system, (2) a fair distribution of resources and opportunities between present and future generations, as well as between agents in the current generation, and (3) an efficient allocation of resources that adequately accounts for natural capital, are thus really predictors of sustainability and not really elements of a definition. Like all predictions, they are uncertain and are subject to much discussion and disagreement.
The second problem is that when one says a system has achieved sustain-ability, one does not mean an infinite life span, but rather a life span consistent with its time and space scale. Figure 1 indicates this relationship by plotting a hypothetical curve of system life expectancy on the y axis as a function of time and space on the x axis. We expect a cell in an organism to have a relatively short life span, the organism to have a longer life span, the species to have an even longer life span, and the planet to have a longer life span. But no system (even the universe itself in the extreme case) is expected to have an infinite life span. A sustainable system in this context is thus one that attains its full expected life span.
Individual humans are sustainable by this definition if they achieve their normal life span. At the population level, average life expectancy is often used as an indicator of health and well-being of the population, but the population itself is expected to have a much longer life span than any individual and would not be