Both the opponents of and believers in Adam Smith's free market capitalism have added to the framework setup in "The Wealth of Nations". Like any good theory, free-market capitalism gets stronger with each reformulation, whether prompted by an addition from a friend or an attack from a foe. Marginal utility, comparative advantage, entrepreneurship, the time-preference theory of interest, monetary theory and many other pieces have been added to the whole since 1776. There is still work to be done as the size and interconnectedness of the world's economies bring up new and unexpected challenges to free-market capitalism. (To read more about this evolution, check out The History Of Economic Thought.)