It restores the faith to encounter someone in the rough and tumble of international relations who stands for something, not for a day and not for a year, as the old song goes, but in this case for most of a lifetime devoted to the pure pursuit of peace.

I speak of Brian Urquhart, a former British army officer who spent six years on the front lines in World War II and the next 41 years making his way up through the bureaucracy of the United Nations. He was first an early planner and finally undersecretary general for special political affairs (peace-keeping operations) when he retired a few weeks ago.

Last week tribute was paid to Urquhart at a world-affairs conference in Boulder, Colo. When I called to ask him about it, he was characteristically diffident. But he id send me a copy of a lecture he gave last month at the International Institute of Strategic Studies in London -- a valedictory, so to speak, on his years of hard work in an international school of hard knocks.

At a time when the United States is embarked on a policy of what some would call "global unilateralism" and others call the Reagan Doctrine, Urquhart's hardheaded appraisal of the futility as well as the utility of multilateralism has a particular relevance.

Of the organization he labored so long for, Urquhart makes no strained defense. Peacekeeping successes are to be found in calamities that haven't happened (World War III, with nuclear weapons) or in things that could have been worse (U.N.-arranged cease-fires in the India-Pakistan War in 1965, the U.N. role in the 1948 Arab-Israeli War, and again in avoiding a U.S.-Soviet confrontation in the Middle East in 1973).

The United Nations, for another example, did not prevent dreadful bloodshed in the Congo in 1961; Urquhart hims nearly beaten to death by Katanganese soldiers. But the 20,000-man U.N. peace- keeping force exercised a decisive restraining influence. So have more than a dozen similar operations over the past 38 years -- in Cyprus, on the Golan Heights and elsewhere. Most of them were mustered and managed under Urquhart's direction.

If the successes are generally relative and can be measured only in grimmer alternatives avoided, the failures are all too visible. Urquhart recorded in his London lecture 135 regional conflicts involving 80 countries and 25 million casualties from the birth of the United Nations until 1979, and noted another 15 such conflicts, including the Iran-Iraq war, since 1979.

But Urquhart argues that even if the United Nations falls far short of its founders' visions, it does provide useful buffer forces and its good offices for mediation. It can keep the fighting from escalating, and it offers a handy face-saver for the great powers by taking the rap for politically unpopular concessions.

Urquhart's point is that the machinery is there; what does not exist is "the will, the mutual confidence or the vision to use it." Worse, he sees the world moving away from the statesmanship of the immediate postwar years. "There is an obvious trend in the opposite, back-to-the-hills direction . . . an anti-internationalism that finds expression in a strangely outmoded chauvinism, in sweeping contempt for international organizations."

A wise Reagan administration would take such an observation personally, though Urquhart is far too practiced an honest broker to name names. At the least, responsible U.S. figures ought to stop talking about gutting the United Nations financially or walking out of it.

"The United Nations was born of a colossal tragedy, and sired by leaders who knew the nature of that tragedy intimately," Urquhart said in London, adding: "Unless we have some excellent alternative, it is not wise to set aside their experience or to let wither the institutions they set up just because the world has become so complex and so difficult."