Formal Fallacy

Taxonomy: Logical Fallacy > Formal Fallacy

Exposition*:

A Formal Fallacy is a type of argument the logical form of which is not validating, that is, there are arguments of that form that are not valid. Formal Fallacy is the most general fallacy for fallacious arguments that are not formally valid, and a given argument will usually commit a more specific formal fallacy―see the Subfallacies, above. A given fallacious argument would be classified as a Formal Fallacy only if it could not be given a more specific classification, such as, undistributed middle, which is a syllogistic fallacy. For this reason, there is no Example of Formal Fallacy given; instead, for examples of formal fallacies, see those given under the Subfallacies.

In modern systems of formal logic there are usually an infinite number of argument forms that are not validating. For this reason, to count as a formal fallacy, a non-validating form of argument needs at least one of the following additional characteristics:

• It is deceptive and likely to be committed, usually by having a logical form that is similar enough to a validating form of argument to be confused with it. The fallacies of propositional logic are of this type.
• It is part of a system of rules such that any argument of a type which the rules can be applied to, and which breaks no rules, is thereby formally valid. In such a system, to commit a fallacy is to break a rule, so that every rule has a corresponding fallacy. Syllogistic fallacies are of this type.

Exposure:

The distinction between a Formal and an Informal Fallacy is that a formal fallacy is based solely on logical form, and an informal fallacy takes into account the non-logical content of the argument. This roughly parallels the distinction between deductive and non-deductive modes of reasoning. Typically, formal fallacies are committed by deductive arguments, whereas informal fallacies occur in arguments that could be at best inductively strong. However, there are exceptions to this pattern, for instance Begging the Question.

*Note: W. Kent Wilson, "Formal Fallacy", in Robert Audi (General Editor), The Cambridge Dictionary of Philosophy, 1995.