Deductive reasoning
Modus tollens

Modus tollens (also known as "the law of contrapositive") is a deductive rule of inference. It validates an argument that has as premises a conditional statement (formula) and the negation of the consequent (¬ Q {\displaystyle \lnot Q}) and as conclusion the negation of the antecedent (¬ P {\displaystyle \lnot P}). In contrast to modus ponens, reasoning with modus tollens goes in the opposite direction to that of the conditional. The general expression for modus tollens is the following:

The following is an example of an argument using modus tollens:
oulipo:avoid_letter[e], punctuation:no_comma, length_constraints:number_words[126]