Modus Tollendo Ponens/Explanation

Proof Rule
The Modus Tollendo Ponens can be expressed in natural language as:

If either of two statements is true, and one of them is not to be true, it follows that the other one is true.


 * Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.


 * -- Sherlock Holmes (Sir Arthur Conan Doyle, 1890: The Sign of the Four, ch. 6)