Affirming the consequent, sometimes called converse error, is a formal fallacy, committed by reasoning in the form:

If P, then Q.
Q.
Therefore, P.

Arguments of this form are invalid, in that arguments of this form do not always give good reason to establish their conclusions, even if their premises are true.

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

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 Bill Gates owns Fort Knox, then he is rich.
Bill Gates is rich.
Therefore, Bill Gates owns Fort Knox.

Arguments of the same form can sometimes seem superficially convincing, as in the following example:

If I have the flu, then I have a sore throat.
I have a sore throat.
Therefore, I have the flu.

But many illnesses cause sore throat, such as the common cold or strep throat.

From Philosophy Word of the Day – Affirming the Consequent