From the time of his first economic works, Marx viewed the labour of any form of society as a set of production activities and analysed the different modes of production throughout history in terms of the specific ways in which these activities are divided up and exchanged. This book aims to demonstrate that the category of living labour, whose central role is a direct result of the definition of social labour as a set of production activities, provides a basis for the originality of the categories of Capital, their mode of presentation and also the nature of Marx's critical method. Within Marxist debates, the purpose of this book is to establish the common ground in Marxist method between (i) the "economic" current of Marxism, whereby the method in Capital sheds light on the objective contradictions of the capitalist mode of production; and (ii) the "philosophical" current, whereby Capital lays the theoretical foundations of a materialist theory of alienation. The book then explores the differences separating Marx's labour theory of value from that of Ricardo, by tracing the emergence of Marx's conception of labour in his early works. The book also underlines the ways in which Marx criticises the fundamental principles of classical economics. These forms are highlighted, in particular, through analyses of questions of unemployment and overpopulation, of the integration of money into economic analysis or of the reproduction of fixed capital. Finally, the book establishes the grounds for Marx's claim for the necessity of crises.