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Lesson 13 min 20 XP

Formal Fallacies

Errors in the structure of an argument — affirming the consequent, denying the antecedent, and undistributed middle — where the form itself is invalid.

When the Structure Itself Is Broken

Unlike informal fallacies (which involve content or relevance problems), formal fallacies are errors in the logical structure of an argument. The premises might be true and relevant, but the conclusion does not follow because the form is invalid.

Affirming the consequent has this form: 'If A then B. B is true. Therefore A is true.' Example: 'If it rains, the ground is wet. The ground is wet. Therefore, it rained.' But the ground could be wet from a sprinkler, a burst pipe, or morning dew. Knowing that the consequent (B) is true does not tell you the antecedent (A) is true, because multiple causes can produce the same effect.

This fallacy is rampant in political reasoning: 'If a country has strong institutions, it will be prosperous. Singapore is prosperous. Therefore, Singapore has strong institutions.' The conclusion might happen to be true, but the argument form is invalid — prosperity can have other causes.