Scottish philosopher and historian David Hume emerged as an economist also with the publication of his Political Discourses. The famous Adam Smith was a friend of his and may have been influenced by Hume: they had similar principles, and both were very good at illustrating and supporting these from history. Although Hume did not formulate a complete system of economic theory, as did Smith in his Wealth of Nations, he introduced several of the new ideas around which the “classical economics” of the 18th century was built. His economic philosophy can be understood from his main arguments: that wealth consists not of money but of commodities; that the amount of money in circulation should be kept related to the amount of goods in the market; and that poor nations impoverish the rest because they do not produce enough to be able to take much part in trade. Beyond this, he urged society to welcome the shift from an agricultural to an industrial economy, without which civilization could not be achieved.