Modus tollendo tollens ( Latin : the way that denies by denying ) is a valid , simple argument form in classical logic sometimes referred to as denying the consequent . It can also be referred to as denying the consequent , and is a valid form of argument , unlike similarly named but invalid arguments such as affirming the consequent or denying the antecedent . Modus tollens is sometimes confused with proof by contradiction or proof by contrapositive .