Mercantilism is an economic theory that holds that the prosperity of a nation depends upon its supply of capital, and that the global volume of trade is "unchangeable." Capital, represented by bullion (gold or silver) held by the state, is best increased through a positive balance of trade with other nations (exports over imports). Mercantilism suggests that the ruling government should advance these goals by playing a protectionist role in the economy, by encouraging exports and discouraging imports, especially through the use of tariffs. The economic policy based upon these ideas is often called the mercantile system.