FRANCIS AVENUE in Cambridge, Massachusetts, is the sort of quintessentially American street that the dreams of the affluent society feed on: a street of unostentatious, comfortable assumption; spacious; lined with trees and big rambling houses of a more or less Victorian inspiration; occasionally a modest car at the curb, a benign pet sniffing the casually tended shrubbery. A vehicle passing slowly does not disturb the impression of contented ease, nor does the handsome family strolling with their dog in the early springtime afternoon toward a birthday party up the street.

And if the birthday party is for the dog and the cake is made of ground round, and if the man is the noted biographer, and if the man is the noted biographer Justin Kaplan (Twain, Whitman) and the woman is the novelist Anne Bernays, the grand-niece of Sigmund Freud . . . well, this is, after all, Cambridge in the early twilight of the 20th century where the conventional wisdom, which the conservative appearance of this street would seem to embody, continues to be severely tried, most particularly by the unusually tall and lanky man who now emerges from one of these houses with his dog straining at the leash.

The man is John Kenneth Galbraith, the writer, teacher, economist and iconoclast, the public man who put the phrase, "the conventional wisdom," into our common speech, who back in the '50s defined for us the affluent society in which enough of us live to make the epithet apt, who continues methodically to question the assumptions on which that society and the life of this very street rest, and who is himself a one-man industrial state, the second largest business in Harvard Square, as he with his disarming lack of false humility puts it.

The true Galbraithean mode is irony. Humility, false or otherwise, is a quality he does not care to cultivate, knowing, as he must, that arrogance lies on its other side. Like De Gaulle, he is a very tall man, 6 feet 8 inches, and as he once told France in a remark meant to fan the small fire of his own assiduously tended mythology, it is very difficult for tall man to avoid looking down on those of conventional stature. Despite appearances, despite the legend his candor and his wry, acerbic wit do much to enhance, Galbraith is not an arrogant man; he is simply larger, smarter and harder working than most, and he, of course, knows it. Not only does he know it; on the whole, he delights in it. A framed piece of needlepoint on the wall above the bookcases in his living room proclaims "Galbraith's First Law: Modesty is a Vastly Overrated Virtue." Walking in the rather rarefied atmosphere of Francis Avenue, he pauses to point out this house or that and to note its distinguished occupants (or former occupants, in the cases of Arthur M. Schlesinger Jr., his close friend whose birthday Galbraith shares, and Daniel Patrick Moynihan). "There is an average of two books per house on this street," he says. "Of course, I bring the average up considerably."

Now in his 73rd year but seeming considerably younger, Galbraith has 20 books behind him, including his trilogy on economics: The Affluent Society (1958), The New Industrial State (1967), and Economics and the the Public Purpose (1973). His 21st is the memoir now appearing, A Life in Our Times, on which he lavished more care to the prose than any thing since The Scotch (1964), which he has hitherto regarded as his best piece of writing. Two more books are in progress. One of them is a collection of essays, The Great Conservative Revolt, putting together in somewhat extended form the pieces that have been appearing the past several months opposite the editorial page of The Washington Post and describing, as he says, "Reagan's rediscovery of the 18th century, with feudal overtones." The other will consider the overall theory and structure of power, its sources, its instruments of enforcement, its countervailing dialectics and dynamics. His editor at Houghton Mifflin, Austin G. Olney, says his company has never lost money on a Galbraith book; indeed, on some the profits have been considerable. Galbraith negotiates his own contracts, naming a sum that may or may not be extravagant, depending on your point of view, but that always includes an added percentage for what Galbraith calls Dynamic Financial Tension (the DFT factor). A friend says, "He knows how to be paid nine different ways for the same word. I've seen it." The remark is made not in resentment or in envy but in awe.

In the meantime, he has accumulated a string of honorary degrees from universities ranging from Mysore -- "A beautiful white robe; I always wear it to commencement because it stands out" -- to Paris to Louvain to California, and a list of credentials that includes but, as they say, is not limited to, distinguished careers in government (with the Office of Price Administration during World War II, and the United States Strategic Bombing Survey of Germany and Japan immediately afterwards), in Democratic politics (working for Roosevelt, for Adlai Stevenson, for both John and Edward Kennedy, for Lyndon Johnson's Great Society programs, for Eugene McCarthy in 1968 and George McGovern in 1972, and being both a very early and very outspoken opponent of the war in Vietnam), and in teaching, almost entirely at Harvard -- "a tolerant place in the main" -- where he is now the emeritus Paul M. Warburg Professor of Economics. "I think I never missed a lecture as a result of politics," he says. And then he was an editor of Fortune for a time in the 1940s and, of course, Ambassador in India from 1961 to 1963. During those years, "I simply excluded myself from the vast routine of the embassy and wrote." The results were Ambassador's Journal (1969), extracted from his diaries, a book on Indian painting with Mohindar Singh Randhawa (1968), and under the pseudonym of Mark Epernay The McLandress Dimension (1963), a satirical work which he wrote on weekends. "I always write on weekends -- don't you?" Except in unusual circumstances -- the India-China border war of 1962, for example -- an ambassdor's job is not particularly taxing, as he has often noted.

When The McLandress Dimension was published, Newsweek cabled Galbraith in India, asking him to confirm or deny that he was Mark Epernay. An honest man, Galbraith mulled the problem over for a bit and cabled back: "Who is Mark Epernay?" Thus was Newsweek satisfied, the Galbraithean integrity unsullied.

As Galbraith himself would point out, should it not be obvious to the most casual observer, he is not only larger, smarter and harder working than the usual run of shad, but he has convictions and the strength to accompany them -- a Voltaire on stilts, as he has been described, cutting through the pomposity of the second rate. The convictions developed along the way from his birthplace in rural southern Ontario to the Ontario Agricultural College and the University of California at Berkeley and beyond, far beyond -- to eminence, to affluence (in a perhaps discomfiting but nonetheless gratifying degree, providing for winters skiing in Gstaad; summers on his farm in Newfane, Vermont; Cambridge and the world in between), and to power, the significant, self-contained power that only a tough, independent mind abrading against the world engenders. But the strength came with the first territory, the rather austere Scottish spiritual landscape of his Elgin County youth. "A long day following a plodding, increasingly reluctant team behind a harrow endlessly back and forth over the uninspiring Ontario terrain persuaded me that all other work was easy," he writes in his memoirs. "My legacy was the inherent insecurity of the farm-reared boy in combination with an aggressive feeling that I owed it all I encountered to make them better informed."

In order to do so, he gets up every morning at half past eight, retreats into whatever study is available, cuts himself off from the telephone, and writes until noon. That is every morning, and, when he is on an airplane, as he frequently is, well he writes there, too. "Once," says Mrs. Galbraith, "on a nice morning in Gstaad, I made him go skiing at 10 o'clock. He felt guilty the rest of the day."

"Unless I get up in the morning and go to the typewriter to work on something," Galbraith says, "I feel positively vacant, ill-at-ease. Nothing fills in time like writing a thousand words or two and polishing them up. Writing forces you to escape from the boredom of your own personality into what you're doing."

The remark may startle those who doubt Galbraith has ever found his own personality less than uniquely absorbing; but one supposes that even Homer nods. In The McLandress Dimension he defines the McLandress Coefficient as "the arithmetic mean or average of the intervals of time during which a subject's thoughts remain centered on some substantive phenomenon other than his own personality." As Mark Epernay, he gave himself the lowest rating of any man then in a senior government position: 1 minute, 15 seconds. "That may be a little high," he says now. He is speaking in the ironic mode for, in fact, Galbraith's coefficient ought to be among the highest, though it would scarcely be in character for him to acknowledge that; it wouldn't fit the legend.

When Galbraith focuses his attention, he focuses it rather more intensely than most, and that is true whether he is attending to a speech for a political candidate, a book he is writing, a class he is teaching, a cause he is advocating, or the self he is contemplating, perhaps with some gratification. It is there at that last still point that man and legend meet in happy harmony, and, if you are Galbraith, it is sometimes difficult to keep your mind from straying back to the diverting complexity of that point. A story is told of a dinner with the author Howard Fast. Galbraith prepared by looking over Fast's recent books, of which there were an adequate number. "Ken took over the conversation," it is reported, "but he directed it toward Howard." Galbraith falls quite naturally into the role of master of ceremonies, fitting for a teacher. "He keeps the hoop rolling," an acquaintance notes, "until he feels it's time for him to roll it away" -- to which one can only note that interesting people tend to find themselves as fascinating as less interesting people find themselves, but with more reason.

"The two most important qualities for a writer are sobriety and solitude," Galbraith says. Sobriety is no problem: he is rather abstemious, is given to few excesses (one being a fondness for smoked salmon), and he conserves his own resources. Solitude is another matter, which is why he finds Switzerland most conducive to working. "Gstaad is the only place in the world where you look at the telephone and wish it would ring," he says. He went there in the spring of 1956 to complete The Affluent Society. "I knew no one, and by the end of the summer the book had written itself." In the winter in Gstaad he writes in the morning, thinks while skiing in the afternoon, and starts off the next day "with a full portfolio of ideas." In Cambridge he usually discusses some business at lunch, spends the afternoon attending to details in his office at Harvard's Littauer Center and the next morning, when he returns to his study on Francis Avenue, finds "I haven't had a thought." Neither in Vermont has work been as successful as in Gstaad, despite the relative seclusion of the farm and the discipline he consistently maintains "of writing something every day -- the main thing. I write a draft, work it over, think it out, work it over again. I may revise five times, and then finally achieve the note of spontaneity."

In writing and in life such seemingly careless ease -- the public note of spontaneity -- is not achieved without private sweat, and without considerable help. In Galbraith's case, the help comes from three remarkably non-deferential women. (He enjoys, it is said, the company and conversation of women more than men.) The first is Emily Wilson, who came into the Galbraith family in 1940 and remained for forty years, managing the household, helping rear the children (there are three surviving sons, Alan, Peter and Jamie, all now living in Washington) and regularly rushing out with cash to pay off the taxi for the arriving Galbraith. She says her former employer is the only man she knows who comes home on a regular basis C.O.D. She once refused to put through a call to him from President Johnson when Galbraith told her he needed an hour's rest. She told the insistent president that she worked for Galbraith, not for him. (The president said later, "Tell that woman I want her here in the White House.") And on another occasion, vividly remembered by the visitor at the door, she shouted upstairs, "Galbraith, get your ass down here.You've got company."

Andrea Williams, a handsome woman who works in the Littauer office on the Harvard Yard, has been his assisstant for 22 years and is, in effect, the editor of his prose as well as the impresario of his professional life. Although Galbraith claims, and doubtless rightly, to be his own best editor, Mrs. Williams is a close second and Galbraith credits her fully, both publicly in his writings and privately in conversation. He refers to their relationship as a partnership and describes her as "a faultless grammarian, unmovable in regard to charity, and she catches the repetitions -- my greatest fault, aside from my impressionistic spelling and punctuation." She is also a Republican, though wavering. When she came to work for him, recently out of Smith and more recently out of a marriage, Galbraith asked if she'd read The Affluent Society, which had just been published. "The first thing I said to him was 'No,'" Mrs. Williams says. "'I browsed through it in a bookstore on my way here, but I thought buying it was a little too excessive a buttering-up.'" She got the job. As to her Republican tendencies, Galbraith told her, "I've always wanted to have a lead on what the other 25 percent of the people in the country are thinking." Clearly they belong together.

And then there is Catherine Atwater Galbraith, his wife, who accompanies her husband almost everywhere and figures more prominently in his life than in his memoirs which are, by and large, those of the public man, not the private. He writes of their marriage: "Kitty had been a graduate student at Radcliffe and a little earlier in Munich, an undergraduate at Smith and at the Sorbonne. She is a wise and affectionate woman of singular beauty, intensely loyal to family and friends, a superb manager of our personal affairs, a brilliant linguist" -- Galbraith is not -- "and student of comparative literature, with no known enemies anywhere in the world, and we lived happily ever after." A sometime teacher of German at Columbia and Harvard though her field is French, she is herself a writer of various articles and the author of India, Now and Through Time, revised, brought up to date and reissued last year. She reads all his final drafts "and even did the income tax once." Kitty Galbraith is an elegant, self-possessed woman with an eye for art and a taste for the acerb gooseberry. The taste seems appropriate. One senses she is not easily intimidated by the towering Galbraith, who rises 16 inches above her, and that in her convictions she can be quite firm; in her life, quite flexible. "I've learned to adapt to all kinds of changes," she says, "which may be why I haven't done as much myself. Every time I settled in to something, we were suddenly going somewhere else. But it's certainly not been dull."

Galbraith has lived long enough, worked hard enough, fought diligently enough to see his own economic ideas move toward becoming the conventionally accepted wisdom, at least in part because of his extraordinary ability to translate complicated economic theories into clear and often witty prose that the ordinarily-educated person can grasp. He has been described as a great teacher -- one of the great teachers of the age, in fact -- but he might more appropriately he described as a preacher: a humane, articulate, liberal preacher with a strong moral commitment to bringing the good life to the the greatest number. His economics are both descriptive and prescriptive, describing what is and asking what ought to be. His final questions are questions of value. Now the faithful -- or at least the fickle -- are leaving his church to worship at the free-market and supply-side altar, the now fashionable cult among whose popular high priests are Jude Wanniski, George Gilder and Milton Friedman. (When Friedman was advising the Israeli government, Gailbraith said, "Any country that has Milton Friedman as an adviser has nothing to fear from a few million Arabs.")

"Reagan has an ingenious way of singling out services to the poor for his cutbacks," Galbraith says, "It's important -- the second most important issue of our time -- to stand against this thrust for the rich as opposed to the poor -- and on conservative grounds." Conservative is emphasized. Despite the charges of his critics on the fringe of the far right, Galbraith has never been remotely radical; his liberalism is adaptive and pragmatic. "Out system survives on the concessions it make to the disadvantaged, and it is not only uncompassionate but reckless to cut back on these funds -- the CETA funds, for instance. We're unleashing the unemployed on the cities, and the cities will be in turmoil." The old guard, the enlightened plutocracy, "all those who justified their wealth by their public service" -- they are indeed passing. "There are no Harrimans, no Bowels, no Nelson Rockefellers in the Reagan administration," Galbraith says. "We have now only people who see money as functional."

The central issue of the time is the control of arms. Last year at the Democratic Convention Galbraith said, and he repeats at the close of his memoirs, "If we fail in the control of the nuclear arms race, all of the other matters we debate in these days will be without meaning. There will be no question of civil rights, for there will be no question of civil rights, for there will be no one to enjoy them. There will be no problem of urban decay, for our cities will be gone . . . let us agree that we tell all of our countrymen, all of our allies, all human beings that we will work to have an end to this nuclear horror that now hovers as a cloud over all mankind."

Such concerns might seem remote from Francis Avenue, Cambridge, from the spacious, comfortable Galbraith house with its panelled walls, its Oriental rugs, its books in their cases, its Indian artifacts and paintings, its portrait of Kitty over the mantleplace, its mementos in Galbraith's uncluttered study of his life and times with the great, the near-great and the merely famous; they are, however, essential to its continued existence, and to the existence of the unpretentious, low-gloss and the and very private life of the highly public person who lives there. Galbraith is, in fact, a stronly private person with few public regrets; whatever personal sorrows there are remain within. His public life, he says, has been "a very superior combination of teaching, writing, and such relationship as I've had to politics. I wouldn't change any of that for the world. But I would be somewhat less diffident about my relationship to politics. I always thought myself on the outer circle, with some exceptions -- when I was with Eugene McCarthy, for one -- and if I were doing it over again I'd move with much more confidence to the center of the circle. Politicians take you on your own estimate of yourself."

It is strange to imagine Galbraith as diffident person, and yet the word pops up when he talks about himself. He was for a long time diffident about his writing, too, accepting "the assumption that being a good economist was not compatible with being a good writer." He is regretful about weakness in languages other English. Otherwise, he keeps his flaws to himself, as is his right. The sacrifices he has made -- the high cost of all this intense effort and the ruthless discipline and denial it must have entailed, both for himself and for others -- are scarcely alluded to, wither in conversion or in his memoris: a few bouts of depression, a few visits to a psychiatrist, an occasional need for sleeping pills, the death from leukemia of a son, Douglas, "intelligent, determined and variously accomplished beyond his years," at the age of 7. "Galbraith is never intimate," a friend says, "though he is one of the kindest and most thoughtful persons around -- just ask the poeple who work for him. Still, people who feel they know him really well realize how little they do know him." That may be -- probably is -- true, but a lot of people like what they know and a lot of people he does know have benefitted from his generosity of spirit, of money and of things.

At the end of one recent evening in the Galbraith house he could be seen from the street sitting in a chair by the window, sipping the remains of his Glenfiddich, and scanning in the lamplight the galleys of an excerpt from his memoirs that features his name in type large enough to be read from the curb. There is a trace of a smile on his face. It does not suggest humility, nor does it suggest arrogance; it is more a smile quiet satisfaction, of justifiable pride.