THE DIFFERENCE between nature and art all but dissolved as Ronald Reagan became the nation's 40th president. It was a scene as theatrical as any in which Mr. Reagan played in Hollywood -- the only thing missing was a sudden shaft of sunlight -- but for all of its larger-than-life qualities it was incontrovertibly real. When Mr. Reagan arrived at the Capitol, America was still be "held hostage" -- a sense of Iran's unspeakable deceitfulness and the United States' painfully cloying vulnerability suffused the air. Virtually as Mr. Reagan took his oath, however, the final knots binding the hostages were coming untied, and, by the time he left the Capitol, America seemed a different place. It seemed, uncannily, precisely the place Mr. Reagan has been saying it is through 16 years of public life, and again in his inaugural address.

President Reagan's America is a nation among nations, a place where freedom has unleashed the "genius of man," a country where "we have every right to dream heroic dreams." There may be a certain automatic qulaity to this vision, as though the government had only to sprinkle the seeds of freedom, jump back and allow prosperity and national power to leap untended from American soil. But there is no denying that this vision has deep roots in American experience and in the American mind and that his evocation of it yesterday was not only politically valid in a general way but also especially appropriate on a day when the hostages were finally being freed. The feeling spread during the Carter years that the American people were being asked to accept certain unnecessary man-made restrictions on their personal circumstances and on the nation's power. As much as anything, Mr. Reagan was elected to remove these restrictions. Who doubts that among Iran's reasons for coming to terms now was a desire to beat him to town?

The first area in which the new president intends to demonstrate the benefits of a reduced (federal) government role is the economy. He is brave and right to set that priority, and he is smart and right to pledge that "no one group [will be] singled out to pay a higher price." Whether the economic corrections he seeks can be achieved, and within a framework of what the political society regards as equity, will surely test to the limits his "government-is-the-problem" ideology and his political skills.

Mr. Reagan has never conceded a contradiction between this ideology and his determination, in the national security area, to strengthen American power. That would seem to require giving Washington greater authority in international dealings, more money and other resources, and more power over people's liberties, as in a draft. No matter. He pledged yesterday to "maintain sufficient strength to prevail if need be, knowing that if we do so we have the best chance of not having to use that strength." He also would regard "will and moral courage" as the country's special "weapon."

It was affecting to hear President Reagan single out by name one American soldier who died in battle in World War I as the moral model for Americans today. The allusion to personal engagement and ultimate sacrifice in a war long past suggests the context of "ask-not" individual endeavor and patriotism in which Mr. Reagan seems most at home. If it has a certain rotogravure quality, its authenticity and evocative power are unarguable.Why, he asked, shouldn't we believe in ourselves and in our capacity to perform great deeds? "After all, we are Americans." He will be testing, we hope successfully, the resiliency of this oldest article of American faith.