The general aim of this work is to critically analyse certain conceptions and political positions of Boaventura de Sousa Santos on the problems of Western modernity and postmodernity. For the Portuguese sociologist, Western modernity is going through an unprecedented crisis of civilisation that can be identified through three fundamental dimensions: the epistemological dimension (the problem of Western scientific knowledge), the theoretical dimension (the problem of the Marxist theory of social emancipation) and the political dimension (the problems of the liberal social contract, social fascism and the disunity of left-wing forces). Postmodernity, however, represents the possibilities and opportunities for overcoming that crisis in view of the epistemological (the ecology of knowledge), theoretical (the new emancipatory political culture) and political (high-intensity democracy) renewals that are underway. The main hypothesis to be presented is that Boaventura Santos' political conceptions and positions – in parallel with his limited understanding of Marxism, the main interlocutor and target of his often mistaken criticism – correspond to the postmodern ideology, which can be characterised, according to Marxism, by epistemological scepticism, contempt for history and political defeatism. In this vein, the theoretical framework adopted here necessarily comes from Marxism, understood, according to Michael Löwy, as a social worldview, that is, an organic, articulated and structured set of values, representations, ideas and cognitive orientations about man in his relationship with nature, society and history, internally unified by a determined perspective and a certain socially conditioned point of view.