With the signing of the INF treaty handled quickly in the opening hours of the third summit between Ronald Reagan and Mikhail Gorbachev, the other great task of this meeting is now under way. It is the defining of Gorbachev, whose intentions and fate now control the future of American-Soviet relations as no single individual ever has before.

I do not mean that the younger, dynamic Soviet leader will inevitably dominate the lame-duck American president, as some believe, or that he is so wily that he will be able to impose his agenda on the United States. The judgment is instead that Reagan has, for better or worse, placed Gorbachev and his ability to change the Soviet system at the center of superpower relations in a gigantic act of faith.

Reagan is a smart politician, so there is a strong political element in his decision to present Gorbachev as the agent of historical change. It helps justify Reagan's own stunning reversal on arms control and his pursuit of relaxed relations with the Russians.

That was then, Reagan seems to be saying about his own past sharp attacks on detente, and this is now. Detente did not work, but "real peace," which is what he is building with Gorbachev, will. The "poor peace" of the past will be replaced by new, improved peace, he came dangerously close to saying at the treaty-signing ceremony.

But there is much more than political need involved in Reagan's own strong personal reaction to this canny, voluble Soviet leader, and it emerged clearly during the initial greetings and speeches at the White House. It involves an instinctive understanding of each other that the two men seem to have developed in their meetings in Geneva, Reykjavik and now in Washington.

That American policy-makers and ordinary citizens are now engaged in intense debate over who Gorbachev really is and what he intends to do, 2 1/2 years after he came to power, is in some ways one of his major accomplishments. We have not accorded the kind of hope that is being stirred by the Washington summit to previous Communist Party general secretaries. By use of the symbols and rhetoric that he understands so well, Reagan is telling the American public that this Russian is special and, until proven otherwise, trustworthy.

The personal relationship established between leaders has always been important, of course. In 1972, despite the professions of noble goals, it was generally understood that detente was a deal between Richard Nixon and Leonid Brezhnev, two cynical insiders intent on maximizing personal and national advantages. Jimmy Carter and Brezhnev, in signing the stillborn SALT II treaty in 1979, created a technical document that lacked the political underpinning it needed for survival.

For Reagan and Gorbachev, there is instead an instinctive understanding of each other, and a certain mutual sympathy that is now clear and at play in shaping the next stages of the superpower dialogue. Summoned to power to bring radical change, they have each confronted systems that turn out to be determined to frustrate the changes they seek.

By according such faith to Gorbachev, Reagan has inevitably alienated his own right wing. He has joined the Soviet leader in stigmatizing nuclear weapons and pledging to work to abolish them in a joint project. Moreover, Reagan seriously undermined the conservative camp's sustained criticism of according "moral equivalency" to the two superpowers by saying yesterday that both nations have been masters and captives of the arms race.

Reagan's lashing back at conservatives who, he says, view war as inevitable is logically consistent with his handling of Gorbachev. The president has detailed his vision of this Soviet leader as one who genuinely wants to avoid war. Reagan's Gorbachev does not have a hidden agenda of expansionism that is advanced in going for the medium-range missile treaty and significant cuts in strategic arms.

Reagan is therefore ahead of many of his previous supporters, and many of his previous critics, in discerning that Gorbachev is that reliable. What this week in Washington will help establish is that Reagan and Secretary of State George Shultz have been right in pushing for this chance to showcase a Russian party boss who appears to be as much an outsider to conventional thinking as is the 40th American president.