Envisioning strategies for sustainable development and its governance are knowledge-intensive processes. Against this background, conflicts about the correct form and actual validity of knowledge supporting sustainable development have arisen. What can be seen as evident-and what not? This article is based on the argument that there are differing modes creating evidence within “epistemic“ and “practice“ communities. Therefore, I propose to decipher knowledge production for sustainable development as processes of social experimentation in Dewey's sense. To do so, I introduce the concept of a “formative public“ for analyzing the cultural and institutional contexts of such processes. The argument is underlined by a focused description of the cases of chemical regulations and climate change politics. The findings support the argument that the politics of sustainable development has to elaborate guidelines and institutional structures for processing knowledge as a social experiment in order to resolve the conflicting ideas mirrored through differing accounts of the evidence.