The true origin and function of money were expounded at least as early as the second century, when the great Roman Paullus wrote: "The origin of buying and selling is in exchange. Formerly there were no coins, and merchandise was in no way distinguished from money. Every man, according to the necessity of the time and of things, exchanged what was useless to him for what was useful, and it was generally the case that what one had abundance of, another was deficient in. But as it did not always easily happen that when one person had what another desired, that other had also what the first desired, a substance was chosen whose general and durable value obviated the difficulties of exchange by being a common measure. This substance, having received a public stamp, has use and value less as a material than as a quantity, and is no longer called merchandise, but money"