It is paradoxical. It also is true. The conservatives' task is to pull this conservative president, and U.S. diplomacy generally, out of the 19th century and into the 20th. This conclusion is compelled by President Reagan's peculiar and opaque rhetoric about a "fresh start" in U.S.-Soviet relations.

This conclusion is paradoxical because conservatives have a retrospective cast of mind. Their cardinal virtue, prudence, involves mining the past for instructive precedents and proven institutions and procedures. Furthermore, the greatest figures of 19th-century diplomacy -- Metternich, Wellington, Bismarck, Disraeli -- are in the conservatives' pantheon.

Nevertheless, the conservatives' problem today is that President Reagan, although a definer of contemporary conservatism, subscribes to a model of diplomacy that reflects the 18th-century liberal mind. This is so even though the model was adopted, not unreasonably, by those 19th-century conservatives.

The problem is the radical newness of a kind of 20th-century regime, and the depressing oldness of rhetoric about a "fresh start." What did Reagan mean by that? Indeed, what could he mean?

He is painfully fond of the least conservative sentiment conceivable, a statement taken from an anti-conservative, Thomas Paine: "We have it in our power to begin the world over again." Any time, any place, that is nonsense. But that may have been how things looked in 18th-century America, on the thinly populated fringe of an unexplored continent, or in revolutionary France, in the first flush of upheaval.

Many people then believed in "fresh starts" because society was not planted thick with institutions; or old institutions suddenly seemed fragile. They believed that people everywhere were similar, essentially good and easily educable. "The present age," said Paine, "will hereafter merit to be called the Age of Reason, and the preso the future as the Adam of a new world." Some people also believed, with Paine, that the "science of government" is "of all things the least mysterious and the most easy to be understood," because "men have but to think and they will neither act wrong nor be misled."

Reagan has repeatedly used Paine's words about beginning the world "over again" concerning domestic policy. One result of his sincere belief in "fresh starts" is the deficit. The theory was that we would cut taxes and then compensate for the lost revenues by cutting spending. We would start "over again," making a "fresh start" in defining federal social roles, as though the New Deal and Great Society never happened. But it is, it seems, impossible to start over again at even, say, 1965 levels of social spending. It is not even possible to make a "fresh start" without Amtrak.

Applied to domestic policy, the idea of a "fresh start" has produced fiscal problems. Applied to foreign policy, it can produce disaster.

Americans believe in "fresh starts," meaning limitless possibility, because they think all people, and all regimes, think "economically," rationally calculating how to enhance their essentially similar interests. That assumption would be true, or true enough, were the world as Paine thought it was. He said there were just two types of government: those "by election and representation" and those "by hereditary succession." He predicted that all governments soon would be representative and then "nations will become acquainted, and prejudices fomented by the intrigues and artifices of courts will cease."

The 19th century was not that serene, but it was relatively tranquil. Irving Kristol, writing in The National Interest, a new foreign policy quarterly of conservative bent, says that 19th-century diplomats represented regimes that regarded one another as permanent presences. They defined national interests in limited and familiar ways that allowed conflicts to be resolved by splitting differences. Sometimes the splitting was done after wars -- but they were limited wars. Governments maneuvered to alter, but not obliterate, the "equilibrium" among powers. The rules of that game of nations allowed for time-outs, and fresh starts.

The rules changed radically with the eruption in this century of totalitarian regimes whose foreign policies reflect domestic arrangements resting on lies and terror. Regimes that derive their claims to legitimacy from ideologies that legitimize limitless violence are not interested in the 19th-century ideal of "equilibrium."

Today's synonym for "equilibrium" is "stability." We seek a "stable" relationship in strategic arms and "stability" in regional conflicts. The Soviet Union derides stability in theory and assails it in practice.

America is a nation of poker players. Poker is a game of fresh starts -- play a hand, shuffle the deck, deal again. The Soviet Union plays chess and pursues endgame.