The Washington argument that ushers in the Reagan-Gorbachev summit is whether credit for the considerable promise of it should go to Ronald Reagan for putting on the pressure that brought Mikhail Gorbachev to the table or to Gorbachev for being flexible, reasonable and ready to take major political risks.

This is in the first instance a narrow, forgettable debate between partisan Reaganites who seek celebration of their man and critics who choke on the thought that Reagan may have done something right after all.

It's also a broad inquiry into the essence of Soviet-American relations. By now we should know there is no single sure way to achieve ''true peace,'' in Reagan's latest formulation of his goal. Nonetheless, the hope persists that if we could somehow get the formula straight, we could bottle it, reproduce it, use it again.

There is a strong case that the Reagan strategy of proclaiming ideological war, building up the American arsenal and trying to roll back the Third World gains Moscow made in the 1970s is working, in part anyway. This strategy convulsed some and troubled others among us. But the Soviets found they could not easily intimidate either Reagan or the European allies or the United States' Third World chargers or their own dissidents and would-be emigrants, and here is Gorbachev touching down for summit No. 3.

This has put some of Reagan's old critics in the awkward position of hailing a prospective goal -- arms control accords, tentative regional easings -- whose means they protested or were skeptical of along the way. Meanwhile, many Reaganites who cheered the means back off from their first major result, the new missile agreement, and mutter darkly about what may yet come. It is one of those delicious turns on which American politics thrives.

There is another sort of turn behind Reagan's ostensible strategy. In a sense he made Gorbachev: he applied the pressure that could not have failed to contribute to the Soviets' decision to go with a younger reformer in order to get the Soviet Union moving again. But Gorbachev could be, if not saving, then embellishing Reagan's presidency by providing the wherewithal for a foreign policy triumph now.

When Reagan pushed, the Kremlin might just have pushed back in the familiar pattern of great-power confrontation. To a point, however, Gorbachev did otherwise. Demonstrably, he has wanted to shift Soviet energies and perhaps later some resources toward domestic renewal, at least in this phase.

Did he make a tactical decision that the Soviet Union needed a breathing space and he had best do what he can to neutralize American conservatives, who make or break de'tente? Did he tap into a kind of unspoken global assumption that, after the careless Soviet imperial surge of the 1970s, the Americans needed to even things out in the 1980s?

Or -- to pass over to the strategic possibilities -- did Gorbachev actually start feeling that it is no longer so important and may be too costly and dangerous to continue fighting the class war around the globe in the old harsh ways? In short, did he find himself, wittingly or not, in the grip of the famous ''mellowing'' that George Kennan posited 40 years ago as the condition that the Soviet Union might eventually come to if the West successfully contained Soviet power?

I promise good answers to these questions in 10 or 20 years. Meanwhile, it is worth contemplating the crucial factor of timing.

Ronald Reagan had the good fortune to arrive in the White House just as -- we all know now -- the Soviet Union was coming into an economic dead end that had fed into an authentic society-wide crisis. Having said for years that communism was a failing system, he went around the country in 1980 insisting that a rapid U.S. arms buildup would strain the defense-burdened Soviet economy and force the Soviets to negotiate arms reductions.

Some of us sophisticates thought he was spouting a right-wing fairy tale, and perhaps he was. But for a similar diagnosis of Soviet rot, Gorbachev is now hailed as a courageous reformer, and for going down the very negotiating path that Reagan charted, Gorbachev is now right up there with Reagan, or ahead, in the who's-a-nice-guy? polls.

I'd settle for saying it takes two.