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Modus Tollens is a rule of inference in propositional logic that states that if we have a Material Condtional that has a false consequent, then we may infer the negation of the antecedent. In other words, if P implies Q is true and Q is false, then we may infer that P is false. In rule form, this is often expressed as

$ \frac {P \rightarrow Q, \lnot Q} {\therefore \lnot P} $