As a result, the credit default market is best described as an insurance market where many of the individual trades are undercapitalized. But even worse, many of the insurers are grossly undercapitalized. In one case in the New York courts, the Swiss banking giant UBS is suing a hedge fund that said it would insure nearly $1.5 billion in bonds but was unable to do so. No wonder  the hedge fund had only $200 million in assets.

If A.I.G. collapsed, its hundreds of billions of dollars of mortgage-related assets would be added to those being sold by other financial institutions. This would just depress values further. The counterparties around the world to A.I.G.’s credit default swaps may be unable to collect on their trades. As a large hedge-fund investor, A.I.G. would suddenly become a large redeemer from hedge funds, forcing fund managers to sell positions and probably driving down prices in the world’s financial markets. More failures, particularly of hedge funds, could follow.

Regulators knew that if Lehman went down, the world wouldn’t end. But Wall Street isn’t remotely prepared for the inestimable damage the financial system would suffer if A.I.G. collapsed.

While Gov. David A. Paterson of New York on Monday allowed A.I.G. to borrow $20 billion from its subsidiaries, that move will only postpone the day of reckoning. The Federal Reserve was also trying to arrange at least $70 billion in loans from investment banks, but it’s hard to see how Wall Street could come up with that much money.

More promisingly, A.I.G. asked the Federal Reserve for a bridge loan. True, there is no precedent for the central bank to extend assistance to an insurance company. But these are unprecedented times, and the Federal Reserve should provide A.I.G. with some form of financial support while the company liquidates its mortgage-related assets in an orderly manner.

The Fed cannot afford to stand on principle. The myth of free markets ended with the takeover of Fannie Mae and Freddie Mac. Actually, it ended with their creation.