Marx’s theory of modern capitalism was constructed in several stages as part of a more general theory of society. The first stage was that in which Marx, during 1842-3, formed his conception of the proletariat as a distinctive element, and a major political force, in the new type of society that was coming into existence in Western Europe. But the proletariat, as Marx noted, was a product of the industrial movement, and in order to understand fully its social situation and historical significance it would be necessary to study in detail the economic structure and development of the modern Western societies. Hence, in a second stage of his work, and guided initially by the studies which Engels had already devoted to political economy,1 Marx embarked upon an extensive reading of the economists-in particular, Say, James Mill, List, Adam Smith and Ricardo-and filled a series of notebooks with critical comments on their writings.