let me give answer which would be a paul romer answer. in physics we learn about the law of conservation of matter. we don't learn that matter gets depleted. so what economic activity consists of -- to the extent that it's physical activity at all, and, of course, a lot of it is becoming again intangible activity communications and so on. but to the extent it's physical activity, it's making some molecules and transforming them to other molecules. so in that sense there is no depletion. we're coming up with new recipes for transforming molecules. if you were to read the interview with paul romer in our book, you'd get a perspective that says that we really don't face an issue of depletion of resources. that that's really not the way the economy works. and, you know -- and i think some of the facts that we mention in the book include fact -- like, for instance, the weight per dollar of gdp is of falling. so the physical components of gdp are falling. relative to the mental components. >> yeah, and i'd also add -- i mean, that's -- there's