Relative Quotes - page 14
If there is some article of very generally recognised value which actually takes its place, as directly significant, on the scales of a great number of people, it may come to be generally accepted, without any special calculation or consideration, by people who are not thinking of any use they may have for it themselves, but are aware that it occupies a sufficiently high relative place on the scales of others to recoup them for what they give in exchange for it. As soon as this custom begins to be well established it will automatically extend and confirm itself, and the commodity in question will become a "currency" or "medium of exchange," the special characteristic of a medium of exchange being that it is accepted by a man who does not want it, or does not want it as much as what he gives for it, in order that he may exchange it for something he wants more.
Philip Wicksteed