food and water can be thought of as a flow--or, more precisely, a critical-zone flow, a current with a volume that must be maintained. By contrast, fossil fuels are like a stock, a fixed amount of a good. Few dispute that the flow of food and water could be interrupted, with terrible effects. But people have disagreed for a century and a half--since the days of Pithole--about whether the world has an adequate stock of fossil fuels.