Adam Smith's The Wealth of Nations in 1776 is usually considered to mark the beginning of classical economics.[2] The fundamental message in Smith's influential book was that the wealth of nations was based not on gold but on trade: That when two parties freely agree to exchange things of value, because both see a profit in the exchange, total wealth increases. Classical economics originally differed from modern libertarian economics in seeing a role for the state in providing for the common good. Smith acknowledged that there were areas where the market is not the best way to serve the common good, and he took it as a given that the greater proportion of the costs supporting the common good should be borne by those best able to afford them.[2]