India, July 25 -- What is money? Economists begin with function.

Money, they say, does three things: it lets us exchange goods and services, acts as a standard measure of value, and it lets us store value. A kind of economic Swiss Army knife, remarkably adaptable and endlessly circulated.

John Maynard Keynes saw money as a bridge between present and future. Milton Friedman warned that it could be a weapon in the wrong hands; that too much of it, too fast, would corrode a system. Hyman Minsky went deeper still: all money is a promise, he said, but not all promises are equal. Some come wrapped in the authority of the state, others in the credibility of a bank, still others in the brute fact of power.

Today, a hundred-rupee note doesn't i...