(Latin for denial mode.) An inference rule that states that if is false and implies , then is also false. This is considered an "unsafe" approach compared to modus ponens because proving a negative is always weaker than proving a positive.
In logic, Modus tollens (Latin for "mode that denies") is the formal name for indirect proof or proof by contrapositive (contrapositive inference), often abbreviated to MT. 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).