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:

\left\{P\rightarrow Q,\neg Q\right\} \models \neg P

In first-order logic:

\models_{\mathfrak{A}}\forall x.\left(P(x)\rightarrow Q(x)\right) \wedge \models_{\mathfrak{A}}\exists x.\left(\neg Q(x)\right) \implies \models_{\mathfrak{A}}\exists x.\left(\neg P(x)\right)
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