modus tollens
From "if P, then Q" and not Q, conclude not P.
Modus tollens ("the way of denying") is the valid form: if P, then Q; not Q; therefore not P. The second premise denies the consequent, and the conclusion denies the antecedent.
Examples
"If the package was delivered, it's on the porch. It's not on the porch. So it wasn't delivered."