A futures contract was drawn up obligating Simon to sell Ehrlich, Harte and Holdren these same quantities of the metals 10 years later, but at 1980 prices. If prices fell, they would pay him. The contract was signed, and Ehrlich and Simon went on attacking each other throughout the 1980's. During that decade the world's population grew by more than 800 million, the greatest increase in history, and the store of metals buried in the earth's crust did not get any larger. Population, when unchecked, increases in a geometrical ratio.
Subsistence increases only in an arithmetical ratio.
The earth has limited resources, and if we don't recycle them we use them up. It must have occurred to Homo habilis while searching for rocks to make the first tools 2.5 million years ago. We borrow it from our children. The idea shapes our personal actions when we bundle newspapers to avoid running out of wood for paper and land for garbage dumps. The counterargument is not nearly as intuitively convincing.