Formal Fallacy

logic

Definition

Formal Fallacy

A formal fallacy is an error in the logical form of an argument. The conclusion does not follow from the premises, and this can be determined by examining the structure of the argument alone, without reference to its content.

Contrast with Informal Fallacies

A informal fallacy involves the content, context, or language of the argument. A formal fallacy involves only the logical structure. Two arguments with the same form are both valid or both fallacious, regardless of what they are about.

Detectability

Formal fallacies can be identified using the rules of deductive logic. No domain knowledge is required. If the logical form is invalid, the argument is fallacious even if every premise is true and the conclusion happens to be true.

Example

Affirming the Consequent

The argument form

$$P\to Q,Q⊢P$$

is invalid. Knowing that $Q$ holds does not establish that $P$ was the reason. This is a formal fallacy because the defect lies entirely in the structure.