Critics have standardly regarded Sartre’s Critique of Dialectical Reason as an abortive attempt to overcome the subjectivist individualism of his early philosophy, motivated by a recognition that Being and Nothingness lacks ethical and political significance, but derailed by Sartre’s Marxism. In this paper I offer an interpretation of the Critique which, if correct, shows it to offer a coherent and highly original account of social and political reality, which merits attention both in its own right and as a reconstruction of the philosophical foundation of Marxism. The key to Sartre’s theory of collective and historical existence in the Critique is a thesis carried over from Being and Nothingness : intersubjectivity on Sartre’s account is inherently aporetic, and social ontology reproduces in magnified form its limited intelligibility, lack of transparency, and necessary frustration of the demands of freedom. Sartre’s further conjecture – which can be formulated a priori but requires a posteriori verification – is that man’s collective historical existence may be understood as the means by which the antinomy within human freedom, insoluble at the level of the individual, is finally overcome. The Critique provides therefore the ethical theory promised in Being and Nothingness .