modus tollens

n. In logic, the idea that if the second part of a conditional statement is false, then the first part is also false. Thus if the statement  "If X then Y" is true and if Y is not true, then we know that X is not true. And so if the statement "Burning things are hot" is true and we know that a thing is not hot, we also know that the thing is not burning. Also called affirming the antecedent and denying the consequent.