ABSTRACT
At the heart of long-lived controversies over sustainability is the substitutability of natural capital and physical capital. Advocates of weak sustainability argue that physical and natural capital are substitutes. As levels of natural capital decline, they believe that technological innovations will allow physical capital to take its place. Thus, the key to sustained welfare is maintenance of the total capital stock. Advocates of strong sustainability prefer not to rely on uncertain technological innovations. They see natural and physical capital as complements, and expect limited amounts of natural capital to form the binding constraint on future welfare. The strong sustainability criterion is the maintenance of sustainable levels of natural capital. Strong sustainability requires the maintenance of aggregate levels of natural capital. It does not stipulate the sustenance of specific components of natural capital or specific physical flows of resources. The harvest of every fishery and forest at a sustainable rate would be a sufficient but unnecessary condition for strong sustainability.
