This paper examines the epistemological arguments about markets and planning that emerged in a series of unpublished exchanges between Hayek and Neurath. The exchanges reveal problems for standard accounts of both the socialist calculation debates and logical empiricism. They also raise questions concerning the sources of ignorance and uncertainty in modern economies, and the role of market and non-market organisations in the distribution and coordination of limited knowledge, which remain relevant to contemporary debates in economics. Hayek had argued that Neurath's work exemplified the errors of rationalism that underpinned the socialist project. In response Neurath highlighted assumptions about the limits of reason and predictability that the two theorists shared and attempted to turn those assumptions back against Hayek in a defence of the possibility of socialist planning. The paper critically compares Neurath's and Hayek's criticisms of rationalism and considers how far Neurath is successful in his attempt to employ Hayek's assumptions against Hayek himself. Copyright © Cambridge University Press.