"Until recently, economists have not been particularly carried away with concern over environmental problems caused by industrial development. ...[T]he few economists ...who have always sounded the alarm ...are somewhat out of the mainstream. These humanist concerns seem to have gone out of style after the age of classical economics. Even the conventional analytical models of contemporary economics seem to prefer to exclude these concepts by ignoring them entirely or by shunting them off into their own branch, called "economic externalities." ...In recent years concern over these ...externalities has grown. The environmentalists are beginning to be included in the mainstream. ...Attempts are even being made to extend the theoretical framework to include the changes in the environment caused by economic activity... The Materials Flow of the Economy... sees the human race living on a 'space ship earth' in which all the inputs and outputs, all the original resources and all the final wastes, must be accounted for. ...[W]hen the materials are returned in the form of smoke, sewage, garbage, junk, heat, noise, and a wide variety of noxious gases... the change is seldom for the better. ...[T]he less production that is needed to maintain an adequate level of affluence, the better. An efficient economy is one that gets big results with little effort. More industries, more mines, more businesses, more employment, and more consumer goods do not always mean more well-being... because all these also mean more destruction of our natural resources and despoilation of our surroundings."