Affirming the Consequent

From “if P then Q” and Q, wrongly concluding that P must be true.

formal

Affirming the consequent is an invalid argument form: from “if P then Q” and the fact that Q is true, it concludes that P is true. But Q may hold for other reasons — the conditional runs only one way, so confirming the consequent does nothing to establish the antecedent.

Examples

“If it's raining, the street is wet. The street is wet, so it must be raining.” — a street cleaner could have wet it.
“If she studied, she'd pass the test. She passed, so she must have studied.” — she might have passed without studying.

Why it works & how to counter

Persuasive because the two facts genuinely are connected. Counter it by listing other ways the consequent could come about — if Q has more than one possible cause, Q doesn't prove P.