Sustainability is an essentially contested idea. Ideas of democracy, justice, truth, and reality themselves are competing philosophical visions of sustainability and a seemingly endless number of fine points on which even those who are in broad agreement can differ. The contested nature of sustainability is a reason to think that it is philosophically important. Functional integrity comes nearest to a satisfactory framework for thinking about sustainability. Legions of resource and development economists leapt to the task, producing a plethora of technical measures to specify sustainability in a manner consistent with existing economic concepts.