The Nature of Logical Fallacies
This book covers a total of 60 formal and informal logical fallacies. But what distinguishes a formal from an informal logical fallacy? Basically, formal fallacies are errors in reasoning where the form of the argument does not always guarantee a true conclusion, while informal fallacies are reasoning errors in an argument’s content or premises. Let’s unpack what these terms mean.
A formal fallacy is not an error of reasoning made by someone wearing a tux and black tie. Rather, it’s an error of reasoning—intentional or unintentional—attributable to the “form” of a deductive argument, independent of its content. (Recall that a deductive argument is a series of premises and a conclusion where, if the premises are assumed to be true, the truth of the conclusion is guaranteed.) Consider the following:
Argument form: This form of argument is called Modus Ponens (Latin for “mode that affirms”) |
Example |
• If P (is true), then Q (is true). |
• If a customer purchases six bottles of our wine (P), then she is eligible to receive a 10 percent discount on the purchase price (Q). |
• P (is true). |
• The customer purchased six bottles of our wine (P is true). |
• Therefore, Q (is true). |
• Therefore, the customer is eligible to receive a 10 percent discount on the purchase price. |
An argument’s form refers to the argument as a whole—its “structure”—and examines how the argument’s structure affects the argument’s validity, independent of its content. In the previous example, the argument’s form is called modus ponens and its structure is defined in the left column. The argument’s content is given in the right-hand column. In this example, assuming true premises, the argument’s form guarantees the truth of the conclusion.
In a formal fallacy, the argument’s structure does not always guarantee that the argument’s conclusion is true. Affirming the Consequent is an invalid argument form and all arguments in this form are invalid because even if all the premises are true, the conclusion is not guaranteed to be true. Consider the following example:
Argument form: This form of argument is called Affirming the Consequent |
Example |
• If P (is true), then Q (is true). |
• If our advertising works, then sales will increase. |
• Q (is true). |
• Sales increased. |
• Then, P (is true). |
• Therefore, our advertising worked. |
Regardless of the content of this argument, the conclusion is not logically guaranteed to be true. Other factors could have caused sales to increase outside of the advertising. For example, competitors could have raised their prices or had distribution problems in getting their product on retailers’ shelves. Thus, sales might have increased, but not necessarily because of the firm’s advertising.
In an informal fallacy, it is not the argument’s form that is in error; rather, we are concerned about the content of an argument and whether the argument’s content gives your audience good reasons to believe the argument. Thus, an informal fallacy is a statement used as a premise that is irrelevant, ambiguous, vague, or offers a poor or bad reason to accept an argument’s conclusion.
Most of the logical fallacies in this book are informal fallacies because they reflect poor reasoning based on the content of an argument. For example, consider the informal fallacy Ad hominem (attacking the person), which we discussed in Chapter 1. It is an informal fallacy because attacking the source or the person making a claim by itself is not a reason to accept an argument’s conclusion. If you want to diminish the credibility of a source or person in an argument, you must provide evidence that that source or person should be discredited—for example, the source or person has a conflict of interest.
In summary, logical fallacies are errors in reasoning, which are characterized as being either formal or informal in nature. Formal fallacies are errors in the form or structure of an argument, irrespective of the argument’s content, such that the argument’s conclusion is not guaranteed to be true. An informal fallacy refers to an argument’s content, irrespective of the argument’s form, which gives one’s audience a poor or irrelevant reason to believe the argument’s conclusion.
Eschewing formal or informal logical fallacies will give your audience good reasons to believe what you have to say. And that’s no fallacy!
Chapter Takeaways
Some arguments assume a given “form.” An argument’s form refers to a specific arrangement and relationship of the argument’s premises and conclusion. This chapter introduced you to two argument forms: Modus Ponens and Affirming the Consequent.
Some argument forms, such as Modus Ponens, guarantee the truth of the argument’s conclusion, assuming true premises. These are called valid deductive arguments.
Some argument forms, such as Affirming the Consequent, do not, and are called formal logical fallacies.
The problem with an informal logical fallacy is not its form, but rather its content. An informal fallacy is a statement used as a premise that is irrelevant, ambiguous, vague, or offers a poor or bad reason to accept an argument’s conclusion. In marketing, most of the fallacies you encounter will be informal versus formal fallacies.
The next chapter will introduce you to several more formal logical fallacies that you are well advised to avoid when making marketing arguments.