A memorial service was held here this week for Charles W. Yost, a remarkably unassuming hero in the trenches of American diplomacy for most of his 73 years.

He had the slim, wiry look of a long-distance runner, just right for a finely paced career that carried him form diplomatic posts in Austria, Thailand, Poland, Greece, and Paris to ambassadorships in Laos, Syria and Morocco, and finally to the post of U.S. representative to the United Nations. He also had the insatiable curiosity of a scientist, the well-stocked mind of a scholar, and the writing style of an avid reader of good writers.

You can discover all this in a book he published in the last year of his life, misleadingly entitled: "History and Memory: "A Stateman's Perceptions of the Twentieth Century." True, it does look back on a long, diverse and distinguished record of service, beginning with Dumbarton Oaks, the San Francisco conference and Potsdam.

But it also looks forward, in a way you can't always expect of a diplomatic memoir. Yost did not just chronicle his experience. He drew on it richly in a final chapter ("The Next Twenty Years; What Is to Be Done?") that ought to be required reading for anybody caught up, one way or another, in the current debate over the proper future course of American foreign policy.

Charley Yost was not easy to summarize. None of the fashionable labels fit -- now hawk or dove. Left or Right, conservative or liberal. A Democrat, he worked for Hubert Humphrey's election in 1968, after retiring from the Foreign Service; in 1969 he was summoned back to duty by Richard Nixon to be U.N. ambassador.

He was, in short, a professional. His balanced detachment forced him into that most treacherous of all positions, the center, where most of the world's hardest and most productive diplomatic work is done. It also left him with little use for the sort of single-minded ideology so much in evidence -- and vogue -- in high places these days.

"The current independence of nations means that any substantial action by one nation against essential interests of others rebounds sooner or later to its own disadvantage," he wrote in that summary chapter. The Soviet occupation of Eastern Europe, he cited by way of example, "has not enchanced but placed in constant jeopardy the security of the U.S.S.R."

Similarly, he argued, the vast security network of pacts and commitments that the United States "flung around the world during the Cold War" made it all the more vulnerable to upheavals in Korea, in Vietnam, in the Middle East, in Cuba. "Realism in international affairs today consists less in imposing limits on others than on oneself, less in containing communism or capitalism than containing one's own excessive ambitions and unwarrented fears," he wrote.

But realism, to Yost, also meant recognizing the "engrained and self-righteous disposition to territorial aggrandizement that makes [Russia] such a frightening and dangerous neighbor and that, if the world and the Soviet Union itself are to escape catastrophe, must be curbed either from within or without."

Without illusions, Yost examined the "five factors," that he believed make the human redicament today uniquely precarious: nuclear weapons; the population explosion; "dangerously diminshed" natural resources on which modern societies depend heavily; "a global juxtaposition of peoples . . . created by the revolution in communications" (airplanes, radio and television, satellites, computers); and, finally, "the drastic compression in time of the social strains and transformation that flow from the coincidence of the several revolutions of modern times."

What is to be donw? A lot, or very little, was Yost's not altogether settling conclusion. It depends on the degree to which mankind "can be persuaded that self-interest now demands much more sophisticated, forbearing, and harmonious accommodations with nature and with our fellow men of all colors and creeds than has been necessary in the past."

Painstakingly and with a clear eye, he explored and identified the pitfalls, limitations and potential of common efforts by allies and adversaries alike to establish some degree of control over nuclear arms, local conflicts, population growth, the ravaging of the environment, the waste of resoures and the world economy in a way that might ease the incendiary disparity between rich and poor.

"There is nothing intrinsically insoluble about any of these problems," Yost, a deeply troubled optimist, wrote at the end. "The question is whether human societies as currently organized, nationally and internationally . . . will make the necessary psychological, political, and structural adaptations in time."

How we answer that question, he continued, will determine whether his wise warning leaves us in a mood "of hope or of despair." As for himself, Yost concluded, "I strongly incline to hope."