Jimmy Carter arrived at the White House knowing it, and was unable to do anything about it. Other presidents before him grasped it at some point during their terms. Last week, the realization came to Ronald Reagan, and he acted on it -- to what end we do not yet know.

The "it" being referred to is the understanding of the extraordinary importance of arms control in this, the fourth decade of the nuclear age.

Eisenhower's "open skies" proposal, Kennedy's test-ban treaty, Johnson's abortive "spirit of Glassboro," Nixon's SALT I treaty, Ford's Vladivostok agreement, Carter's failed try for SALT II -- the history of the modern presidency is studded with efforts to apply rational limits to the insanity of the nuclear-arms race.

There is something in the subject itself -- the primal fear of radiation and incineration, the dream of nuclear power being harnessed to the peaceful uses of mankind -- that makes ordinary speakers eloquent and superior speakers sublime. So it was Wednesday with President Reagan at the National Press Club.

Watching him, one knew all the reasons for skepticism. The talk was designed to calm European anti-nuclear demonstrations and to help West German Chancellor Helmut Schmidt withstand the wave of propaganda surrounding the visit to Bonn by Soviet President Leonid Brezhnev. Even the mid-morning delivery time was dictated by the desire to beam the message to the broadest European audience, watching the evening newscasts seven hours ahead of us.

The specifics of the Reagan proposal came as no surprise. The offer to withhold emplacement of a new generation of American nuclear weapons in Europe in return for the dismantling of the Soviet missiles now threatening Europe has been resisted by many in his own administration and was foredoomed to immediate rejection by the Russians.

Those facts -- well-publicized before the speech -- somehow did not dim its impact. His words touched chords that could not -- and should not -- be stilled by the interposition of such calculated qualifiers.

"There is no reason," the president said, "why people in any part of the world should have to live in permanent fear of war or its specter. I believe the time has come for all nations to act in a responsible spirit. . . . I believe the time is right to move forward on arms control. . . ."

The pople of this nation and the world desperately want to believe what the president said, that "nothing will have a higher priority" than the goal of nuclear disarmament. That human impulse imposes itself on presidents, whatever their other commitments, and becomes even more the central theme of their efforts as they look to history for their final vindication.

It is that impulse that makes even cynics become believers on the issue of arms control. In that context, it was significant, I think, that President Reagan closed his speech with a quotation from John F. Kennedy. He chose a passage from the ninth month of the Kennedy administration, a speech to the United Nations delivered in the bleak period of verbal confrontation with Soviet leader Nikita Khrushchev and the threat of armed conflict over Berlin.

There was both pessimism and cynicism in the air when Kennedy spoke. As his aide and biographer, Theodore C. Sorensen, has written, Kennedy's "initial interest in disarmament was largely for propaganda reasons -- a desire to influence neutral and world opinion. He told his disarmament planners, as they were preparing for the spring 1962 Geneva disarmament conference that he wanted them to meet the sweeping, oversimplified Soviet proposals with counter-proposals that 'were not so complex and cautious as to lack all force and appeal.'"

"But," Sorensen writes, "he increasingly recognized that there was no ultimate security in armaments, that tensions and danger were rising even as our nuclear stockpiles rose. Gradually and still skeptically he began to believe that disarmament was really achievable . . . . and that his administration's own plan . . . was a good beginning toward a goal as he did not expect to achieve in his political lifetime."

It took almost two more years before Kennedy was ready to outline in his American University speech the proposal for moving from "a strategy for annihilation toward a strategy for peace" that produced the nuclear test-ban treaty just weeks before his death. Today, 18 years after he left the White House for the last time, he is remembered as much for that speech and that treaty as for any of his other accomplishments.

Peace is the dream of all mankind. That is the realization that now spurs Ronald Reagan and that could crown his presidency -- if he and we are lucky.