What is the Difference Between Formal and Informal Fallacy

The main difference between formal and informal fallacy is that the error in formal fallacies lies in their logical structure, whereas the error in informal fallacies lies in their content.

In short, formal fallacy and informal fallacy are two types of reasoning errors that occur in arguments. In fact, they can weaken the validity and soundness of any argument. Moreover, such fallacies can occur accidentally or can be used deliberately to manipulate or deceive people.

Key Areas Covered

1. What is Formal Fallacy
     – Definition, Features 
2. What is Informal Fallacy
     – Definition, Features 
3. Similarities – Formal and Informal Fallacy
     – Outline of Common Features
4. Difference Between Formal and Informal Fallacy
     – Comparison of Key Differences

Key Terms

Formal Fallacy, Informal Fallacy, Ambiguity Fallacies, Fallacies of Presumption, Relevance Fallacies,

What is Formal Fallacy

Formal fallacy is a deductive argument that is invalid. It’s a pattern of reasoning that is always wrong. Formal fallacy always has a flaw in its logical structure. Therefore, we also call it as logical fallacy. In formal fallacy, the conclusion is not supported by the premises. The deduction is wrong, so there is no logical argument.

For example,

Premise: Some girls have long hair.

Premise:  Anne is a girl.

Conclusion: Anne has long hair.

In the above example, the argument is true but invalid. Let’s now look at an argument that is false and invalid:

Premise: Dogs are herbivores.

Premise:  Shadow is a dog.

Conclusion: Shadow is a herbivore.  

What is Informal Fallacy

An informal fallacy is an error in reasoning found in the content of the argument. Unlike in formal fallacy, the error in informal fallacy is not in the form or logic of the argument. Informal fallacies often involve using irrelevant information in arguments or information based on assumptions that later prove to be false. They are widespread in everyday conversations and can take many forms. We can observe informal fallacies in political speeches, advertisements, newspaper articles, internet forums, and social networking sites. They are often dependent on misuse of language and misuse of evidence.

Furthermore, there are three general categories of informal fallacy as relevance fallacies, ambiguity fallacies, and fallacies of presumption. Relevance fallacies use information that is irrelevant to the conclusion. Ambiguity fallacies, on the other hand, use unclear terms and propositions so that it becomes impossible to understand the precise meaning of the argument. In fallacies of presumption, the conclusion depends on certain assumptions not clearly stated in the premises.

Similarities Between Formal and Informal Fallacy

• Formal fallacy and informal fallacy are two types of reasoning errors that occur in arguments.
• Moreover, they can weaken the validity and soundness of any argument.
• These fallacies can occur accidentally or can be used deliberately to manipulate or deceive people.
• In addition, they can damage the credibility and reputation of the person making the argument.

Difference Between Formal and Informal Fallacy

Definition

A formal fallacy is an error in reasoning due to its defective logical structure, while an informal fallacy is an error in reasoning found in the content of the argument, not in its form or logic.

Logic

The error in formal fallacies lies in their logical structure, but informal fallacies may be logically correct.

Type of Error

A formal fallacy always has a flaw in its logical structure, and its deduction is wrong, whereas an informal fallacy has errors because it may involve irrelevant information, ambiguous arguments, incorrect assumptions, etc.

Conclusion

In brief, formal fallacy and informal fallacy are two types of reasoning errors that occur in arguments. The main difference between formal and informal fallacy is that the error in formal fallacies lies in their logical structure, whereas the error in informal fallacies lies in their content. It’s always to avoid such fallacies in your arguments since they can damage your credibility and deceive your audience.  

Reference:

1. “What Is Formal Fallacy?” Cliffs Notes.
2. “Formal and Informal Fallacies.” Lumen: English 112: Exposition and Persuasion.
3. Rodrigues, Cassiano Terra. “Informal Fallacies.” Introduction to Philosophy Logic, Rebus Community, 18 Nov. 2020.

Image Courtesy:

1. “Logical Fallacies Fallacy Icon” By SkepticalScience – Own work (CC BY-SA 4.0) via Commons Wikimedia