A
fallacy is an
argument that uses poor
reasoning. An argument can be fallacious whether or not its conclusion is true. A fallacy can be either formal or informal. An error that stems from a poor logical form is sometimes called a
formal fallacy or simply an invalid argument. An
informal fallacy is an error in reasoning that does not originate in improper logical form. Arguments committing informal fallacies may be formally valid, but still fallacious.
Fallacies of
presumption fail to prove the conclusion by assuming the conclusion in the proof. Fallacies of
weak inference fail to prove the conclusion with insufficient evidence. Fallacies of
distraction fail to prove the conclusion with irrelevant evidence, like emotion. Fallacies of
ambiguity fail to prove the conclusion due to vagueness in words, phrases, or grammar.
Some fallacies are committed intentionally (to manipulate or persuade by deception), others unintentionally due to carelessness or ignorance.