The best statement of laissez-faire was made in 1776 by a Scottish philosopher, Adam Smith, in his Wealth of Nations. In the process of enunciating three basic principles of economics, Smith presented a strong attack on mercantilism. He condemned the mercantilist use of tariffs to protect home countries. If one country can supply another country with a product cheaper than the latter can make it, it is better to purchase than to produce it. To Smith, free trade was a fundamental economic principle. Smith's second principle was his labour theory of value. He claimed that gold and silver were not the source of a nation's true wealth, but unlike the Physiocrats, he did not believe that soil was either. Labour constituted the true wealth of a nation, he believed. Smith believed that the state should not interfere in economic matters. He believed the government had three basic functions: to protect society from invasion, defend individuals from injustice and oppression, and keep up certain public works that private individuals could not afford.