Affirming the Consequent

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An Affirming the Consequent is an argument pattern that ...



  • (Wikipedia, 2015) ⇒ Retrieved:2015-1-2.
    • Affirming the consequent, sometimes called converse error or fallacy of the converse, is a formal fallacy of inferring the converse from the original statement. The corresponding argument has the general form:
      1. If P, then Q.
      2. Q.
      3. Therefore, P.
    • An argument of this form is invalid, i.e., the conclusion can be false even when statements 1 and 2 are true. Since P was never asserted as the only sufficient condition for Q, other factors could account for Q (while P was false).

      To put it differently, if P implies Q, the only inference that can be made is non-Q implies non-P. (Non-P and non-Q designate the opposite propositions to P and Q.) This is known as logical contraposition. Symbolically:

      [math]\displaystyle{ (P \to Q)\leftrightarrow (\neg Q \to \neg P) }[/math]

      The name affirming the consequent derives from the premise Q, which affirms the "then" clause of the conditional premise.