This is not altogether different to my question regarding technology in that technology is defined in most textbooks quite simply by the production function, the current state of knowledge as how to best combine factor resources, such as capital, labour and land, to produce desired goods and services. You can find many discussions on Georgescu-Roegens critique in the environmental/ecological economics literature right up to the present. Unfortunately, most economists continue to disregard the role of natural resources in production. Check out the ideas of weak sustainability vs. strong sustainability (though Im not a big fan of the terminology). The former assumes a Solow/Stiglitz/Hartwick endless substitutability between natural capital and manufactured capital via the Cobb-Douglass production function. The latter a Georgescu-Roegen/Daly/Ayres non-substitutability based on thermodynamic laws. I think this is a debate that really needs to penetrate the mainstream, especially given the current concerns of many geologists over oil. We have to check to see if we have been too complacent with our substitutability assumptions. Adam McHugh