Thinking about such processes in ecological terms reminds us that wealth never really consists of things; it consists of control over the energy flows that make, move, mine, and transform things. Wealth is a sort of compressed sunlight, just as matter is really congealed energy. Mobilizing this compressed energy from the rest of the population, along with the flows of resources that it made possible, became the fundamental task for rulers and governments, and that task would shape all aspects of the evolution and history of agrarian civilizations.

— Origin Story: A Big History of Everything by David Christian