Fallacy 1 of 3

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Affirming the Consequent

F001 • Formal - Propositional Logic

Also known as: Converse Error, Fallacy of the Converse

Difficulty 1/10Low LoadCommon

Definition

Inferring the truth of the antecedent from the truth of the consequent in a conditional statement.

Why Invalid

The consequent can be true for reasons other than the antecedent being true. The conditional only guarantees Q when P is true, not vice versa.

Examples

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Scope of Applicability

  1. Identify conditional statement (if P then Q)
  2. Check if consequent (Q) is affirmed
  3. Check if conclusion affirms antecedent (P)
  4. Verify no additional information makes inference valid

Common Mistakes

  • Confusing with valid modus ponens (If P then Q; P; therefore Q)
  • Not recognizing when additional context makes it reasonable

Related Concepts

Denying the AntecedentCommutation of Conditionals

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F002 Denying the Antecedent
Return to 1.1 If-Then Reasoning