong reasons—sort of like optical illusions for the mind. Some, however, are as clearly wrong as a pig roast at a bar mitzvah (yet still fool too many people).  For example,

“Don’t grow a mustache, because Hitler had a mustache. Therefore, you will be like Hitler!”

After reading this book, you can probably match about a dozen fallacies with the above argument. The error in reasoning should be apparent—sharing a physical characteristic with a fascist dictator will not make you a fascist dictator.

Logical Fallacies are Common and Worthy of Identifying by Name

Over the years, I have received questions from perhaps hundreds of students of logical fallacies who have presented what met all the other criteria of logical fallacies but was unique, very specific, or already fit nicely under a more general category of logical fallacy. For example, the appeal to emotion fallacy is a general category of fallacies, and there are many in that category such as appeal to anger, appeal to pity, appeal to fear, and many more. These are all common enough to be worthy of their own fallacy. But what about "appeal to indignation"? This certainly could be fallacious, but so rare that it's just not worth naming since it fits under the appeal to emotion fallacy. If there is a general category under which the rare fallacy fits, it is less likely to be named.

Dr. Bo's Three Criteria for a Logical Fallacy

In this book, we are using what is referred to as the argument conception of fallacies (Hanson, 2015). That is, what we are identifying as a "logical fallacy" goes beyond the standard conception of "fallacy" where the error in reasoning must apply to argumentation. More specifically,

  1. It must be an error in reasoning not a factual error.
  2. It must be commonly applied to an argument either in the form of the argument or in the interpretation of the argument.
  3. It must be deceptive in that it often fools the average adult.

Therefore, we will define a logical fallacy as a concept within argumentation that commonly leads to an error in reasoning due to the deceptive nature of its presentation. Logical fallacies can comprise fallacious arguments that contain one or more non-factual errors in their form or deceptive arguments that often lead to fallacious reasoning in their evaluation.

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