" Two statements are logically equivalent if it is impossible for one to be true and the other false. They are either both true or both false. One of the most important logical equivalences is called contraposition. It tells us that any statement of the form “If P, then Q” is logically equivalent to “If not-Q, then not-P.” The example in the text of statements A and B is an example of contraposition. "

― William Lane Craig , On Guard: Defending Your Faith with Reason and Precision