The question of whether or not there will be substantial physical and social limits to economic growth cannot be answered with any degree of certainty. While one can be certain that spring will follow winter, and autumn will follow summer, the same kind of certainty does not exist when forecasting the state of the environment in the future. The type of demands upon the environment will depend as today upon the form and extent of social activity. It is necessary to state the obvious, namely that different societies both today and in the past have made different demands and impacts upon the environment. If, for example, present-day energy consumption in different countries is compared, one tends to find that high levels of consumption occur in countries with high levels of economic activity. Nonetheless, there is a good deal of variation between countries with similar levels of economic activity. G. Foley comments:

Energy demands in the future will depend to a considerable extent upon the form of economic development. Using energy-accounting techniques, P. Chapman, G. Leach and others have demonstrated vast differences in energy consumption for different ways of producing similar objectives in transport, agriculture, heating houses, packaging of goods, etc.