1.4 Fallacies – The Basics

A fallacy is a defect in an argument that involves mistaken reasoning; sometimes fallacies are committed purposefully, to influence or mislead the reader or listener.

  • formal fallacy is one that can be detected by examining the form of an argument.
  • An informal fallacy is one that can only be detected by examining the content of the argument.

1.4.1 Formal Fallacies

The scope of this course does not encompass details of the many argument structures, or forms. Correspondingly, there will not be extensive consideration of formal fallacies, those committed when form is defective. We will, none-the-less, look briefly at two examples of formal fallacies, each of which can result from invalid (defective!) use of an argument form that we visited briefly in our examination of the deductive argument types modus ponens and modus tollens:

Affirming the Consequent

This fallacy might be seen as a flawed (invalid!) attempt to use the modus ponensargument form. Recall that one of the premises in modus ponens affirms the antecedent of the hypothetical premise. In effect, with modus ponens, the antecedent necessitates the consequent. In the fallacious example below, however, the consequent is affirmed instead of the antecedent:

Premise 1: If I’m cleaning the kitchen, then I’m not reading my book.
Premise 2: I’m not reading my book.
Conclusion: Thus, I’m cleaning the kitchen.

This reasoning is defective; think about it. The consequent cannot necessitate the antecedent. Not being engaged in reading the book does not, by necessity, infer that I am cleaning the kitchen. (Maybe I’m sleeping or out for a run.)

Denying the Antecedent

This fallacy can be seen as a defective (invalid!) use of the modus tollens argument form. Recall that one of the premises in modus tollens denies the consequent of the hypothetical premise. In the fallacious example below, however, the antecedent, is denied instead of the consequent:

Premise 1: If I’m cleaning the kitchen, then I’m not reading my book.
Premise 2: I’m not cleaning the kitchen. (The denial of “cleaning” is “not cleaning.”)
Conclusion: Thus, I am reading my book. (The denial of “not reading” is “reading.”)

This too is defective reasoning, if you think about it. Not being engaged in kitchen cleaning does not by necessity, infer that I am reading. I could be doing anything besides cleaning the kitchen.

1.4.2 Informal Fallacies

An informal fallacy is one that can be detected by examining the content of the argument rather than the form. While informal fallacies can sometimes be attributed to hasty or negligent reasoning, more often they are committed with the clear intent to mislead the listener or audience, to justify belief in a claim that is not true. Further, these fallacies may arise in an atmosphere charged with emotion.

Informal fallacies are attributed not just to arguments with actual premise-conclusion form, but also to wider use of language that is intended to establish a claim or make a point.

There are many accounts (lists, enumerations) of informal fallacies, not only in logic texts but in materials from other disciplines concerned with communication. Lists of fallacies sometimes use different descriptive names for the same basic fallacy. For example, “Do you still beat your wife?” might be referred to as a “complex question,” “compound, question” or “loaded question” fallacy, depending upon where you read about it. Whatever it is called, its intent to mislead through implicitly inserting information that is not overtly stated.

When reviewing the following material on informal fallacies, watch for some that correspond to defective use of argument types (inductive ones especially) that we considered here in the section on “Argument Types.”


Read this presentation on common informal fallacies[CC-BY-NC-ND]

Supplemental resources (bottom of the page) provide further insight on and additional examples of informal fallacies.


Apply your knowledge of common informal fallacies committed in arguments and in wider use of language. Consider what you read and/or hear on news media, social platforms, or wherever you spend time paying attentions to what others are saying and writing. Look/listen for arguments or language that make claims that seem misleading.

In your Discussion post: (for at least one defective argument or claim:) (1) describe the argument or claim; (2) state where you observed it, and (3) identify the fallacy that characterizes the misguided reasoning.

Note: Post your response in the appropriate Discussion topic.

Complete the Unit Test by the date on the Schedule of Work.

Supplemental Resources

Informal Fallacies – further examples:

inFact: Logical Fallacies 1
inFact: Logical Fallacies 2
inFact: Logical Fallacies 3