Suppose we are tradesmen. It matters little to any of us what commodities he takes in exchange for goods (other than commodities he himself can use). But if he takes what others refuse he is stuck with something useless, and if he refuses what others take he needlessly inconveniences his customers and himself. Each must choose what he will take according to his expectations about what he can spend -- that is, about what others will take: gold and silver if he can spend gold and silver, U.S. notes if he can spend U.S. notes, Canadian pennies if he can spend Canadian pennies, wampum if he can spend wampum, goats if he can spend goats, whatever may come along if he can spend whatever may come along, nothing if he can spend nothing. -- David Lewis, Convention , p. 7 Lewis is correct: the essence of money is a convention. Menger may or may not have been correct about how money came into being, but once it did so, the essence of somethings being money is, "I should accept this for my...