It was Wednesday afternoon more than midway through the summit, and the treaty to do away with medium- and shorter-range U.S. and Soviet missiles had been signed the day before. The president had been meeting for two days formally and informally with the Soviet general secretary, with more encounters to come. But already Ronald Reagan was on a roll.

That much was evident in his manner as he took time off to chat in his Oval Office with this columnist and three colleagues. Maybe it was not quite the happiest day of his life, but it was "quite a day." He "felt good." And much the most important thing about it were his reasons why.

Part of it was the seven years of hard diplomatic slogging that produced the treaty. Part of it was the prospect for "improved relations in other places." He was asked if he was smiling out of any sense that "the West was winning," as evidenced by the Soviet struggle to bring about internal reforms. But that was not part of it; he didn't want to talk in those terms when "we are signing mutually satisfactory" agreements. And, besides, it was something a great deal more profound.

In his two previous meetings with Mikhail Gorbachev at Geneva and Reykjavik "but even more in this last meeting with the general secretary," the president said, he had come to perceive a fundamental change in the Soviet Union's view of its role in the world.

Quickly he insisted that "this doesn't mean I am dropping my guard." What it does seem to mean, however, is that Reagan is substantially revising his own perception of what he has commonly called the "Evil Empire" and the "focus of evil in the world" and of the social-economic system that he once consigned to "the ash heap of history."

He takes none of that back. On the contrary, he thinks his harsh judgments and basic beliefs are simply being proved right: dealing from strength has paid off. Hanging tough on his zero-zero INF proposal and the deployment of U.S. Pershing IIs and cruise missiles in Europe is what brought the Soviets back to the bargaining table on INF, he insists; his own walkout from Reykjavik on the sticking point of his Strategic Defense Initiative brought the Soviets back once again.

So it's not that Reagan is changing; it's the Soviets. But the result is a change in the way Reagan talks about the Soviets and communism -- a new way of talking that would have turned off many of his most ardent early supporters just as his own show of affability toward Gorbachev is enraging the true believers in his arch-conservative constituency today.

He is cautious about all this -- but convincing: "Possibly the fundamental change is that in the past Soviet leaders have openly expressed" the objective of a "one-world communist state . . . their obligation to expand and make the whole world that way -- and I no longer feel that way. I think we have a potential here of {Soviet} recognition that we have two systems, that they are competitive, that {they} aren't alike, have different values. . . but a desire to prove that we can live together in peace."

This, the president went on, is "what I've been seeing" in various conversations with Gorbachev: "The Soviet Union is expressing a belief in our, yes, competitive society, but {in} living in peace in a world together." He doesn't think "either of us is going to drop our guard," and he concedes that Gorbachev has not explicitly acknowledged to him that the Soviets have abandoned global expansionism.

What impresses Reagan, however, is that Gorbachev is "the first and only leader that has never stood up before" a Communist Party gathering "and openly stated that {global} goal, as have the others." The president cites other signs of accommodation in the problems Gorbachev is willing to discuss: a withdrawal from Afghanistan; the Soviet role in Nicaragua; a seemingly real desire to help bring an end to the Iraq-Iran war; an awareness of the need to deal with the overwhelming Soviet conventional superiority in Europe before there can be movement on removing the "theater" or battlefield nuclear weapons in Europe that Reagan believes will be sufficient to "equalize" the conventional imbalance in Europe without INF.

"This is the fourth Soviet leader in my term as president and {Gorbachev} is the first . . . that has openly discussed 'glasnost,' that there are flaws in their economic system . . . which may partially confirm some of the things I said before about their system." Without going into specifics, he said that "some of the things {Gorbachev} wants to do have been real departures from what has been {their} general policy. He's loyal to their philosophy but he believes there are things that need correcting."

Perhaps even more significant is what Gorbachev has apparently been saying to Reagan about disarmament. "He has talked voluntarily about what he sees as a need for all of us to reduce military forces, not just nuclear power, that they are a drain on the citizenry and that we would all be better off if we didn't have so much."

Can Reagan sell all this to those on the disenchanted right who are calling him things like a "useful idiot" for the communists and who think their "ideological leader has softened up"? he is asked.

"When I get my temper back, I'll make it clear to them I haven't softened up," he replied. And he is prepared to offer the INF treaty itself as evidence that by his dealings from strength the Soviets "got the idea that we weren't just hungry for a de'tente and would sign anything . . . and they came back."