32 Repository of Logical Fallacies
Kirsten DeVries; Core Curriculum; and Christina Frasier
Learning Objectives
- Recognize distinctions between types of fallacies and how they are presented in discourse.
- Identify manipulative approaches to argumentation.
- Understand the composition of logical reasoning through syllogism.
Formal and Informal Fallacies
This section of the handbook introduces the concept of formal and informal fallacies while reviewing ethos, logos, and pathos. Whereas the chapter above grouped examples of fallacies by the their errors in terms of logos, pathos, and ethos, this chapter groups fallacies around the following characteristics:
- Fallacies of irrelevance
- Fallacies of presumption
- Fallacies of ambiguity
- Fallacies of inconsistency
But before we jump into descriptions of the fallacies and examples, we first define the differences between formal and informal fallacies.
What are fallacies?
Fallacies are errors or tricks of reasoning. We call a fallacy an error of reasoning if it occurs accidentally; we call it a trick of reasoning if a speaker or writer uses it in order to deceive or manipulate his audience. Fallacies can be either formal or informal.
Whether a fallacy is an error or a trick, whether it is formal or informal, its use undercuts the validity and soundness of any argument. At the same time, fallacious reasoning can damage the credibility of the speaker/writer and improperly manipulate the emotions of the audience/reader.
What is a formal fallacy?
Most formal fallacies are errors of logic: the conclusion doesn’t really “follow from” (is not supported by) the premises. Either the premises are untrue or the argument is invalid. Below is an example of an invalid deductive argument.
Example
Premise: All black bears are omnivores.
Premise: All raccoons are omnivores.
Conclusion: All raccoons are black bears.
Raccoons also are a subset of omnivores. But these two subsets do not overlap, and that fact makes the conclusion illogical. The argument is invalid—that is, the relationship between the premises doesn’t support the conclusion.
Why is it important to recognize formal fallacies?
“Raccoons are black bears” is instantaneously recognizable as fallacious and may seem too silly to be worth bothering about. However, that and other forms of poor logic play out on a daily basis, and they have real world consequences. Below is an example of a fallacious argument:
Example
Premise: All Iranians are Muslims.
Conclusion: All Iranians are Arabs.
This argument fails on two levels. First, the premises are untrue because although many Arabs and Iranians are Muslim, not all are. Second, the two ethnic groups are sets that do not overlap; nevertheless, the two groups are confounded because they (largely) share one quality in common. One only has to look at comments on the web to realize that the confusion is widespread and that it influences attitudes and opinions about U.S. foreign policy.
What is an informal fallacy?
Infor