But the plumbing metaphor is misleading. Credit is not a flow. It is not something that can be forced downstream by clearing a pipe. Credit is a contract. It requires a borrower as well as a lender, a customer as well as a bank. And the borrower must meet two conditions. One is creditworthiness, meaning a secure income and, usually, a house with equity in it. Asset prices therefore matter. With a chronic oversupply of houses, prices fall, collateral disappears, and even if borrowers are willing they can't qualify for loans. The other requirement is a willingness to borrow, motivated by what Keynes called the "animal spirits" of entrepreneurial enthusiasm. In a slump, such optimism is scarce. Even if people have collateral, they want the security of cash. And it is precisely because they want cash that they will not deplete their reserves by plunking down a payment on a new car.