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)