Affirming the consequent
Affirming the consequent, sometimes called converse error, fallacy of the converse or confusion of necessity and sufficiency, is a formal fallacy of inferring the converse from the original statement. The corresponding argument has the general form:
- If P, then Q.
- 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:
One way to demonstrate the invalidity of this argument form is with a counterexample with true premises but an obviously false conclusion. For example:
- If someone owns Fort Knox, then he or she is rich.
- Bill Gates is rich.
- Therefore, Bill Gates owns Fort Knox.
Owning Fort Knox is not the only way to be rich. Any number of other ways exist to be rich.
However, one can affirm with certainty that "if someone is not rich" (non-Q), then "this person does not own Fort Knox" (non-P). This is the contrapositive of the first statement, and it must be true if and only if the original statement is true.
Arguments of the same form can sometimes seem superficially convincing, as in the following example:
- Confusion of the inverse
- Denying the antecedent
- ELIZA effect
- Fallacy of the single cause
- Fallacy of the undistributed middle
- Inference to the best explanation
- Modus ponens
- Modus tollens
- Post hoc ergo propter hoc
- Necessity and sufficiency
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