The grounds, upon which the general power to coin money, and regulate the value of foreign and domestic coin, is granted to the national government, cannot require much illustration in order to vindicate it. The object of the power is to produce uniformity of value throughout the Union, and thus to preclude us from the embarrassments of a perpetually fluctuating and variable currency. Money is the universal medium or common standard, by a comparison with which the value of all merchandise may be ascertained, or, it is a sign, which represents the respective values of all commodities

Joseph Story, Commentaries on the Constitution 3:§§ 1112--17