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What Is a Formal Fallacy?

Angela Farrer
Angela Farrer

A formal fallacy is a specific type of error in a deductive argument, and it is identifiable by examination of the argument's overall structure. These fallacious arguments are different from an informal fallacy that often needs closer analysis of the subject details. The average formal fallacy may appear to be a valid logical argument due to the truth in at least one of the beginning premises, but the defect is often found in the argument's conclusion. This kind of conclusion is considered invalid when it does not follow correct and established rhetorical patterns.

In order for a fallacy to be considered formal, it needs to occur within a deductive rather than an inductive argument. The contributing statements that make up this kind of argument are known as premises in the study of rhetoric. A deductive argument begins with a series of premises that lead to one conclusion while an inductive one separates different premises from a beginning conclusion. The premises of a deductive argument need to contain truth before a valid conclusion can be drawn from them. Some of the most common cases of formal fallacies result from mistakes in the logic of at least one premise.

Woman standing behind a stack of books
Woman standing behind a stack of books

Although the conclusion of a formal fallacy can sometimes appear to be true and reasonable, a technical flaw in one of the preceding statements can sometimes lead to a conclusion that does not make logical sense. A discovered formal fallacy is generally given four different designations: true or false and valid or invalid. This type of variation can result in argumentation that is valid but still untrue. A small change in the wording of a false but valid argument can sometimes render it true and valid. The altered argument then becomes a formally valid statement rather than a formal fallacy.

When this type of mistake in reasoning is done on purpose, the resulting fallacy is also sometimes called a sophism. Some of these purposeful fallacious arguments are made to deceive listeners and to lead their reasoning in the wrong direction. This use of a formal fallacy is often considered a breach of the ethics that are also an important part of truthful and concrete logic. Other formal fallacies can be made by accident as a result of incorrect or careless thought processes due to limited critical reasoning abilities. This instance of an unintended fallacy can usually be corrected when the original speaker's attention is called to it.

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