Marx's social philosophy is a theory of freedom in the spirit of the Enlightenment, which holds that only the rational society makes the good life possible. By linking moral independence and individual fulfilment to rational conduct and reasoned debate, the Enlightenment inaugurated critical social theory, the study of the degree to which the social conditions necessary for rational living have been achieved. Marx's innovation in this programme is to have seen that the achievement of reason in society means that social theory must issue in political strategy if it is not to become an apology for the unreasonable conditions it detects (Marcuse 1999: 252–7). Marx's social philosophy accordingly developed a theory of the historical sequence of modes of production, which matured through several phases, as his research turned from practical politics to political economy and back again. Throughout, it is the relation between praxis, structure and history that guides Marx's conception of social struggles and political freedom.

This chapter explores the central ideas of Marx – alienation, the labour theory of value, the contradictions of capitalism, the state and ideology, classes and revolution – as the essential background to understanding Marxism. I trace Marx's development from the materialist inversion of Hegelian philosophy of the “young Marx” to the more scientific theory of the “mature Marx”, indicating breaks and continuities between the “two Marxes”. I close with an extended reflection on the tension between descriptive-explanatory and normative-evaluative accounts in Marx.