laws. Other definitions may thus be superfluous, or mere glosses on the immutable thermodynamic reality. There are numerous other definitions and systems of operationalization for sustainability, and there has been competition for influence between them, with the unfortunate result that, in the minds of some observers at least, sustainability has no agreed-upon definition. Natural Step approach Following the Brundtland Commission's report, one of the first initiatives to bring scientific principles to the assessment of sustainability was by Swedish cancer scientist Karl-Henrik Robèrt. Robèrt coordinated a consensus process to define and operationalize sustainability. At the core of the process lies a consensus on what Robèrt came to call the natural step framework. The framework is based on a definition of sustainability, described as the system conditions of sustainability (as derived from System theory). In the natural step framework, a sustainable society does not systematically increase concentrations of substances extracted from the Earth's crust, or substances produced by society; that does not degrade the environment and in which people have the capacity to meet their needs worldwide. Ecological footprint approach Ecological footprint accounting, based on the biological concept of carrying capacity, tracks the amount of land and water area a human population demands for producing the biological resources the population consumes, for absorbing its waste, and for accommodating its built infrastructure, all under prevailing technology. This amount then is compared to available biocapacity, in the world or in that region. The biocapacity represents the area able to regenerate resources and assimilate waste. Global Footprint Network publishes every year results for all nations captured in UN statistics. The algorithms of ecological footprint accounts have been used in combination with the emergy methodology (S. Zhao, Z. Li and W. Li 2005), and a sustainability index has been derived from the latter. They have also been combined with a measure of quality of life, for instance through the "Happy Planet Index" (HPI) calculated for 178 nations (Marks et al., 2006). The Happy Planet Index calculates how many happy life years each country is able to generate per global hectare of ecological footprint. One of the striking conclusions to emerge from ecological footprint accounting is that it would be necessary to have 4 or 5 back-up planets engaged in nothing but agriculture for all those alive today to live a western lifestyle. The Footprint analysis is closely related to the I = PAT equation that, itself,