Samuelson's Economics has probably been the most influential textbook in the field, certainly since Alfred Marshall's Principles of Economics (1890). In 1947 Samuelson published Foundations of Economic Analysis, which drastically redirected the advanced study of economics toward greater and more productive use of mathematics, both to integrate seemingly diverse branches of economics into a few core concepts and to sort out key differences underlying seemingly divergent propositions. As if to compensate for this abstract work, Samuelson in 1948 published a comprehensive, clearly written introductory text. Now in its 15th edition, it has sold upwards of 4 million new copies, and many times that on the used book market; it has been translated into more than 30 languages, some of them in pirated editions. Since economics is one of the most commonly taught courses in American colleges, few texts have been studied by so many. Samuelson was also influenced, but not dominated, by the Keynesian revolution of the 1930s, incorporating the relatively new subject of macroeconomics, with its implications for the role of government in stabilizing the economy.