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.