Matter taken up into the economy can be recycled, using energy; but energy, used once, is forever unavailable to us at that level again. The law of entropy commands a one-way flow downward from more to less useful forms. An animal can’t live perpetually on its own excreta. Neither can you fill the tank of your car by pushing it backwards. Thus, Georgescu-Roegen, paraphrasing the economist Alfred Marshall, said: “Biology, not mechanics, is our Mecca.”

Following Soddy, Georgescu-Roegen and other ecological economists argue that wealth is real and physical. It’s the stock of cars and computers and clothing, of furniture and French fries, that we buy with our dollars. The dollars aren’t real wealth, but only symbols that represent the bearer’s claim on an economy’s ability to generate wealth. Debt, for its part, is a claim on the economy’s ability to generate wealth in the future. “The ruling passion of the age,” Soddy said, “is to convert wealth into debt”  to exchange a thing with present-day real value (a thing that could be stolen, or broken, or rust or rot before you can manage to use it) for something immutable and unchanging, a claim on wealth that has yet to be made. Money facilitates the exchange; it is, he said, “the nothing you get for something before you can get anything.”

Problems arise when wealth and debt are not kept in proper relation. The amount of wealth that an economy can create is limited by the amount of low-entropy energy that it can sustainably suck from its environment  and by the amount of high-entropy effluent from an economy that the environment can sustainably absorb. Debt, being imaginary, has no such natural limit. It can grow infinitely, compounding at any rate we decide.

Whenever an economy allows debt to grow faster than wealth can be created, that economy has a need for debt repudiation. Inflation can do the job, decreasing debt gradually by eroding the purchasing power, the claim on future wealth, that each of your saved dollars represents. But when there is no inflation, an economy with overgrown claims on future wealth will experience regular crises of debt repudiation  stock market crashes, bankruptcies and foreclosures, defaults on bonds or loans or pension promises, the disappearance of paper assets.

It’s like musical chairs  in the wake of some shock (say, the run-up of the price of gas to $4 a gallon), holders of abstract debt suddenly want to hold money or real wealth instead. But not all of them can. One person’s loss causes another’s, and the whole system cascades into crisis. Each and every one of the crises that has beset the American economy in recent years has been, at heart, a crisis of debt repudiation. And we are unlikely to avoid more of them until we stop allowing claims on income to grow faster than income.