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.