Any commitment to education for sustainability assumes that teaching and learning can reduce or prevent damage to the biosphere. However, such an assumption remains deeply problematic. David Gruenewald's is only one of several possible epistemological positions. This article discusses the variety of such positions, arguing for that which might variously be interpreted as either pragmatic, postfoundationalist, or poststructuralist, and concluding that we can only act in good faith as regards environmental sustainability, without certainty about what we are trying to conserve or how we might go about it. By contrast, Gruenewald has made unwarranted absolutist assumptions, both about the relationship of ecological and social issues and about Foucault.