Conventional approaches in resource and environmental economics generally treat investments in natural capital and other assets as largely fungible. In contrast, intergenerational equity and nonfungibility are at the center of positions on sustainability espoused by philosophers and ecologists. In the proposed project, researchers seek to clarify these disciplinary differences in concept and to identify a useful common ground for public policy debates about the management of the natural environment. In considering these issues, an interdisciplinary approach that recognizes the importance of differing perspectives on values is required. The research will be conducted jointly by an economist and an environmental philosopher, with additional contributions from experts on ecological systems. The researchers will compare two conceptual approaches for expressing sustainability constraints. The first is a general "hybrid" system that gives different weight to economic tradeoffs and ecological constraints in different circumstances. The second approach lets unidimensional shadow prices guide decisions, but those shadow prices are constructed to reflect obligations to future generations and limits on resource/capital substitutability.