Now as Daly said, man-made capital and natural capital are not substitutable. This is obvious, but since most economists say they are substitutable, it has to be insisted on. Put simply, you can't substitute more sawmills for fewer forests. If you're building a house you can juggle the number of power saws and carpenters, which means they're substitutable, but you can't build it with half the amount of lumber, no matter how many saws or carpenters you have. Try it and you have a house of air. And that's where we live now.