a tertium quid, or universal equivalent. And in order to have a really universal equivalent it is necessary that use-value should be eliminated from it, since such an equivalent is required to express not the diverse qualities of all the various commodities, but the relative quantity of embodied human labour which they severally contain.
Money as a mere measure of value is imaginary and ideal, but the bodily form of it must express quantitatively equivalent abstract value—i.e. labour—and takes the form of the precious metals, finally of gold.
Gold has come to be the bodily form taken by the universal measure of value, partly because of its natural qualities—portability, durability, etc., but chiefly because the course of history has invested it with this function; and also because its value, instead of changing from, say, week to week, as is the case with other commodities, changes rather from century 249 to century, so that it may be considered stable relatively to them, just as one speaks of indigo as a permanent dye, which it is relatively to other dyes, although none are absolutely permanent.194