“[Aristotle] shows how currency serves as a measure...[I]f men always needed immediately the goods they have among themselves, they would have no need of any exchange except of thing for thing, e.g., wine for grain. But sometimes one man (who has a surplus of wine at present) does not need the grain that another man has (who is in need of wine), but perhaps later he will need the grain or some other product. In this way then for the necessity of future exchange, money or currency is, as it were, a surety that if a man has no present need but may want in the future, the thing he needs will be available when he presents the currency.”