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In general, the state produces warfare, and monopolies; it is society which produces prosperity.
The problem with fiat money is that some party will benefit from tightening the money supply and other parties will benefit from loosening it. Administrators will generally pay more attention to people from the same social circles (CEOs, politicians, etc.) than to others, so even completely honest administrators will skew their policies to the advantage of the rich, and dishonest ones may manipulate supplies to benefit their associates.
The problem with the gold standard, or with bimetallism for that matter, is that people can't extract more gold as fast as they can produce other commodities. The supply of gold is more-or-less stable, while the supply of other commodities is usually increasing (oil is an exception), so the buying power of gold will usually increase just for sitting on it.