Hayek's epistemic arguments against planning were aimed not just against socialism but also the tradition of ecological economics. The concern with the physical preconditions of economic activity and defence of non-monetary measures in economic choice were expressions of the same rationalist illusion about the scope of human knowledge that underpinned the socialist project. Neurath's commitment to physicalism, in natura calculation and planning typified these errors. Neurath responded to these criticisms in unpublished notes and correspondence with Hayek. These high-lighted the epistemological premises his work shared with Hayek's, representing a response to Hayek from Hayek's own assumptions. This paper examines the cogency and continuing relevance of the arguments in this debate. © Cambridge Political Economy Society 2004; all rights reserved.