What Is a Logical Fallacy?

Understanding Defective Arguments

Logic Sleuth
Logic Sleuth. H. Armstrong Roberts/ClassicStock/Archive Photos/Getty

Fallacies are defects in an argument - other than false premises - which cause an argument to be invalid, unsound or weak. Fallacies can be separated into two general groups: formal and informal. A formal fallacy is a defect which can be identified merely by looking at the logical structure of an argument rather than any specific statements. Informal fallacies are defects which can be identified only through an analysis of the actual content of the argument.


Formal Fallacies

Formal fallacies are only found only in deductive arguments with identifiable forms. One of the things which makes them appear reasonable is the fact that they look like and mimic valid logical arguments, but are in fact invalid. Here is an example:

  1. All humans are mammals. (premise)
  2. All cats are mammals. (premise)
  3. All humans are cats. (conclusion)

Both premises in this argument are true but the conclusion is false. The defect is a formal fallacy, and can be demonstrated by reducing the argument to its bare structure:

  1. All A are C
  2. All B are C
  3. All A are B

It does not matter what A, B, and C stand for — we could replace them with "wines," "milk" and "beverages." The argument would still be invalid and for the exact same reason. As you see, it can be helpful to reduce an argument to its structure and ignore content in order to see if it is valid.

Informal Fallacies

Informal fallacies are defects which can be identified only through an analysis of the actual content of the argument rather than through its structure.

Here is an example:

  1. Geological events produce rock. (premise)
  2. Rock is a type of music. (premise)
  3. Geological events produce music. (conclusion)

The premises in this argument are true, but clearly, the conclusion is false. Is the defect a formal fallacy or an informal fallacy? To see if this is actually a formal fallacy, we have to break it down to its basic structure:              

  1. A = B
  2. B = C
  3. A = C

This structure is valid; therefore the defect cannot be a formal fallacy and must instead be an informal fallacy identifiable from the content. When we examine the content we find that a key term, "rock," is being used with two different definitions (the technical term for this sort of fallacy is ).

Informal fallacies can work in several ways. Some distract the reader from what is really going on. Some, like in the above example, make use of or ambiguity to cause confusion. Some appeal to rather than logic and reason.

Categories of Fallacies

There are many ways to categorize fallacies. Aristotle was the first to try to systematically describe and categorize them, identifying thirteen fallacies divided into two groups. Since then, many more have been described and the categorization has become more complicated. The categorization used here should prove useful but it is not the only valid way of organizing fallacies.

Fallacies of Grammatical Analogy
Arguments with this defect have a structure which is grammatically close to arguments which are valid and make no fallacies. Because of this close similarity, a reader can be distracted into thinking that a bad argument is actually valid.

Fallacies of Ambiguity
With these fallacies, some sort of ambiguity is introduced either in the premises or in the conclusion itself. This way, an apparently false idea can be made to appear true so long as the reader does not notice the problematic definitions.


Fallacies of Relevance
These fallacies all make use of premises which are logically irrelevant to the final conclusion.


Fallacies of Presumption
Logical fallacies of presumption arise because the premises already assume what they are supposed to prove. This is invalid because there is no point in trying to prove something you already assume to be true and no one who needs to have something proven to them will accept a premise which already assumes the truth of that idea.


Fallacies of Weak Induction
With this type of fallacy, there may be an apparent logical connection between the premises and the conclusion but if that connection is real then it is too weak to support the conclusion.


Resources on Fallacies

A Concise Introduction to Logic, by Patrick J. Hurley. Published by Wadsworth.
This is one of the premier introductions to logic for students in college - but it is probably something that everyone should consider getting. It could be considered a manual of required reading before graduating to adulthood. It's easy to read and understand and it gives a very good explanation of the basics of arguments, fallacies, and logic.

Elements of Logic, by Stephen F. Barker. Published by McGraw-Hill.
This book isn't quite as comprehensive as Hurley's, but it still provides quite a lot of information at a level that should be understandable to most people.

Introduction to Logic and Critical Thinking, by Merrilee H. Salmon. Published by Harcourt Brace Jovanovich.
This book was designed for both college and high school level logic classes. It has less information than the above books.

With Good Reason: An Introduction to Informal Fallacies, by S. Morris Engel.Published by St. Martin's Press.
This is another good book dealing with logic and arguments and is particularly valuable because it focuses primarily upon informal fallacies.

The Power of Logical Thinking, by Marilyn vos Savant. Published by St. Martin's Press.
This book explains a lot about clear, logical thinking - but focuses more on statistics and how to use numbers properly. This is important because most people are as clueless about numbers as they are about basic logic.

The Encyclopedia of Philosophy, edited by Paul Edwards."
This 8 volume set, later reprinted in 4 volumes, is a fantastic reference for anyone wishing to learn more about philosophy. Unfortunately, it is out of print and not cheap, but worth it if you can find it used for under $100.

The Fallacy Files, by Gary N. Curtis.
Developed after many years of work, this site presents each fallacy with its own page of explanation, along with a couple of examples. He also updates the site with fallacies found in the news or recent books.​