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.