The Wealth of Nations by Part 1 Chapter 5 Page 47

they get something more than this price. But when, under all those occasional fluctuations, the market price either of gold or silver bullion continues for several years together steadily and constantly, either more or less above, or more or less below the mint price, we may be assured that this steady and constant, either superiority or inferiority of price, is the effect of something in the state of the coin, which, at that time, renders a certain quantity of coin either of more value or of less value than the precise quantity of bullion which it ought to contain.

The constancy and steadiness of the effect supposes a proportionable constancy and steadiness in the cause.

The money of any particular country is, at any particular time and place, more or less an accurate measure of value