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The following is an example of an argument using modus ponens:
# If it is raining, then there are clouds in the sky.
# It is raining.
# Thus, there are clouds in the sky.

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 () and as conclusion the negation of the antecedent (). In contrast to modus ponens,

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