Affirming the Consequent
LSAT glossary · Logical Reasoning
Definition
A formal fallacy in conditional logic: from P → Q and Q, invalidly concluding P. Affirming the necessary condition tells you nothing about the sufficient — Q could be true for many reasons besides P. Example: from "If it rains, the ground is wet" and "The ground is wet," it is INVALID to conclude "It is raining" (the sprinklers could be on). This fallacy is one of the LSAT's most frequently tested errors, appearing both as a flaw to identify in Logical Reasoning and as an invalid inference to avoid in Logical Reasoning.
Affirming the Consequent in plain English
Affirming the Consequent is a tempting logic TRAP — it feels right but is actually broken. It happens when you know "if A, then B," you see that B is true, and you wrongly leap to "so A must be true." The flaw is that B can be caused by lots of things, not just A. "If it rains, the ground gets wet" plus "the ground is wet" does NOT prove it rained — someone could have left the sprinklers on. The LSAT loves testing whether you can catch this: a guaranteed result showing up never proves which specific cause produced it.
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