The myth runs as follows: the god who fashioned human beings mixed different metals into their souls. Those fitted to rule were made with gold; those fitted for military service, with silver; farmers and craftsmen, with bronze and iron. These are the natural differences that explain why people belong to different social roles — and why the arrangement is ordained rather than merely imposed.
Any functioning city requires a shared account of why its authority is legitimate. Pure reason cannot provide this on its own — citizens need a story they can feel as well as understand. The noble lie provides a foundation for civic loyalty and acceptance of hierarchy that naked force cannot supply. Plato is not arguing for cynical propaganda; the rulers are to believe the myth too.
The passage has haunted Plato's reputation. Karl Popper treated it as the seed of totalitarian propaganda — the philosopher-elite lying to the masses for their own good. Others read it as irony, a Socratic provocation designed to make us uncomfortable with any founding myth, including our own. The question it raises is perennial: can any political community survive on transparent reason alone, or does it always need a sustaining narrative to hold it together?
The "noble lie" (gennaion pseudos) appears in Book III of The Republic. The word gennaion means noble or well-born — Plato implies the lie is noble in purpose even if not in method.