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.")
As an argument[edit]
A modus tollens argument has the following form:
- P1: If X, then Y.
- P2: Not Y.
- C1: Therefore, not X.
For example:
- P1: If it is raining, the ground is wet.
- P2: The ground is not wet.
- C1: Therefore, it is not raining.
As a rule of inference[edit]
In propositional logic:
In first-order logic:
External links[edit]
- See the Wikipedia article on Modus tollens.