Denying the Antecedent
LSAT glossary · Logical Reasoning
Definition
A formal fallacy in conditional logic: from P → Q and ¬P, invalidly concluding ¬Q. Denying the sufficient condition tells you nothing about the necessary — Q could still be true through some other sufficient cause. Example: from "If it rains, the ground is wet" and "It is not raining," it is INVALID to conclude "The ground is dry" (the sprinklers could still be on). Along with Affirming the Consequent, this is one of the two formal fallacies that appear constantly on the LSAT and must be reflexively recognized.
Denying the Antecedent in plain English
Denying the Antecedent is the mirror-image trap to its cousin — also convincing, also wrong. It happens when you know "if A, then B," you learn A is false, and you wrongly conclude "so B is false too." The problem is that A was just ONE way to get B, not the only way. "If it rains, the ground gets wet" plus "it's not raining" does NOT prove the ground is dry — the sprinklers could still be soaking it. The lesson: knocking out one possible cause never rules out the result, because other causes might still be doing the job.
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