Coming Spaceship Earth, economist and systems scientist Kenneth E. Boulding argued that mankind would soon have to adapt to economic principles much different than the past 'open earth' of illimitable plains and exploitative behaviour. On the basis of the thermodynamic principle of the conservation of matter and energy, Boulding developed the view that the flow of natural resources through the economy is a rough measure of the Gross national product (GNP); and, consequently, that society should start regarding the GNP as a cost to be minimized rather than a benefit to be maximized. Therefore, mankind would have to find its place in a cyclical ecological system without unlimited reservoirs of anything, either for extraction or for pollution — like a spaceman on board a spaceship. Boulding was not the first to make use of the 'Spaceship Earth' metaphor, but he was the one who combined this metaphor with the analysis of natural resource flows through the economy. In his 1971 magnum opus on The Entropy Law and the Economic Process, Romanian American economist Nicholas Georgescu-Roegen integrated the thermodynamic concept of entropy with economic analysis, and argued that all natural resources are irreversibly degraded when put to use in economic activity. What happens in the economy is that all matter and energy is transformed from states available for human purposes (valuable natural resources) to states unavailable for human purposes (valueless waste and pollution). In the history of economic thought, Georgescu-Roegen was also the first economist of some standing to theorise on the premise that all of earth's mineral resources will eventually be exhausted at some point (see below). Also in 1971, pioneering ecologist and general systems analyst Howard T. Odum published his book on Environment, Power and Society, where he described human society in terms of ecology. He formulated the maximum power principle, according to which all organisms, ecosystems and human societies organise themselves in order to maximize their use of available energy for survival. Odum pointed out that those human societies with access to the higher quality of energy sources enjoyed an advantage over other societies in the Darwinian evolutionary struggle. Odum later co-developed the concept of emergy (i.e., embodied energy) and made other valuable contributions to ecology and systems analysis. His work provided the biological term 'ecology' with its broader societal meaning used today. In 1972, scientist and systems analyst Dennis Meadows and his team of researchers had their