The aim of this paper is to show how Marx's Economic and Philosophic Manuscripts of 1844, which stand as his first systematic study of classical economists, transform the concept of labour as it had been developed by political economy against mercantilism and physiocracy. The current interpretations of the Marxian theory of value are first reviewed. The analysis of the Manuscripts then shows that the contribution of Hegelian philosophy lies in the definition of social labour in total opposition to the orthodox conception. This issue leads to a reexamination of the significance of these Manuscripts for the Marxian concept of value and its source, general and abstract labour.