Precious metal was removed from them, leaving them still useful as an identifiable coin in the marketplace, for a certain number of units in trade, but which no longer had the quantity of metal supplied by the coin's minter. It was observed as early as Oresme, Copernicus and then in 1558 by Sir Thomas Gresham, that "bad" money drives out "good" in any marketplace; (Gresham's Law states "Where legal tender laws exist, bad money drives out good money"). A more precise definition follows that: "A currency that is artificially overvalued by law will drive out of circulation a currency that is artificially undervalued by that law."