Modus Tollens
 Definition: Let $P$ and $Q$ be statements. Then the rule of inference known as Modus Tollens is given by the argument $((P \rightarrow Q) \wedge \neg Q) \rightarrow \neg P$.
For example, let $P$ and $Q$ be the following statements:
• $P$: Bob has studied for the test.
• $Q$: Bob will pass the test.