We need to understand what philosophers in the Middle Ages, from Italy to India to China, already understood perfectly well: Money is not a thing, and is certainly not a scarce resource. Money is a promise. And it is a promise we keep to those we value and break to those we do not. In Greece, Ireland, Portugal, and Spain, sovereign-debt default seems ever more likely. If it occurs, then what will happen? Certain promises will be kept, and others will be broken. As we learn from politicians every day, it is rarely possible to keep all promises exactly as one has made them. Today, in the United Kingdom, many politicians are saying, “I know I was elected on a solemn pledge not to raise tuition fees, but now that I’m in power I realize that was unrealistic. We will have to triple them."

Last year the National Debt Clock in New York City ran out of digits. The billboard-size electronic counter, mounted on a wall near Times Square, overflowed when the public debt reached $10 trillion, or 1013 dollars. The crisis was resolved by squeezing another digit into the space occupied by the dollar sign. Now a new clock is on order, with room for growth; it won’t fill up until the debt reaches a quadrillion (1015) dollars.

The incident of the Debt Clock brings to mind a comment made by Richard Feynman in the 1980s—back when mere billions still had the power to impress:

There are 1011 stars in the galaxy. That used to be a huge number. But it’s only a hundred billion. It’s less than the national deficit! We used to call them astronomical numbers. Now we should call them economical numbers.

The important point here is not that high finance is catching up with the sciences; it’s that the numbers we encounter everywhere in daily life are growing steadily larger. Computer techn