Modus Tollens
LSAT glossary · Logical Reasoning
Definition
A valid inference form in conditional logic: from P → Q and ¬Q, conclude ¬P. Often called "denying the necessary," this is the contrapositive in action: if the necessary condition fails, the sufficient condition could not have held. Example: from "If it rains, the ground is wet" and "The ground is not wet," Modus Tollens validly concludes "It is not raining." Modus Tollens has the same logical force as the original conditional and is essential for working contrapositives on the LSAT.
Modus Tollens in plain English
Modus Tollens is the clever "run it backwards" move: if you know "if A, then B," and you discover B did NOT happen, then A couldn't have happened either. Using "if it rains, the ground gets wet" — if you walk outside and the ground is bone dry, you can be certain it did NOT rain, because rain would have forced wet ground. The key insight is that the guaranteed result is missing, so the thing that guarantees it must be missing too. It's like an alarm that ALWAYS rings when there's a fire: no ringing means no fire, period.
Learn these first
Sign up free: all 104 LSAT terms as flashcards, plus practice questions with full explanations →
Related LSAT terms
LSAT practice questions · LSAT flashcards · LSAT study guide