“Marx’s labour theory of value is a further development and perfection of the labour theory of value as it emanated from the ‘classical’ school of political economy, and especially of Ricardo’s version. But the changes Marx brought into that theory were manifold. One especially was to be decisive: the use of the concept of abstract social labour as the foundation of his theory of value. It is for this reason that Marx cannot be considered as in any way an ‘advanced neo-Ricardian’. ‘Labour quantities as the essence of value’ is something quite different from ‘labour quantities as numéraire ’ –a common measuring rod of the value of all commodities. The distinction between concrete labour, which determines the use-value of commodities, and abstract labour, which determines their value, is a revolutionary step forward beyond Ricardo of which Marx was very proud; indeed he considered it his main achievement, together with the discovery of the general category of surplus-value, encompassing profit, rent and interest. It is based on an understanding of the peculiar structure of a society of commodity-producers, that is of the key problem of how to relate to each other the segments of the global labour potential of society which have taken the form of private labour. It represents, therefore, together with Marx’s concept of necessary labour and surplus labour (necessary product and surplus product), the key nexus between economic theory and the science of social revolution, historical materialism.”
Ernest Mandel, Introduction to Capital Volume I