"Economic capital should be stable. The widely accepted definition of economic sustainability is 'maintenance of capital,' or keeping capital intact. Thus Hicks' definition of income – 'the amount one can consume during a period and still be as well off at the end of the period' can define economic sustainability, as it devolves on consuming interest, rather than capital. Historically, economics has rarely been concerned with natural capital (e.g. intact forests, healthy air). To the traditional economic criteria of allocation and efficiency must now be added a third, that of scale (Daly, 1992). The scale criterion would constrain throughput growth the flow of material and energy (natural capital) from environmental sources to sinks. Economics values things in money terms, and is having major problems valuing natural capital, intangible, intergenerational, and especially common access resources, such as air. Because people and irreversibles are at stake, economics needs to use anticipation and the precautionary principle routinely, and should err on the side of caution in the face of uncertainty and risk".

this is the branch of sustainability that is concerned with the health of nature. Since the welfare of people and communities depends ultimately on the health of nature it is a fundamentally important concept.

developing a country and meeting people's needs in ways that do not damage or use up the environmental resources (clean water, air, healthy soils and ecosystems) on which sustainable development and quality of life ultimately rest, so that we can continue to develop and meet human needs for generations to come