IT IS A special moment in East-West affairs. For

the better part of a year a new American president has been insisting on a pause in direct Soviet- American dealings in order to put them on a new basis. Mr. Reagan has done this by his speeches, his defense program and his diplomacy. And now he has turned to Moscow, saying, in his latest letter to Soviet leader Leonid Brezhnev, that the United States seeks "a truly stable and constructive relationship" built on a respite in Moscow's "unremitting and comprehensive military buildup," on its abandonment of the "pursuit of unilateral advantage" in the Third World, and on a hands-off policy in Poland. To reach that "constructive relationship," Mr. Reagan offers a "dialogue . . . on critical geopolitical issues," negotiations leading to "genuine arms reductions" and, for openers, talks on missiles in Europe.

Such is the program the president summarized on the eve of his administration's first high-level contacts with the Kremlin. The Soviet leadership, accepting that the time was right to summarize its own program, did so in Foreign Minister Andrei Gromyko's address to the United Nations on Tuesday--the day before the first of two scheduled meetings with Secretary of State Alexander Haig. Mr. Gromyko challenged what he described as the Reagan attempt to assert "American leadership of the world," said Moscow will not forgo "legitimate interests of our own, including commitments to our allies" and denounced "interference in the internal affairs" of Poland. He added that the Soviet Union seeks "normal businesslike relations with the United States" and is ready for talks on the "limitation of strategic weapons" and on missiles in Europe, too.

In brief, on general issues there is the familiar stuff of East-West confrontation, but on one specific issue -- missiles in Europe--there is a readiness to negotiate, though even on that issue an initial gap is evident. Washington wants to talk about "theater nuclear forces," principally the big new Soviet SS20s, while Moscow wants to talk about "nuclear weapons in Europe," including "forward-based systems of the United States." The Euromissile talks, Mr. Haig and Mr. Gromyko agreed, are to start on Nov. 30.

These talks will be closely watched, not merely because their subject is vital to Europe but also because of the clues they may provide to Mr. Reagan's whole approach to East-West affairs. A large question has been raised by his statements on the matter. Does he see negotiations, as past administrations have, as a way to try to manage American differences with a country that is powerful and dangerous yet capable of defining its own interest in a certain stability? Or does he see in negotiations a means by which to enforce the strategic advantage ("margin of safety") which the West must have because of the controlling nature of the Soviet Union as an aggressive revolutionary power?

The Soviet Union is intensely curious about this question and is, presumably, prepared to seek advantage either way. The European allies are more than curious. Gratified as they are that Mr. Reagan has moved to a table, they are still not sure whether he accepts or values their deep stake in steady East-West ties. For Mr. Reagan this will be the first test --a demanding and revealing one--of the validity of his Soviet policy.