Affirming the consequent

Formal fallacies → Propositional

Affirming the consequent is when you infer that a cause must have happened because its usual effect happened—even though the same effect can have other causes. In short: "If A then B; B is true; therefore A is true" is invalid. It feels convincing because we often reason correctly from cause to effect, but here we are reasoning backward without enough information. The conclusion doesn't follow, because B might have been brought about by something else.

Examples

  • If it rains, the street gets wet. The street is wet. So it must have rained.

  • If the battery is flat, the car won't start. The car won't start. So the battery must be flat.

  • If the policy were harmful, unemployment would rise. Unemployment has risen. So the policy must be harmful.

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