This article analyses the recurring topics in the epistemology of the leading 20th-century French sociologist and political theorist Raymond Aron, drawing on his doctoral dissertation 'Introduction a la philosophie de l'histoire' (1938) and on a range of works he published in his later years. The author first discusses six different reasons for Aron's conspicuous absence from many contemporary handbooks on the social sciences: his deliberate avoidance of developing a system in his work, his disinclination towards abstract theoretising, his lack of interest in empirical research, and his refusal to specialise in one field, and also the changes that occurred in the social scientific context in which his work was received and changes in the surrounding political and social circumstances, most notably the collapse of the communist regimes. The author notes that a major feature in Aron's epistemological thought was his neo-Kantian awareness of the limits of strictly scientific knowledge, which he identified with the domain of causal analysis. The second crucial theme, recurring throughout Aron's work, is the indispensability of philosophy for providing the foundations for social scientific analysis, always in need of being positioned with respect to values. His enduring interest in international relations and contemporary history is taken as an indication of the third basic element of his epistemology: a passion for the analysis of singular events. The author concludes that, given his preoccupation with the singular and the particular, the key, albeit somehow implicit, aspect of his epistemology is the capacity for judgment in the Kantian sense.