This chapter offers a new interpretation of Raymond Aron’s doctoral thesis, Introduction à la philosophie de l’histoire (1938). Described by Aron as having established the basis of all his subsequent political thought, the Introduction contains a pluralist critique of Marx’s philosophy of history which doubles as a normative justification for political liberalism. Anticipating the ‘epistemology of doubt’ characteristic of later cold war liberalism, the book also served as the philosophical basis for Aron’s ethic of intellectual responsibility. Yet the extent to which the Introduction’s historical relativism undermines its ethical and normative arguments has been widely debated. Through an analysis of Aron’s previously under-explored interpretations of Dilthey and Heidegger, the chapter argues that scholarly disagreement on this issue reflects the Introduction’s ambiguous epistemological agnosticism. The chapter concludes with a discussion of the influence of the Introduction to the Philosophy of History on Jean-Paul Sartre's Critique of Dialectical Reason.