Difference Between Formal and Informal Fallacy

Valid reasoning and argument involve a logical flow of ideas with no grammatical errors or ambiguous sentences. There are several instances of such mishaps in everyday language. Formal fallacy and informal fallacy are two common types of a reasoning error. While they may seem similar, formal fallacy and informal fallacy vary on several grounds. 

Formal Fallacy vs Informal Fallacy

The main difference between formal fallacy and informal fallacy is that formal fallacy deals with the logic of the technical structure. On the other hand, informal fallacy deals with the logic of the meaning of language. Here, the emphasis is not on the form of the argument but its meaning.  

Formal Fallacy vs Informal Fallacy

Formal Fallacy refers to a deductive argument that has an illogical structure. The pattern of reasoning for formal fallacies is always wrong. Thus, the formal fallacy is also known as the logical fallacy. The distinctive feature of formal fallacy is that the conclusion is not supported by the premises. An example is some girls have long hair and Meghan is a girl. Thus, Meghan has long hair. 

An informal fallacy is a reasoning error found in the content of an argument. The distinctive feature of informal fallacies is that they use irrelevant information in arguments or the information used to make an argument is based on assumptions that are later proven to be false. Informal fallacies are common occurrences in political speeches, internet forums, and newspaper articles. 

Comparison Table Between Formal Fallacy and Informal Fallacy 

Parameters of ComparisonFormal Fallacy Informal Fallacy
Definition A formal fallacy is a reasoning error that occurs due to defective logical structure. An informal fallacy is a reasoning error in the content of the argument. 
ErrorThe main error is in the logical structure. There is no error in the logical structure. 
Specification The deduction in formal fallacy is always wrong. Informal fallacy involves errors like irrelevant information and incorrect assumptions. 
Feature The structure of formal fallacy is invalid. The premises of informal fallacy are incorrect or irrelevant. 
Example All black bears are carnivores. Because nobody has ever proved to me there is God, there is no God. 

What is Formal Fallacy?

Formal fallacy refers to a pattern of reasoning rendered invalid by a flaw in its logical structure. A feature of formal fallacy is that it is a deductive argument that is invalid. Although the argument may have true premises, it may have false conclusions. There are several everyday examples of formal fallacy. For example, all-white bears are pandas is a formal fallacy. 

A formal fallacy is a fallacy where deduction goes wrong. Consequently, it is no longer a logical process. The form, arrangement or technical structure is generally faulty in a formal fallacy. The main question in such fallacies is not whether the presented information is true or false, but whether the argument is correct or incorrect, valid or invalid. 

Informal fallacy, the concluding argument may be objectively true. However, the argument may be formally invalid. Other names of a formal fallacy are a deductive fallacy and logical fallacy. A formal fallacy can also be present in a non-deductive argument. Consider, for instance, an inductive argument that incorrectly conveys principles of probability. Such situations are also included under a formal fallacy. 

There are several examples of formal fallacy. For instance, consider two sentences: all birds have beaks and that creature has a beak. Thus, that creature is a bird. Another example is that most Rimnars are Jornars, and most Jornars are Dimnars. Thus, most Rimnars are Dimnars.  Therefore, formal fallacies have several common occurrences.  

What is an Informal Fallacy?

Informal fallacy refers to an incorrect argument in natural language. The content and context of informal fallacies are usually false or based on assumptions. There are several different types of informal fallacies that have been identified over the years. Some common informal fallacies are the fallacy of equivocation, the fallacy of amphiboly, fallacies of composition and division. 

Relevance fallacies are a type of informal fallacies that use information irrelevant to the conclusion. On the other hand, ambiguity fallacies employ unclear terms and prepositions. Consequently, it becomes impossible to understand the idea behind sentences and arguments. In presumption fallacies, the conclusion depends on certain assumptions that are not clearly stated in the premises. 

Informal fallacy generally occurs in situations of unclear expression. The main focus of informal fallacy is on the meaning of the argument. There are several indicators of an informal fallacy. For instance, misconceptions due to underlying presumptions or just plain illogical sequences of thought are categorized as an informal fallacy. 

An example of informal fallacy is that logic teaches an individual how to argue, and people argue entirely too much. Thus, we don’t need to provide teaching on logic. Therefore, informal fallacies occur in everyday life. 

Main Differences Between Formal Fallacy and Informal Fallacy

  1. While formal fallacy always has a flaw in its logical structure, an informal fallacy consists of errors in its content. 
  2. The deductive argument of a formal fallacy is invalid. On the other hand, informal fallacy may consist of inappropriate information or ambiguous arguments. 
  3. Formal fallacies can be identified by examining the structure of the argument. In contrast, informal fallacy can be identified by analyzing the content of the premises. 
  4. The formal fallacy may contain errors in its arrangement or conclusive argument. In contrast, informal fallacy may involve the misuse of language or plain illogical consequences of thought. 
  5. An example of formal fallacy is that all racoons are omnivores. In contrast, an example of informal fallacy is that since feathers are light, and what is light can not be dark, feathers can not be dark.  


Therefore, formal fallacy and informal fallacy vary on several grounds. While the major flaw in a formal fallacy is the illogical sentence structure, the major flaw in informal fallacy is grammatical errors, ambiguous information, and misstatement of fact or opinion. 

Formal fallacy and informal fallacy are similar in the context that the presence of either of them can weaken the validity and soundness of any argument. Such fallacies may occur unintentionally or maybe intentionally written down to deceive people. Nonetheless, the differences between formal fallacy and informal fallacy outweigh the similarities.  


  1. https://www.triviumpursuit.com/articles/formal_informal_fallacies.php 
  2. https://en.wikipedia.org/wiki/Informal_fallacy 
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