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The tertiary-source fallacy (TSF), dictionary fallacy, encyclopedia fallacy, or style-guide fallacy is the idea that tertiary sources—such as encyclopedias, dictionaries, and sometimes review articles—are more reliable than primary and secondary sources, and therefore that they trump other arguments and evidence. Often this is paired with an argument that a given tertiary source provides the only correct version or interpretation of the facts. However, tertiary sources are of highly variable quality, and are not uniformly "better" or "more reliable" than primary or secondary sources. There are important reasons for this.
It is not fallacious to cite a tertiary source in an article or offer it as evidence in a discussion. But advancing its view as if it ended the discussion, as if other facts and reason cannot surmount this source, is fallacious. Specifically, it is a form of the argument to authority fallacy.