Modus Ponens
LSAT glossary · Logical Reasoning
Definition
A valid inference form in conditional logic: from P → Q and P, conclude Q. Often called "affirming the sufficient": once the sufficient condition is established as true, the necessary condition must also be true. Example: from "If it rains, the ground is wet" and "It is raining," Modus Ponens validly concludes "The ground is wet." Modus Ponens is one of only two valid direct inferences from a conditional (the other is Modus Tollens); the inverse moves — affirming the necessary or denying the sufficient — are formal fallacies.
Modus Ponens in plain English
Modus Ponens is just the most natural piece of reasoning you already do every day, given a fancy Latin name. It says: if you know "if A, then B," and you find out A is actually true, then B must be true too. For example, "if it rains, the ground gets wet" plus "it's raining right now" lets you safely conclude the ground is wet. It's basically pulling the trigger — once the front part of an "if-then" happens, the back part is locked in, and this is one of the few moves in logic that's always 100 percent valid.
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