The early stages of the Iraq war may have been a watershed in American optimism. The happy talk was so extreme it is now difficult to believe it was sincere: “we will be greeted as liberators”; “mission accomplished”; the insurgency is “in the last throes.” Most wildly optimistic of all was the goal: a military action transforming the Middle East into pro-American democracies.

The gap between predictions and reality has left Americans deeply discouraged. So has much of what has happened, or not happened, at the same time. Those who believed New Orleans would rebound quickly after Hurricane Katrina have seen their hopes dashed. Those counting on solutions to health care, energy dependence or global warming have seen no progress. It is no wonder the nation is in a gloomy mood; 71 percent of respondents in a recent Associated Press-Ipsos poll said the country is on the wrong track.

These are ideal times for the release of “Pessimism: Philosophy, Ethic, Spirit,” by Joshua Foa Dienstag, a U.C.L.A. political theorist. Mr. Dienstag aims to rescue pessimism from the philosophical sidelines, where it has been shunted by optimists of all ideologies. The book is seductive, because pessimists are generally more engaging and entertaining than optimists, and because, as the author notes, “the world keeps delivering bad news.” It is almost tempting to throw up one’s hands and sign on with Schopenhauer.

Pessimism, however, is the most un-American of philosophies. This nation was built on the values of reason and progress, not to mention the “pursuit of happiness.” Pessimism as philosophy is skeptical of the idea of progress. Pursuing happiness is a fool’s errand. Pessimism is not, as is commonly thought, about being depressed or misanthropic, and it does not hold that humanity is headed for disaster. It simply doubts the most basic liberal principle: that applying human reasoning to the world’s problems will have a positive effect.

The biggest difference between optimists and pessimists, Mr. Dienstag argues, is in how they view time. Optimists see the passing of time as a canvas on which to paint a better world. Pessimists see it as a burden. Time ticks off the physical decline of one’s body toward the inevitability of death, and it separates people from their loved ones. “All the tragedies which we can imagine,” said Simone Weil, the French philosopher who starved herself to death at age 34, “return in the end to the one and only tragedy: the passage of time.”

Optimists see history as the story of civilization’s ascent. Pessimists believe, Mr. Dienstag notes, in the idea that any apparent progress has hidden costs, so that even when the world seems to be improving, “in fact it is getting worse (or, on the whole, no better).” Polio is cured, but AIDS arrives. Airplanes make travel easy, but they can drop bombs or be crashed into office towers. There is no point in seeking happiness. When joy “actually makes its appearance, it as a rule comes uninvited and unannounced,” insisted Schopenhauer, the dour German who was pessimism’s leading figure.

As politicians, pessimists do not believe in undertaking great initiatives to ameliorate unhappiness, since they are skeptical they will work. They are inclined to accept the world’s evil and misery as inevitable. Mr. Dienstag tries to argue that pessimists can be politically engaged, and in modest ways they can be. Camus joined the French Resistance. But pessimism’s overall spirit, as Camus noted, “is not to be cured, but to live with one’s ailments.”

President Clinton was often mocked for his declarations that he still believed “in a place called Hope.” But he understood that instilling hope is a critical part of leadership. Other than a few special interest programs — like cutting taxes on the wealthy and giving various incentives to business — it is hard to think of areas in which the Bush administration has raised the nation’s hopes and met them. This president has, instead, tried to focus the American people on the fear of terrorism, for which there is no cure, only bad choices or something worse.

Part of Mr. Bush’s legacy may well be that he robbed America of its optimism — a force that Franklin Delano Roosevelt and other presidents, like Ronald Reagan, used to rally the country when it was deeply challenged. The next generation of leaders will have to resell discouraged Americans on the very idea of optimism, and convince them again that their goal should not be to live with their ailments, but to cure them.