Modus Tollens
GMAT glossary · Verbal Reasoning
Definition
Valid inference: If A then B; not B; therefore not A.
Modus Tollens in plain English
Modus tollens is the mirror image of the previous rule, and it's the clever detective move. Start with "if it rains, the ground gets wet." Now suppose you look outside and the ground is dry, that's "not wet." You can conclude for certain: it did NOT rain. The trick is that if a cause always produces an effect, then the effect being missing proves the cause never happened. Sherlock Holmes basically lives on this: rule out what didn't happen by noticing what its consequences failed to show.
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