The True Purpose of Money
MONEY is a uniform standard of exchange that has value only insofar as it is accepted by enough people such that it can effectively serve its purpose--to facilitate exchange of people's labor and the fruits of people's labor.
How many people is enough? A self sustaining economy has enough people. In other words, a community (however small or large) has enough people if collectively it produces everything its individual constituents need to survive and thrive.