With an estimated $4 trillion in housing wealth and $9 trillion in stock-market wealth destroyed so far in the United States, there is little doubt that we are witnessing a classic debt-deflation bust at work, characterized by falling prices, frozen credit markets and plummeting asset values.

Those who want to understand the mechanism might ponder Irving Fisher's comment in 1933: When it comes to booms gone bust, "over-investment and over-speculation are often important; but they would have far less serious results were they...