Spices brought to Europe from the Eastern world were some of the most valuable commodities for their weight, sometimes rivaling gold. From 1070 onward, kingdoms in West Africa became significant members of global trade. This came initially through the movement of gold and other resources sent out by Muslim traders on the Trans-Saharan trading network. Later, West Africa exported gold, spices, cloth, and slaves to European traders such as the Portuguese, Dutch, and English. This was often in exchange for cloth, iron, or cowrie shells which were used locally as currency. Founded in 1352, the Bengal Sultanate was a major trading nation in the world and often referred to by the Europeans as the richest country to trade with. In the 16th and 17th centuries, the Portuguese gained an economic advantage in the Kingdom of Kongo due to different philosophies of trade. Whereas Portuguese traders concentrated on the accumulation of capital, in Kongo spiritual meaning was attached to many objects of trade. According to economic historian Toby Green, in Kongo "giving more than receiving was a symbol of spiritual and political power and privilege." In the 16th century, the Seventeen Provinces were the center of free trade, imposing no exchange controls, and advocating the free movement of goods. Trade in the East Indies was dominated by Portugal in the 16th century, the Dutch Republic in the 17th century, and the British in the 18th century. The Spanish Empire developed regular trade links across both the Atlantic and the Pacific Oceans. In 1776, Adam Smith published the paper "An Inquiry into the Nature and Causes of the Wealth of Nations". It criticized Mercantilism, and argued that economic specialization could benefit nations just as much as firms. Since the division of labour was restricted by the size of the market, he said that countries having access to larger markets would be able to divide labour more efficiently and thereby become more productive. Smith said that he considered all rationalizations of import and export controls "dupery", which hurt the trading nation as a whole for the benefit of specific industries. In 1799, the Dutch East India Company, formerly the world's largest company, became bankrupt, partly due to the rise of competitive free trade. In 1817, David Ricardo, James Mill and Robert Torrens showed that free trade would benefit the industrially weak as well as the strong, in the famous theory of comparative advantage. In Principles of Political Economy and Taxation Ricardo advanced the doctrine still considered the most counterintuitive in economics:  The ascendancy of free trade was primarily based on national advantage in the mid 19th century.