Socialism: Utopian and Scientific of thousands of copies. Throughout the decade of the 1860s, Karl Marx, close personal friend and political associate of Friedrich Engels, dedicated himself to the study of economics, culminating in the publication of the first volume of "Das Kapital" in 1867. Marx's lengthy and ponderous volume was extremely difficult for the average to penetrate, however, leading Engels to suggest to Marx in a letter of September 16, 1868 that a short popularized version of "Das Kapital" for a working class audience was urgently needed. "If it is not written, some Moses or other will come along and do it and
Karl Marx value advocated by David Ricardo. The work was enthusiastically received, and the edition sold out quickly. The successful sales of "A Contribution to the Critique of Political Economy" stimulated Marx in the early 1860s to finish work on the three large volumes that would compose his major life's work – "Das Kapital" and the "Theories of Surplus Value", which discussed the theoreticians of political economy, particularly Adam Smith and David Ricardo. "Theories of Surplus Value" is often referred to as the fourth volume of "Das Kapital" and constitutes one of the first comprehensive treatises on the history of economic thought.