Definition
A fallacy is a mistake in reasoning or a false belief — a claim that feels true, sounds authoritative, or is widely repeated, yet does not hold up when checked against evidence. In formal logic the word names a specific defect in an argument's structure. In everyday use, and in The Book of Common Fallacies, it is applied far more broadly to any belief that fails scrutiny, whether the error lies in the logic, the facts, or the source.
What unites these uses is the gap between confidence and correctness. A fallacy is not simply a wrong answer; it is a wrong answer that travels well, because it is convenient, memorable, or comfortable to believe.
Why it matters
How it works
A fallacy spreads when the cost of believing it is low and the appeal of believing it is high. The claim circulates through repetition: each retelling adds social proof without adding evidence. Because the belief is rarely tested, no one encounters the contradicting fact, and the error becomes self-sustaining.
Breaking the cycle means treating a claim's popularity as irrelevant to its truth, tracing it back to a primary source, and asking what evidence would be required to support it. When that evidence is missing, the claim is exposed as a fallacy regardless of how widely it is held.