PUBLIC PHILOSOPHER; Selected Letters Of Walter Lippmann.Edited by John Morton Blum. Ticknor & Fields. 652 pp. $29.95.

BEFORE THIS collection of "selected" letters of Walter Lippmann came along I thought I knew him well enough. There was the syndicated column. There was a fine collection of his early writings in his twenties. There were the big books exploring heavy philosophical propositions having to do with the American polity, the play of public opinion in policymaking, the conduct of diplomacy, the role of a free press, morality in public affairs. There were the celebrated television interviews towards the end of his public career, and finally Ronald Steel's excellent biography whose title (Walter Lippmann and the American Century) conveys a certain sense of the sweep of the man's impact on his times. I should addthat I knew him as a friend and mentor. So I come to a review of this book as a Lippmann buff. But that only reinforces the point I would make.

It does not matter as you hop-skip through this extraordinary collection of letters -- to presidents and potentates and a lot of people you have never heard of -- in what way Lippmann may have influenced your thinking or your life. Whether he impressed you or enraged you or figured not at all -- whether you are too young to have known much about him or old enough to have known a lot -- you will be rewarded by reading his out- going mail.

Lippmann was born in 1889 and died in 1974. According to John Morton Blum, the able editor and annotator of this collection whose notes provide just the right amount of context, Lippmann wrote some 20,000 letters in his lifetime -- many of them having to do with mundane, business matters. Those published here have to do with, well, ideas. They begin in 1907 with a letter to a young lady friend in which Lippmann reveals his early socialist dreams, his sense of what life is all about: "We must make our choice of the largest life and then live it supremely well." The last of the letters in this collection was written in 1969. An unknown undergraduate at the University of Maryland had asked his opinion about her conviction that "the moment for revolution had come." Lippmann's thoughtful response is wonderfully revealing of the transformation over 62 years of a fire-breathing socialist who believed it was possible to "build a citadel of human joy on the slum of misery" to a relatively conservative fellow with a restrained view that society can be improved and reformed but not transformed by disruption of the established institutions. He told the young student that he too had been a revolutionary, as a young man. He agreed that "life in the modern world is far from being the good life."

It is this evolution in Lippmann's thinking that makes what would otherwise be a bedside collection of correspondence -- one that can be opened at random and perused with profit -- into a book with a theme, a plot and more fascinating characters than you can imagine even in a volume of 652 pages. Blum's introduction would be a fine essay in itself; it is, in addition, an invaluable guide. He tells us what to look for: A picture of an uncommonly interesting, endlessly curious, often intellectually arrogant, tightly disciplined philosopher/journalist who was constitutionally incapable of dodging a controversy or giving way to his deep doubts about the governability of mankind and the management of an orderly world.

The value of these letters does not lie in what they confirm: That Lippmann was wrong about a lot of things just as he has turned out to have been brilliantly prescient about a lot of other things. Still less do we need the letters to raise the question of whether Walter Lippmann would have been a greater philosopher, or a finer professor of political science, or more powerful journlist or perhaps a distinguished statesman if he had stuck to just one of these lines of work instead of at one time or another practicing all four -- sometimes simultaneously in a way that violated conventional rules. The collection includes all manner of criticism or praise, privately conveyed to everybody from John Maynard Keynes to Felix Frankfurter, Cordell Hull, J. William Fulbright, John Foster Dulles, Dean Acheson, Wendell Willkie, John F. Kennedy, Dwight Eisenhower -- to presidents, prime ministers, senators, governors, mayors -- even while he was seemingly taking a proper distance from the high and mighty in the public appraisals he was offering his readers in his published writings.

BUT NEVER MIND the propriety of it. The difference between public and private judgments is what's especially valuable about these letters. They are, by their private nature, less Olympian; the tone is chatty, the hard edge of the public writings is softened by the intimacy of private correspondence.

Not that Lippmann necessarily thought one way in public and another in private. It is simply that he wrote differently; the letters are more like conversations. One thinks of Lippmann as a stern geopolitical pontificator. Here we find moving, thoughtful, gentle letters of condolence. He could write one to Adlai Stevenson for the loss of an election and another to the widow of a former professor for the loss of her husband -- "the greatest teacher I have ever known." When he heard that Alice Longworth was not pleased with a book she had just published, he told her she was wrong, it was fascinating and that she must write another, but that frankly she had been "a bit of a coward" about what she had left out.

He despaired, regularly, over congressional meddling at the expense of "predominant executive leadership," but in a letter to George Kennan he characteristically added: "I don't mean to throw up my hands." That's what has always struck me most forcefully about this most prolific public commentator. He regularly expressed the dimmest views of the future and of the workings of democracy; of the incapacity of the electorate to make sensible informed decisions by a majority vote; of the inability of a free press to inform the public adequately or present a balanced, truthful picture of events. You wondered when a man in such a frame of mind would "throw up his hands," turn cranky, give up.

But he never quite did, however strong his sense of nothing working right. He dealt with his later disillusionments in the way that he dealt with the first. In a series of letters in 1912, he explained to a friend why he was abandoning a job as assistant to the socialist mayor of Schenectady and beginning to rethink his socialist beliefs. The ideology at issue is not as interesting as his attitude. The job in Schenectady was becoming something of a rut and "I shall always fight an easy rut." And again: "I have never cared for an upholstered life, and, please God, I never shall. The protected existence, as I see it, is to refuse the risks, to be prudent and acquiescent, to sit tight, perhaps to climb cautiously, but never to plunge . . . I'd rather be squashed at the bottom of the heap than planted at the top. The thing that is easy to do isn't worth doing when you've done it." He was 23 years old at the time.

The rest of these letters over the next 60 years are in that spirit: An endless succession of intellectual plunges, none of them easy and many inconsistent or unpredictable. Throughout, it is the intellectual energy, sparking in so many different directions and striking so many different subjects, that astounds. More so than the public writings do these collected letters, by the privacy of their nature and their unselfconscious confidentiality, convey the nature of what Robert Frost would have called Lippmann's "lover's quarrel with the world."