That last bit contains a lot of truth. If we could see a century or two into the future we would be amazed (shocked?) at how our descendants will deal with energy. We take for granted that things will continue on indefinitely, when in fact we are enjoying a bubble of cheap energy that future generations will look back on with envy. 

Imagine a world that never had any appreciable amount of fossil fuels. How would life have been different for us? For one thing, transportation would be very expensive. So would many chemicals and materials that we toss without a second thought. Prices would guide our actions, as they do today, but on a much higher level. Anything with a lot of embodied energy would be selected against; concrete might never have become a common building material, for example. Rationing would be an ever-present concern.

Would such a society have ever advanced to the point that it could design and build nuclear plants, solar farms, or even commercial-scale wind turbines? Maybe, but it might take the equivalent of an Apollo program to get it done. We are fortunate to have had cheap energy and have attained a standard of living unimaginable just a century or two ago. As our population grows and as supplies of key resources dwindle, we will see just how much we can conserve and recycle. We will, over time, become more like that world that never had fossil fuels -- with the exception that we have this moment in time to develop renewables on the cheap. We might want to pick up the pace a little.

BTW: "Sustainable" is a word used a lot here lately. You could say that sustainable agriculture is any form of agriculture that turns a profit, thus sustaining the farmer for another year, but this is short-sighted. Upon a little more reflection, we might add that we should also replace nutrients, conserve our topsoil, and show at least some concern for water quality. But can any system that relies heavily on depleting resources -- petroleum, rock phosphate, shrinking aquifers -- truly be called sustainable?