Modus tollens
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Modus tollens ("mode of taking") is a logical argument, or rule of inference. (Compare with modus ponens, or "mode of putting.")
[edit] As an argument
A modus tollens argument has the following form:
If P, then Q.
Not Q.
Therefore, not P.
For example:
If it is raining, the ground is wet.
The ground is not wet.
Therefore, it is not raining.
[edit] As a rule of inference
In propositional logic:
In first-order logic: