David Ricardo on money

Strictly speaking, there can be no permanent measure of value. A measure of value should itself be invariable; but this is not the case with either gold or silver, they being subject to fluctuations as well as other commodities. Experience has indeed taught us, that though the variations in the value of gold or silver may be considerable, on a comparison of distant periods, yet for short spaces of time their value is tolerably fixed. It is this property, among their other excellencies, which fits them better than any other commodity for the uses of money. Either gold or silver may therefore, in the point of view in which we are considering them, be called a measure of value.

-- David Ricardo, The High Price of Bullion: A Proof of the Depreciation of Bank Notes, 1810, pg. 13 (footnote)