The four most dangerous words for investors are: This time is different.

In 1999, technology companies with no earnings or sales were valued at billions of dollars. But this time was different, investors told themselves. The Internet could not be missed at any price.

They were wrong. In 2000 and 2001, technology stocks plunged, erasing trillions of dollars in wealth.

Now investors have again convinced themselves that this time is different, that the credit crisis will push economies worldwide into the deepest recession since the Great Depression. Fear runs even deeper today than greed did a decade ago.

But in their panic, investors are ignoring 60 years of history. Since the Depression, governments have become far more aggressive about intervening when credit markets seize up or economies struggle. And those interventions have generally succeeded. The recessions since World War II, while hardly easy, have been far less painful than the Depression.

Now some veteran investors, including G. Kenneth Heebner, a mutual fund manager with one of the best long-term track records on Wall Street, say that the sell-off has gone much too far and that stocks are poised to rally powerfully if the downturn is less severe than investors fear.

"The fact is, there are a lot of tremendous bargains out there," said Heebner, who manages about $10 billion in several mutual funds. Indeed, by many measures, stocks are as affordable as they have been in the last 25 years.

He pointed to Chesapeake Energy, a natural gas producer that he owns in his CGM Focus mutual fund. In July, Chesapeake traded for $63 a share. On Friday, it fell as low as $11.99.

He said that investors with a stomach for risk and a long time horizon should consider following the American investor Warren Buffett, who in the past three weeks has invested $8 billion in Goldman Sachs and General Electric.

Heebner said he expected world economies to contract over the next year. But he said the market plunge was no longer being driven by rational analysis. Stocks are probably falling because of a combination of panic and forced selling by hedge funds that must meet margin calls from their lenders, he said.

Heebner's funds have not avoided the carnage this year. The CGM Focus fund is down about 42 percent so far in 2008. But his long- term track record is impressive. In the decade that ended Dec. 31, 2007, CGM Focus rose 26 percent a year, including reinvested dividends, making it among the best-performing mutual funds.

Heebner is not alone in his optimism.