Jay Forrester's Economic Dynamics was published in 1971 and The Limits to Growth by Dennis Meadows and his associates appeared a year later. The publication of those two books gave rise to twenty years of intense research into the economics of exhaustible resources, research which everywhere has had a substantial impact both on public debate and on academic curricula. And now, just as that line of research is losing steam, economists are focussing on problems associated with the degradation of the natural environment, problems which call for models which, in their formal structure, are quite...